@node Sockets, Low-Level Terminal Interface, Pipes and FIFOs, Top @c %MENU% A more complicated IPC mechanism, with networking support @chapter Sockets This chapter describes the GNU facilities for interprocess communication using sockets. @cindex socket @cindex interprocess communication, with sockets A @dfn{socket} is a generalized interprocess communication channel. Like a pipe, a socket is represented as a file descriptor. Unlike pipes sockets support communication between unrelated processes, and even between processes running on different machines that communicate over a network. Sockets are the primary means of communicating with other machines; @code{telnet}, @code{rlogin}, @code{ftp}, @code{talk} and the other familiar network programs use sockets. Not all operating systems support sockets. In the GNU library, the header file @file{sys/socket.h} exists regardless of the operating system, and the socket functions always exist, but if the system does not really support sockets these functions always fail. @strong{Incomplete:} We do not currently document the facilities for broadcast messages or for configuring Internet interfaces. The reentrant functions and some newer functions that are related to IPv6 aren't documented either so far. @menu * Socket Concepts:: Basic concepts you need to know about. * Communication Styles::Stream communication, datagrams and other styles. * Socket Addresses:: How socket names (``addresses'') work. * Interface Naming:: Identifying specific network interfaces. * Local Namespace:: Details about the local namespace. * Internet Namespace:: Details about the Internet namespace. * Misc Namespaces:: Other namespaces not documented fully here. * Open/Close Sockets:: Creating sockets and destroying them. * Connections:: Operations on sockets with connection state. * Datagrams:: Operations on datagram sockets. * Inetd:: Inetd is a daemon that starts servers on request. The most convenient way to write a server is to make it work with Inetd. * Socket Options:: Miscellaneous low-level socket options. * Networks Database:: Accessing the database of network names. @end menu @node Socket Concepts @section Socket Concepts @cindex communication style (of a socket) @cindex style of communication (of a socket) When you create a socket, you must specify the style of communication you want to use and the type of protocol that should implement it. The @dfn{communication style} of a socket defines the user-level semantics of sending and receiving data on the socket. Choosing a communication style specifies the answers to questions such as these: @itemize @bullet @item @cindex packet @cindex byte stream @cindex stream (sockets) @strong{What are the units of data transmission?} Some communication styles regard the data as a sequence of bytes with no larger structure; others group the bytes into records (which are known in this context as @dfn{packets}). @item @cindex loss of data on sockets @cindex data loss on sockets @strong{Can data be lost during normal operation?} Some communication styles guarantee that all the data sent arrives in the order it was sent (barring system or network crashes); other styles occasionally lose data as a normal part of operation, and may sometimes deliver packets more than once or in the wrong order. Designing a program to use unreliable communication styles usually involves taking precautions to detect lost or misordered packets and to retransmit data as needed. @item @strong{Is communication entirely with one partner?} Some communication styles are like a telephone call---you make a @dfn{connection} with one remote socket and then exchange data freely. Other styles are like mailing letters---you specify a destination address for each message you send. @end itemize @cindex namespace (of socket) @cindex domain (of socket) @cindex socket namespace @cindex socket domain You must also choose a @dfn{namespace} for naming the socket. A socket name (``address'') is meaningful only in the context of a particular namespace. In fact, even the data type to use for a socket name may depend on the namespace. Namespaces are also called ``domains'', but we avoid that word as it can be confused with other usage of the same term. Each namespace has a symbolic name that starts with @samp{PF_}. A corresponding symbolic name starting with @samp{AF_} designates the address format for that namespace. @cindex network protocol @cindex protocol (of socket) @cindex socket protocol @cindex protocol family Finally you must choose the @dfn{protocol} to carry out the communication. The protocol determines what low-level mechanism is used to transmit and receive data. Each protocol is valid for a particular namespace and communication style; a namespace is sometimes called a @dfn{protocol family} because of this, which is why the namespace names start with @samp{PF_}. The rules of a protocol apply to the data passing between two programs, perhaps on different computers; most of these rules are handled by the operating system and you need not know about them. What you do need to know about protocols is this: @itemize @bullet @item In order to have communication between two sockets, they must specify the @emph{same} protocol. @item Each protocol is meaningful with particular style/namespace combinations and cannot be used with inappropriate combinations. For example, the TCP protocol fits only the byte stream style of communication and the Internet namespace. @item For each combination of style and namespace there is a @dfn{default protocol}, which you can request by specifying 0 as the protocol number. And that's what you should normally do---use the default. @end itemize Throughout the following description at various places variables/parameters to denote sizes are required. And here the trouble starts. In the first implementations the type of these variables was simply @code{int}. On most machines at that time an @code{int} was 32 bits wide, which created a @emph{de facto} standard requiring 32-bit variables. This is important since references to variables of this type are passed to the kernel. Then the POSIX people came and unified the interface with the words "all size values are of type @code{size_t}". On 64-bit machines @code{size_t} is 64 bits wide, so pointers to variables were no longer possible. The Unix98 specification provides a solution by introducing a type @code{socklen_t}. This type is used in all of the cases that POSIX changed to use @code{size_t}. The only requirement of this type is that it be an unsigned type of at least 32 bits. Therefore, implementations which require that references to 32-bit variables be passed can be as happy as implementations which use 64-bit values. @node Communication Styles @section Communication Styles The GNU library includes support for several different kinds of sockets, each with different characteristics. This section describes the supported socket types. The symbolic constants listed here are defined in @file{sys/socket.h}. @pindex sys/socket.h @comment sys/socket.h @comment BSD @deftypevr Macro int SOCK_STREAM The @code{SOCK_STREAM} style is like a pipe (@pxref{Pipes and FIFOs}). It operates over a connection with a particular remote socket and transmits data reliably as a stream of bytes. Use of this style is covered in detail in @ref{Connections}. @end deftypevr @comment sys/socket.h @comment BSD @deftypevr Macro int SOCK_DGRAM The @code{SOCK_DGRAM} style is used for sending individually-addressed packets unreliably. It is the diametrical opposite of @code{SOCK_STREAM}. Each time you write data to a socket of this kind, that data becomes one packet. Since @code{SOCK_DGRAM} sockets do not have connections, you must specify the recipient address with each packet. The only guarantee that the system makes about your requests to transmit data is that it will try its best to deliver each packet you send. It may succeed with the sixth packet after failing with the fourth and fifth packets; the seventh packet may arrive before the sixth, and may arrive a second time after the sixth. The typical use for @code{SOCK_DGRAM} is in situations where it is acceptable to simply re-send a packet if no response is seen in a reasonable amount of time. @xref{Datagrams}, for detailed information about how to use datagram sockets. @end deftypevr @ignore @c This appears to be only for the NS domain, which we aren't @c discussing and probably won't support either. @comment sys/socket.h @comment BSD @deftypevr Macro int SOCK_SEQPACKET This style is like @code{SOCK_STREAM} except that the data are structured into packets. A program that receives data over a @code{SOCK_SEQPACKET} socket should be prepared to read the entire message packet in a single call to @code{read}; if it only reads part of the message, the remainder of the message is simply discarded instead of being available for subsequent calls to @code{read}. Many protocols do not support this communication style. @end deftypevr @end ignore @ignore @comment sys/socket.h @comment BSD @deftypevr Macro int SOCK_RDM This style is a reliable version of @code{SOCK_DGRAM}: it sends individually addressed packets, but guarantees that each packet sent arrives exactly once. @strong{Warning:} It is not clear this is actually supported by any operating system. @end deftypevr @end ignore @comment sys/socket.h @comment BSD @deftypevr Macro int SOCK_RAW This style provides access to low-level network protocols and interfaces. Ordinary user programs usually have no need to use this style. @end deftypevr @node Socket Addresses @section Socket Addresses @cindex address of socket @cindex name of socket @cindex binding a socket address @cindex socket address (name) binding The name of a socket is normally called an @dfn{address}. The functions and symbols for dealing with socket addresses were named inconsistently, sometimes using the term ``name'' and sometimes using ``address''. You can regard these terms as synonymous where sockets are concerned. A socket newly created with the @code{socket} function has no address. Other processes can find it for communication only if you give it an address. We call this @dfn{binding} the address to the socket, and the way to do it is with the @code{bind} function. You need be concerned with the address of a socket if other processes are to find it and start communicating with it. You can specify an address for other sockets, but this is usually pointless; the first time you send data from a socket, or use it to initiate a connection, the system assigns an address automatically if you have not specified one. Occasionally a client needs to specify an address because the server discriminates based on address; for example, the rsh and rlogin protocols look at the client's socket address and only bypass password checking if it is less than @code{IPPORT_RESERVED} (@pxref{Ports}). The details of socket addresses vary depending on what namespace you are using. @xref{Local Namespace}, or @ref{Internet Namespace}, for specific information. Regardless of the namespace, you use the same functions @code{bind} and @code{getsockname} to set and examine a socket's address. These functions use a phony data type, @code{struct sockaddr *}, to accept the address. In practice, the address lives in a structure of some other data type appropriate to the address format you are using, but you cast its address to @code{struct sockaddr *} when you pass it to @code{bind}. @menu * Address Formats:: About @code{struct sockaddr}. * Setting Address:: Binding an address to a socket. * Reading Address:: Reading the address of a socket. @end menu @node Address Formats @subsection Address Formats The functions @code{bind} and @code{getsockname} use the generic data type @code{struct sockaddr *} to represent a pointer to a socket address. You can't use this data type effectively to interpret an address or construct one; for that, you must use the proper data type for the socket's namespace. Thus, the usual practice is to construct an address of the proper namespace-specific type, then cast a pointer to @code{struct sockaddr *} when you call @code{bind} or @code{getsockname}. The one piece of information that you can get from the @code{struct sockaddr} data type is the @dfn{address format designator}. This tells you which data type to use to understand the address fully. @pindex sys/socket.h The symbols in this section are defined in the header file @file{sys/socket.h}. @comment sys/socket.h @comment BSD @deftp {Data Type} {struct sockaddr} The @code{struct sockaddr} type itself has the following members: @table @code @item short int sa_family This is the code for the address format of this address. It identifies the format of the data which follows. @item char sa_data[14] This is the actual socket address data, which is format-dependent. Its length also depends on the format, and may well be more than 14. The length 14 of @code{sa_data} is essentially arbitrary. @end table @end deftp Each address format has a symbolic name which starts with @samp{AF_}. Each of them corresponds to a @samp{PF_} symbol which designates the corresponding namespace. Here is a list of address format names: @table @code @comment sys/socket.h @comment POSIX @item AF_LOCAL @vindex AF_LOCAL This designates the address format that goes with the local namespace. (@code{PF_LOCAL} is the name of that namespace.) @xref{Local Namespace Details}, for information about this address format. @comment sys/socket.h @comment BSD, Unix98 @item AF_UNIX @vindex AF_UNIX This is a synonym for @code{AF_LOCAL}. Although @code{AF_LOCAL} is mandated by POSIX.1g, @code{AF_UNIX} is portable to more systems. @code{AF_UNIX} was the traditional name stemming from BSD, so even most POSIX systems support it. It is also the name of choice in the Unix98 specification. (The same is true for @code{PF_UNIX} vs. @code{PF_LOCAL}). @comment sys/socket.h @comment GNU @item AF_FILE @vindex AF_FILE This is another synonym for @code{AF_LOCAL}, for compatibility. (@code{PF_FILE} is likewise a synonym for @code{PF_LOCAL}.) @comment sys/socket.h @comment BSD @item AF_INET @vindex AF_INET This designates the address format that goes with the Internet namespace. (@code{PF_INET} is the name of that namespace.) @xref{Internet Address Formats}. @comment sys/socket.h @comment IPv6 Basic API @item AF_INET6 This is similar to @code{AF_INET}, but refers to the IPv6 protocol. (@code{PF_INET6} is the name of the corresponding namespace.) @comment sys/socket.h @comment BSD @item AF_UNSPEC @vindex AF_UNSPEC This designates no particular address format. It is used only in rare cases, such as to clear out the default destination address of a ``connected'' datagram socket. @xref{Sending Datagrams}. The corresponding namespace designator symbol @code{PF_UNSPEC} exists for completeness, but there is no reason to use it in a program. @end table @file{sys/socket.h} defines symbols starting with @samp{AF_} for many different kinds of networks, most or all of which are not actually implemented. We will document those that really work as we receive information about how to use them. @node Setting Address @subsection Setting the Address of a Socket @pindex sys/socket.h Use the @code{bind} function to assign an address to a socket. The prototype for @code{bind} is in the header file @file{sys/socket.h}. For examples of use, see @ref{Local Socket Example}, or see @ref{Inet Example}. @comment sys/socket.h @comment BSD @deftypefun int bind (int @var{socket}, struct sockaddr *@var{addr}, socklen_t @var{length}) The @code{bind} function assigns an address to the socket @var{socket}. The @var{addr} and @var{length} arguments specify the address; the detailed format of the address depends on the namespace. The first part of the address is always the format designator, which specifies a namespace, and says that the address is in the format of that namespace. The return value is @code{0} on success and @code{-1} on failure. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{socket} argument is not a valid file descriptor. @item ENOTSOCK The descriptor @var{socket} is not a socket. @item EADDRNOTAVAIL The specified address is not available on this machine. @item EADDRINUSE Some other socket is already using the specified address. @item EINVAL The socket @var{socket} already has an address. @item EACCES You do not have permission to access the requested address. (In the Internet domain, only the super-user is allowed to specify a port number in the range 0 through @code{IPPORT_RESERVED} minus one; see @ref{Ports}.) @end table Additional conditions may be possible depending on the particular namespace of the socket. @end deftypefun @node Reading Address @subsection Reading the Address of a Socket @pindex sys/socket.h Use the function @code{getsockname} to examine the address of an Internet socket. The prototype for this function is in the header file @file{sys/socket.h}. @comment sys/socket.h @comment BSD @deftypefun int getsockname (int @var{socket}, struct sockaddr *@var{addr}, socklen_t *@var{length-ptr}) The @code{getsockname} function returns information about the address of the socket @var{socket} in the locations specified by the @var{addr} and @var{length-ptr} arguments. Note that the @var{length-ptr} is a pointer; you should initialize it to be the allocation size of @var{addr}, and on return it contains the actual size of the address data. The format of the address data depends on the socket namespace. The length of the information is usually fixed for a given namespace, so normally you can know exactly how much space is needed and can provide that much. The usual practice is to allocate a place for the value using the proper data type for the socket's namespace, then cast its address to @code{struct sockaddr *} to pass it to @code{getsockname}. The return value is @code{0} on success and @code{-1} on error. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{socket} argument is not a valid file descriptor. @item ENOTSOCK The descriptor @var{socket} is not a socket. @item ENOBUFS There are not enough internal buffers available for the operation. @end table @end deftypefun You can't read the address of a socket in the file namespace. This is consistent with the rest of the system; in general, there's no way to find a file's name from a descriptor for that file. @node Interface Naming @section Interface Naming Each network interface has a name. This usually consists of a few letters that relate to the type of interface, which may be followed by a number if there is more than one interface of that type. Examples might be @code{lo} (the loopback interface) and @code{eth0} (the first Ethernet interface). Although such names are convenient for humans, it would be clumsy to have to use them whenever a program needs to refer to an interface. In such situations an interface is referred to by its @dfn{index}, which is an arbitrarily-assigned small positive integer. The following functions, constants and data types are declared in the header file @file{net/if.h}. @comment net/if.h @deftypevr Constant size_t IFNAMSIZ This constant defines the maximum buffer size needed to hold an interface name, including its terminating zero byte. @end deftypevr @comment net/if.h @comment IPv6 basic API @deftypefun {unsigned int} if_nametoindex (const char *@var{ifname}) This function yields the interface index corresponding to a particular name. If no interface exists with the name given, it returns 0. @end deftypefun @comment net/if.h @comment IPv6 basic API @deftypefun {char *} if_indextoname (unsigned int @var{ifindex}, char *@var{ifname}) This function maps an interface index to its corresponding name. The returned name is placed in the buffer pointed to by @code{ifname}, which must be at least @code{IFNAMSIZ} bytes in length. If the index was invalid, the function's return value is a null pointer, otherwise it is @code{ifname}. @end deftypefun @comment net/if.h @comment IPv6 basic API @deftp {Data Type} {struct if_nameindex} This data type is used to hold the information about a single interface. It has the following members: @table @code @item unsigned int if_index; This is the interface index. @item char *if_name This is the null-terminated index name. @end table @end deftp @comment net/if.h @comment IPv6 basic API @deftypefun {struct if_nameindex *} if_nameindex (void) This function returns an array of @code{if_nameindex} structures, one for every interface that is present. The end of the list is indicated by a structure with an interface of 0 and a null name pointer. If an error occurs, this function returns a null pointer. The returned structure must be freed with @code{if_freenameindex} after use. @end deftypefun @comment net/if.h @comment IPv6 basic API @deftypefun void if_freenameindex (struct if_nameindex *@var{ptr}) This function frees the structure returned by an earlier call to @code{if_nameindex}. @end deftypefun @node Local Namespace @section The Local Namespace @cindex local namespace, for sockets This section describes the details of the local namespace, whose symbolic name (required when you create a socket) is @code{PF_LOCAL}. The local namespace is also known as ``Unix domain sockets''. Another name is file namespace since socket addresses are normally implemented as file names. @menu * Concepts: Local Namespace Concepts. What you need to understand. * Details: Local Namespace Details. Address format, symbolic names, etc. * Example: Local Socket Example. Example of creating a socket. @end menu @node Local Namespace Concepts @subsection Local Namespace Concepts In the local namespace socket addresses are file names. You can specify any file name you want as the address of the socket, but you must have write permission on the directory containing it. @c XXX The following was said to be wrong. @c In order to connect to a socket you must have read permission for it. It's common to put these files in the @file{/tmp} directory. One peculiarity of the local namespace is that the name is only used when opening the connection; once open the address is not meaningful and may not exist. Another peculiarity is that you cannot connect to such a socket from another machine--not even if the other machine shares the file system which contains the name of the socket. You can see the socket in a directory listing, but connecting to it never succeeds. Some programs take advantage of this, such as by asking the client to send its own process ID, and using the process IDs to distinguish between clients. However, we recommend you not use this method in protocols you design, as we might someday permit connections from other machines that mount the same file systems. Instead, send each new client an identifying number if you want it to have one. After you close a socket in the local namespace, you should delete the file name from the file system. Use @code{unlink} or @code{remove} to do this; see @ref{Deleting Files}. The local namespace supports just one protocol for any communication style; it is protocol number @code{0}. @node Local Namespace Details @subsection Details of Local Namespace @pindex sys/socket.h To create a socket in the local namespace, use the constant @code{PF_LOCAL} as the @var{namespace} argument to @code{socket} or @code{socketpair}. This constant is defined in @file{sys/socket.h}. @comment sys/socket.h @comment POSIX @deftypevr Macro int PF_LOCAL This designates the local namespace, in which socket addresses are local names, and its associated family of protocols. @code{PF_Local} is the macro used by Posix.1g. @end deftypevr @comment sys/socket.h @comment BSD @deftypevr Macro int PF_UNIX This is a synonym for @code{PF_LOCAL}, for compatibility's sake. @end deftypevr @comment sys/socket.h @comment GNU @deftypevr Macro int PF_FILE This is a synonym for @code{PF_LOCAL}, for compatibility's sake. @end deftypevr The structure for specifying socket names in the local namespace is defined in the header file @file{sys/un.h}: @pindex sys/un.h @comment sys/un.h @comment BSD @deftp {Data Type} {struct sockaddr_un} This structure is used to specify local namespace socket addresses. It has the following members: @table @code @item short int sun_family This identifies the address family or format of the socket address. You should store the value @code{AF_LOCAL} to designate the local namespace. @xref{Socket Addresses}. @item char sun_path[108] This is the file name to use. @strong{Incomplete:} Why is 108 a magic number? RMS suggests making this a zero-length array and tweaking the following example to use @code{alloca} to allocate an appropriate amount of storage based on the length of the filename. @end table @end deftp You should compute the @var{length} parameter for a socket address in the local namespace as the sum of the size of the @code{sun_family} component and the string length (@emph{not} the allocation size!) of the file name string. This can be done using the macro @code{SUN_LEN}: @comment sys/un.h @comment BSD @deftypefn {Macro} int SUN_LEN (@emph{struct sockaddr_un *} @var{ptr}) The macro computes the length of socket address in the local namespace. @end deftypefn @node Local Socket Example @subsection Example of Local-Namespace Sockets Here is an example showing how to create and name a socket in the local namespace. @smallexample @include mkfsock.c.texi @end smallexample @node Internet Namespace @section The Internet Namespace @cindex Internet namespace, for sockets This section describes the details of the protocols and socket naming conventions used in the Internet namespace. Originally the Internet namespace used only IP version 4 (IPv4). With the growing number of hosts on the Internet, a new protocol with a larger address space was necessary: IP version 6 (IPv6). IPv6 introduces 128-bit addresses (IPv4 has 32-bit addresses) and other features, and will eventually replace IPv4. To create a socket in the IPv4 Internet namespace, use the symbolic name @code{PF_INET} of this namespace as the @var{namespace} argument to @code{socket} or @code{socketpair}. For IPv6 addresses you need the macro @code{PF_INET6}. These macros are defined in @file{sys/socket.h}. @pindex sys/socket.h @comment sys/socket.h @comment BSD @deftypevr Macro int PF_INET This designates the IPv4 Internet namespace and associated family of protocols. @end deftypevr @comment sys/socket.h @comment X/Open @deftypevr Macro int PF_INET6 This designates the IPv6 Internet namespace and associated family of protocols. @end deftypevr A socket address for the Internet namespace includes the following components: @itemize @bullet @item The address of the machine you want to connect to. Internet addresses can be specified in several ways; these are discussed in @ref{Internet Address Formats}, @ref{Host Addresses} and @ref{Host Names}. @item A port number for that machine. @xref{Ports}. @end itemize You must ensure that the address and port number are represented in a canonical format called @dfn{network byte order}. @xref{Byte Order}, for information about this. @menu * Internet Address Formats:: How socket addresses are specified in the Internet namespace. * Host Addresses:: All about host addresses of Internet host. * Protocols Database:: Referring to protocols by name. * Ports:: Internet port numbers. * Services Database:: Ports may have symbolic names. * Byte Order:: Different hosts may use different byte ordering conventions; you need to canonicalize host address and port number. * Inet Example:: Putting it all together. @end menu @node Internet Address Formats @subsection Internet Socket Address Formats In the Internet namespace, for both IPv4 (@code{AF_INET}) and IPv6 (@code{AF_INET6}), a socket address consists of a host address and a port on that host. In addition, the protocol you choose serves effectively as a part of the address because local port numbers are meaningful only within a particular protocol. The data types for representing socket addresses in the Internet namespace are defined in the header file @file{netinet/in.h}. @pindex netinet/in.h @comment netinet/in.h @comment BSD @deftp {Data Type} {struct sockaddr_in} This is the data type used to represent socket addresses in the Internet namespace. It has the following members: @table @code @item sa_family_t sin_family This identifies the address family or format of the socket address. You should store the value @code{AF_INET} in this member. @xref{Socket Addresses}. @item struct in_addr sin_addr This is the Internet address of the host machine. @xref{Host Addresses}, and @ref{Host Names}, for how to get a value to store here. @item unsigned short int sin_port This is the port number. @xref{Ports}. @end table @end deftp When you call @code{bind} or @code{getsockname}, you should specify @code{sizeof (struct sockaddr_in)} as the @var{length} parameter if you are using an IPv4 Internet namespace socket address. @deftp {Data Type} {struct sockaddr_in6} This is the data type used to represent socket addresses in the IPv6 namespace. It has the following members: @table @code @item sa_family_t sin6_family This identifies the address family or format of the socket address. You should store the value of @code{AF_INET6} in this member. @xref{Socket Addresses}. @item struct in6_addr sin6_addr This is the IPv6 address of the host machine. @xref{Host Addresses}, and @ref{Host Names}, for how to get a value to store here. @item uint32_t sin6_flowinfo This is a currently unimplemented field. @item uint16_t sin6_port This is the port number. @xref{Ports}. @end table @end deftp @node Host Addresses @subsection Host Addresses Each computer on the Internet has one or more @dfn{Internet addresses}, numbers which identify that computer among all those on the Internet. Users typically write IPv4 numeric host addresses as sequences of four numbers, separated by periods, as in @samp{128.52.46.32}, and IPv6 numeric host addresses as sequences of up to eight numbers separated by colons, as in @samp{5f03:1200:836f:c100::1}. Each computer also has one or more @dfn{host names}, which are strings of words separated by periods, as in @samp{mescaline.gnu.org}. Programs that let the user specify a host typically accept both numeric addresses and host names. To open a connection a program needs a numeric address, and so must convert a host name to the numeric address it stands for. @menu * Abstract Host Addresses:: What a host number consists of. * Data type: Host Address Data Type. Data type for a host number. * Functions: Host Address Functions. Functions to operate on them. * Names: Host Names. Translating host names to host numbers. @end menu @node Abstract Host Addresses @subsubsection Internet Host Addresses @cindex host address, Internet @cindex Internet host address @ifinfo Each computer on the Internet has one or more Internet addresses, numbers which identify that computer among all those on the Internet. @end ifinfo @cindex network number @cindex local network address number An IPv4 Internet host address is a number containing four bytes of data. Historically these are divided into two parts, a @dfn{network number} and a @dfn{local network address number} within that network. In the mid-1990s classless addresses were introduced which changed this behavior. Since some functions implicitly expect the old definitions, we first describe the class-based network and will then describe classless addresses. IPv6 uses only classless addresses and therefore the following paragraphs don't apply. The class-based IPv4 network number consists of the first one, two or three bytes; the rest of the bytes are the local address. IPv4 network numbers are registered with the Network Information Center (NIC), and are divided into three classes---A, B and C. The local network address numbers of individual machines are registered with the administrator of the particular network. Class A networks have single-byte numbers in the range 0 to 127. There are only a small number of Class A networks, but they can each support a very large number of hosts. Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191. Class C networks are the smallest; they have three-byte network numbers, with the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes of an Internet address specify a network. The remaining bytes of the Internet address specify the address within that network. The Class A network 0 is reserved for broadcast to all networks. In addition, the host number 0 within each network is reserved for broadcast to all hosts in that network. These uses are obsolete now but for compatibility reasons you shouldn't use network 0 and host number 0. The Class A network 127 is reserved for loopback; you can always use the Internet address @samp{127.0.0.1} to refer to the host machine. Since a single machine can be a member of multiple networks, it can have multiple Internet host addresses. However, there is never supposed to be more than one machine with the same host address. @c !!! this section could document the IN_CLASS* macros in . @c No, it shouldn't since they're obsolete. @cindex standard dot notation, for Internet addresses @cindex dot notation, for Internet addresses There are four forms of the @dfn{standard numbers-and-dots notation} for Internet addresses: @table @code @item @var{a}.@var{b}.@var{c}.@var{d} This specifies all four bytes of the address individually and is the commonly used representation. @item @var{a}.@var{b}.@var{c} The last part of the address, @var{c}, is interpreted as a 2-byte quantity. This is useful for specifying host addresses in a Class B network with network address number @code{@var{a}.@var{b}}. @item @var{a}.@var{b} The last part of the address, @var{b}, is interpreted as a 3-byte quantity. This is useful for specifying host addresses in a Class A network with network address number @var{a}. @item @var{a} If only one part is given, this corresponds directly to the host address number. @end table Within each part of the address, the usual C conventions for specifying the radix apply. In other words, a leading @samp{0x} or @samp{0X} implies hexadecimal radix; a leading @samp{0} implies octal; and otherwise decimal radix is assumed. @subsubheading Classless Addresses IPv4 addresses (and IPv6 addresses also) are now considered classless; the distinction between classes A, B and C can be ignored. Instead an IPv4 host address consists of a 32-bit address and a 32-bit mask. The mask contains set bits for the network part and cleared bits for the host part. The network part is contiguous from the left, with the remaining bits representing the host. As a consequence, the netmask can simply be specified as the number of set bits. Classes A, B and C are just special cases of this general rule. For example, class A addresses have a netmask of @samp{255.0.0.0} or a prefix length of 8. Classless IPv4 network addresses are written in numbers-and-dots notation with the prefix length appended and a slash as separator. For example the class A network 10 is written as @samp{10.0.0.0/8}. @subsubheading IPv6 Addresses IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data. A host address is usually written as eight 16-bit hexadecimal numbers that are separated by colons. Two colons are used to abbreviate strings of consecutive zeros. For example, the IPv6 loopback address @samp{0:0:0:0:0:0:0:1} can just be written as @samp{::1}. @node Host Address Data Type @subsubsection Host Address Data Type IPv4 Internet host addresses are represented in some contexts as integers (type @code{uint32_t}). In other contexts, the integer is packaged inside a structure of type @code{struct in_addr}. It would be better if the usage were made consistent, but it is not hard to extract the integer from the structure or put the integer into a structure. You will find older code that uses @code{unsigned long int} for IPv4 Internet host addresses instead of @code{uint32_t} or @code{struct in_addr}. Historically @code{unsigned long int} was a 32-bit number but with 64-bit machines this has changed. Using @code{unsigned long int} might break the code if it is used on machines where this type doesn't have 32 bits. @code{uint32_t} is specified by Unix98 and guaranteed to have 32 bits. IPv6 Internet host addresses have 128 bits and are packaged inside a structure of type @code{struct in6_addr}. The following basic definitions for Internet addresses are declared in the header file @file{netinet/in.h}: @pindex netinet/in.h @comment netinet/in.h @comment BSD @deftp {Data Type} {struct in_addr} This data type is used in certain contexts to contain an IPv4 Internet host address. It has just one field, named @code{s_addr}, which records the host address number as an @code{uint32_t}. @end deftp @comment netinet/in.h @comment BSD @deftypevr Macro {uint32_t} INADDR_LOOPBACK You can use this constant to stand for ``the address of this machine,'' instead of finding its actual address. It is the IPv4 Internet address @samp{127.0.0.1}, which is usually called @samp{localhost}. This special constant saves you the trouble of looking up the address of your own machine. Also, the system usually implements @code{INADDR_LOOPBACK} specially, avoiding any network traffic for the case of one machine talking to itself. @end deftypevr @comment netinet/in.h @comment BSD @deftypevr Macro {uint32_t} INADDR_ANY You can use this constant to stand for ``any incoming address'' when binding to an address. @xref{Setting Address}. This is the usual address to give in the @code{sin_addr} member of @w{@code{struct sockaddr_in}} when you want to accept Internet connections. @end deftypevr @comment netinet/in.h @comment BSD @deftypevr Macro {uint32_t} INADDR_BROADCAST This constant is the address you use to send a broadcast message. @c !!! broadcast needs further documented @end deftypevr @comment netinet/in.h @comment BSD @deftypevr Macro {uint32_t} INADDR_NONE This constant is returned by some functions to indicate an error. @end deftypevr @comment netinet/in.h @comment IPv6 basic API @deftp {Data Type} {struct in6_addr} This data type is used to store an IPv6 address. It stores 128 bits of data, which can be accessed (via a union) in a variety of ways. @end deftp @comment netinet/in.h @comment IPv6 basic API @deftypevr Constant {struct in6_addr} in6addr_loopback This constant is the IPv6 address @samp{::1}, the loopback address. See above for a description of what this means. The macro @code{IN6ADDR_LOOPBACK_INIT} is provided to allow you to initialize your own variables to this value. @end deftypevr @comment netinet/in.h @comment IPv6 basic API @deftypevr Constant {struct in6_addr} in6addr_any This constant is the IPv6 address @samp{::}, the unspecified address. See above for a description of what this means. The macro @code{IN6ADDR_ANY_INIT} is provided to allow you to initialize your own variables to this value. @end deftypevr @node Host Address Functions @subsubsection Host Address Functions @pindex arpa/inet.h @noindent These additional functions for manipulating Internet addresses are declared in the header file @file{arpa/inet.h}. They represent Internet addresses in network byte order, and network numbers and local-address-within-network numbers in host byte order. @xref{Byte Order}, for an explanation of network and host byte order. @comment arpa/inet.h @comment BSD @deftypefun int inet_aton (const char *@var{name}, struct in_addr *@var{addr}) This function converts the IPv4 Internet host address @var{name} from the standard numbers-and-dots notation into binary data and stores it in the @code{struct in_addr} that @var{addr} points to. @code{inet_aton} returns nonzero if the address is valid, zero if not. @end deftypefun @comment arpa/inet.h @comment BSD @deftypefun {uint32_t} inet_addr (const char *@var{name}) This function converts the IPv4 Internet host address @var{name} from the standard numbers-and-dots notation into binary data. If the input is not valid, @code{inet_addr} returns @code{INADDR_NONE}. This is an obsolete interface to @code{inet_aton}, described immediately above. It is obsolete because @code{INADDR_NONE} is a valid address (255.255.255.255), and @code{inet_aton} provides a cleaner way to indicate error return. @end deftypefun @comment arpa/inet.h @comment BSD @deftypefun {uint32_t} inet_network (const char *@var{name}) This function extracts the network number from the address @var{name}, given in the standard numbers-and-dots notation. The returned address is in host order. If the input is not valid, @code{inet_network} returns @code{-1}. The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore. @end deftypefun @comment arpa/inet.h @comment BSD @deftypefun {char *} inet_ntoa (struct in_addr @var{addr}) This function converts the IPv4 Internet host address @var{addr} to a string in the standard numbers-and-dots notation. The return value is a pointer into a statically-allocated buffer. Subsequent calls will overwrite the same buffer, so you should copy the string if you need to save it. In multi-threaded programs each thread has an own statically-allocated buffer. But still subsequent calls of @code{inet_ntoa} in the same thread will overwrite the result of the last call. Instead of @code{inet_ntoa} the newer function @code{inet_ntop} which is described below should be used since it handles both IPv4 and IPv6 addresses. @end deftypefun @comment arpa/inet.h @comment BSD @deftypefun {struct in_addr} inet_makeaddr (uint32_t @var{net}, uint32_t @var{local}) This function makes an IPv4 Internet host address by combining the network number @var{net} with the local-address-within-network number @var{local}. @end deftypefun @comment arpa/inet.h @comment BSD @deftypefun uint32_t inet_lnaof (struct in_addr @var{addr}) This function returns the local-address-within-network part of the Internet host address @var{addr}. The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore. @end deftypefun @comment arpa/inet.h @comment BSD @deftypefun uint32_t inet_netof (struct in_addr @var{addr}) This function returns the network number part of the Internet host address @var{addr}. The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore. @end deftypefun @comment arpa/inet.h @comment IPv6 basic API @deftypefun int inet_pton (int @var{af}, const char *@var{cp}, void *@var{buf}) This function converts an Internet address (either IPv4 or IPv6) from presentation (textual) to network (binary) format. @var{af} should be either @code{AF_INET} or @code{AF_INET6}, as appropriate for the type of address being converted. @var{cp} is a pointer to the input string, and @var{buf} is a pointer to a buffer for the result. It is the caller's responsibility to make sure the buffer is large enough. @end deftypefun @comment arpa/inet.h @comment IPv6 basic API @deftypefun {const char *} inet_ntop (int @var{af}, const void *@var{cp}, char *@var{buf}, size_t @var{len}) This function converts an Internet address (either IPv4 or IPv6) from network (binary) to presentation (textual) form. @var{af} should be either @code{AF_INET} or @code{AF_INET6}, as appropriate. @var{cp} is a pointer to the address to be converted. @var{buf} should be a pointer to a buffer to hold the result, and @var{len} is the length of this buffer. The return value from the function will be this buffer address. @end deftypefun @node Host Names @subsubsection Host Names @cindex hosts database @cindex converting host name to address @cindex converting host address to name Besides the standard numbers-and-dots notation for Internet addresses, you can also refer to a host by a symbolic name. The advantage of a symbolic name is that it is usually easier to remember. For example, the machine with Internet address @samp{158.121.106.19} is also known as @samp{alpha.gnu.org}; and other machines in the @samp{gnu.org} domain can refer to it simply as @samp{alpha}. @pindex /etc/hosts @pindex netdb.h Internally, the system uses a database to keep track of the mapping between host names and host numbers. This database is usually either the file @file{/etc/hosts} or an equivalent provided by a name server. The functions and other symbols for accessing this database are declared in @file{netdb.h}. They are BSD features, defined unconditionally if you include @file{netdb.h}. @comment netdb.h @comment BSD @deftp {Data Type} {struct hostent} This data type is used to represent an entry in the hosts database. It has the following members: @table @code @item char *h_name This is the ``official'' name of the host. @item char **h_aliases These are alternative names for the host, represented as a null-terminated vector of strings. @item int h_addrtype This is the host address type; in practice, its value is always either @code{AF_INET} or @code{AF_INET6}, with the latter being used for IPv6 hosts. In principle other kinds of addresses could be represented in the database as well as Internet addresses; if this were done, you might find a value in this field other than @code{AF_INET} or @code{AF_INET6}. @xref{Socket Addresses}. @item int h_length This is the length, in bytes, of each address. @item char **h_addr_list This is the vector of addresses for the host. (Recall that the host might be connected to multiple networks and have different addresses on each one.) The vector is terminated by a null pointer. @item char *h_addr This is a synonym for @code{h_addr_list[0]}; in other words, it is the first host address. @end table @end deftp As far as the host database is concerned, each address is just a block of memory @code{h_length} bytes long. But in other contexts there is an implicit assumption that you can convert IPv4 addresses to a @code{struct in_addr} or an @code{uint32_t}. Host addresses in a @code{struct hostent} structure are always given in network byte order; see @ref{Byte Order}. You can use @code{gethostbyname}, @code{gethostbyname2} or @code{gethostbyaddr} to search the hosts database for information about a particular host. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls. You can also use @code{getaddrinfo} and @code{getnameinfo} to obtain this information. @comment netdb.h @comment BSD @deftypefun {struct hostent *} gethostbyname (const char *@var{name}) The @code{gethostbyname} function returns information about the host named @var{name}. If the lookup fails, it returns a null pointer. @end deftypefun @comment netdb.h @comment IPv6 Basic API @deftypefun {struct hostent *} gethostbyname2 (const char *@var{name}, int @var{af}) The @code{gethostbyname2} function is like @code{gethostbyname}, but allows the caller to specify the desired address family (e.g.@: @code{AF_INET} or @code{AF_INET6}) of the result. @end deftypefun @comment netdb.h @comment BSD @deftypefun {struct hostent *} gethostbyaddr (const char *@var{addr}, size_t @var{length}, int @var{format}) The @code{gethostbyaddr} function returns information about the host with Internet address @var{addr}. The parameter @var{addr} is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The @var{length} argument is the size (in bytes) of the address at @var{addr}. @var{format} specifies the address format; for an IPv4 Internet address, specify a value of @code{AF_INET}; for an IPv6 Internet address, use @code{AF_INET6}. If the lookup fails, @code{gethostbyaddr} returns a null pointer. @end deftypefun @vindex h_errno If the name lookup by @code{gethostbyname} or @code{gethostbyaddr} fails, you can find out the reason by looking at the value of the variable @code{h_errno}. (It would be cleaner design for these functions to set @code{errno}, but use of @code{h_errno} is compatible with other systems.) Here are the error codes that you may find in @code{h_errno}: @table @code @comment netdb.h @comment BSD @item HOST_NOT_FOUND @vindex HOST_NOT_FOUND No such host is known in the database. @comment netdb.h @comment BSD @item TRY_AGAIN @vindex TRY_AGAIN This condition happens when the name server could not be contacted. If you try again later, you may succeed then. @comment netdb.h @comment BSD @item NO_RECOVERY @vindex NO_RECOVERY A non-recoverable error occurred. @comment netdb.h @comment BSD @item NO_ADDRESS @vindex NO_ADDRESS The host database contains an entry for the name, but it doesn't have an associated Internet address. @end table The lookup functions above all have one in common: they are not reentrant and therefore unusable in multi-threaded applications. Therefore provides the GNU C library a new set of functions which can be used in this context. @comment netdb.h @comment GNU @deftypefun int gethostbyname_r (const char *restrict @var{name}, struct hostent *restrict @var{result_buf}, char *restrict @var{buf}, size_t @var{buflen}, struct hostent **restrict @var{result}, int *restrict @var{h_errnop}) The @code{gethostbyname_r} function returns information about the host named @var{name}. The caller must pass a pointer to an object of type @code{struct hostent} in the @var{result_buf} parameter. In addition the function may need extra buffer space and the caller must pass an pointer and the size of the buffer in the @var{buf} and @var{buflen} parameters. A pointer to the buffer, in which the result is stored, is available in @code{*@var{result}} after the function call successfully returned. If an error occurs or if no entry is found, the pointer @code{*@var{result}} is a null pointer. Success is signalled by a zero return value. If the function failed the return value is an error number. In addition to the errors defined for @code{gethostbyname} it can also be @code{ERANGE}. In this case the call should be repeated with a larger buffer. Additional error information is not stored in the global variable @code{h_errno} but instead in the object pointed to by @var{h_errnop}. Here's a small example: @smallexample struct hostent * gethostname (char *host) @{ struct hostent hostbuf, *hp; size_t hstbuflen; char *tmphstbuf; int res; int herr; hstbuflen = 1024; /* Allocate buffer, remember to free it to avoid memory leakage. */ tmphstbuf = malloc (hstbuflen); while ((res = gethostbyname_r (host, &hostbuf, tmphstbuf, hstbuflen, &hp, &herr)) == ERANGE) @{ /* Enlarge the buffer. */ hstbuflen *= 2; tmphstbuf = realloc (tmphstbuf, hstbuflen); @} /* Check for errors. */ if (res || hp == NULL) return NULL; return hp; @} @end smallexample @end deftypefun @comment netdb.h @comment GNU @deftypefun int gethostbyname2_r (const char *@var{name}, int @var{af}, struct hostent *restrict @var{result_buf}, char *restrict @var{buf}, size_t @var{buflen}, struct hostent **restrict @var{result}, int *restrict @var{h_errnop}) The @code{gethostbyname2_r} function is like @code{gethostbyname_r}, but allows the caller to specify the desired address family (e.g.@: @code{AF_INET} or @code{AF_INET6}) for the result. @end deftypefun @comment netdb.h @comment GNU @deftypefun int gethostbyaddr_r (const char *@var{addr}, size_t @var{length}, int @var{format}, struct hostent *restrict @var{result_buf}, char *restrict @var{buf}, size_t @var{buflen}, struct hostent **restrict @var{result}, int *restrict @var{h_errnop}) The @code{gethostbyaddr_r} function returns information about the host with Internet address @var{addr}. The parameter @var{addr} is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The @var{length} argument is the size (in bytes) of the address at @var{addr}. @var{format} specifies the address format; for an IPv4 Internet address, specify a value of @code{AF_INET}; for an IPv6 Internet address, use @code{AF_INET6}. Similar to the @code{gethostbyname_r} function, the caller must provide buffers for the result and memory used internally. In case of success the function returns zero. Otherwise the value is an error number where @code{ERANGE} has the special meaning that the caller-provided buffer is too small. @end deftypefun You can also scan the entire hosts database one entry at a time using @code{sethostent}, @code{gethostent} and @code{endhostent}. Be careful when using these functions because they are not reentrant. @comment netdb.h @comment BSD @deftypefun void sethostent (int @var{stayopen}) This function opens the hosts database to begin scanning it. You can then call @code{gethostent} to read the entries. @c There was a rumor that this flag has different meaning if using the DNS, @c but it appears this description is accurate in that case also. If the @var{stayopen} argument is nonzero, this sets a flag so that subsequent calls to @code{gethostbyname} or @code{gethostbyaddr} will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call. @end deftypefun @comment netdb.h @comment BSD @deftypefun {struct hostent *} gethostent (void) This function returns the next entry in the hosts database. It returns a null pointer if there are no more entries. @end deftypefun @comment netdb.h @comment BSD @deftypefun void endhostent (void) This function closes the hosts database. @end deftypefun @node Ports @subsection Internet Ports @cindex port number A socket address in the Internet namespace consists of a machine's Internet address plus a @dfn{port number} which distinguishes the sockets on a given machine (for a given protocol). Port numbers range from 0 to 65,535. Port numbers less than @code{IPPORT_RESERVED} are reserved for standard servers, such as @code{finger} and @code{telnet}. There is a database that keeps track of these, and you can use the @code{getservbyname} function to map a service name onto a port number; see @ref{Services Database}. If you write a server that is not one of the standard ones defined in the database, you must choose a port number for it. Use a number greater than @code{IPPORT_USERRESERVED}; such numbers are reserved for servers and won't ever be generated automatically by the system. Avoiding conflicts with servers being run by other users is up to you. When you use a socket without specifying its address, the system generates a port number for it. This number is between @code{IPPORT_RESERVED} and @code{IPPORT_USERRESERVED}. On the Internet, it is actually legitimate to have two different sockets with the same port number, as long as they never both try to communicate with the same socket address (host address plus port number). You shouldn't duplicate a port number except in special circumstances where a higher-level protocol requires it. Normally, the system won't let you do it; @code{bind} normally insists on distinct port numbers. To reuse a port number, you must set the socket option @code{SO_REUSEADDR}. @xref{Socket-Level Options}. @pindex netinet/in.h These macros are defined in the header file @file{netinet/in.h}. @comment netinet/in.h @comment BSD @deftypevr Macro int IPPORT_RESERVED Port numbers less than @code{IPPORT_RESERVED} are reserved for superuser use. @end deftypevr @comment netinet/in.h @comment BSD @deftypevr Macro int IPPORT_USERRESERVED Port numbers greater than or equal to @code{IPPORT_USERRESERVED} are reserved for explicit use; they will never be allocated automatically. @end deftypevr @node Services Database @subsection The Services Database @cindex services database @cindex converting service name to port number @cindex converting port number to service name @pindex /etc/services The database that keeps track of ``well-known'' services is usually either the file @file{/etc/services} or an equivalent from a name server. You can use these utilities, declared in @file{netdb.h}, to access the services database. @pindex netdb.h @comment netdb.h @comment BSD @deftp {Data Type} {struct servent} This data type holds information about entries from the services database. It has the following members: @table @code @item char *s_name This is the ``official'' name of the service. @item char **s_aliases These are alternate names for the service, represented as an array of strings. A null pointer terminates the array. @item int s_port This is the port number for the service. Port numbers are given in network byte order; see @ref{Byte Order}. @item char *s_proto This is the name of the protocol to use with this service. @xref{Protocols Database}. @end table @end deftp To get information about a particular service, use the @code{getservbyname} or @code{getservbyport} functions. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls. @comment netdb.h @comment BSD @deftypefun {struct servent *} getservbyname (const char *@var{name}, const char *@var{proto}) The @code{getservbyname} function returns information about the service named @var{name} using protocol @var{proto}. If it can't find such a service, it returns a null pointer. This function is useful for servers as well as for clients; servers use it to determine which port they should listen on (@pxref{Listening}). @end deftypefun @comment netdb.h @comment BSD @deftypefun {struct servent *} getservbyport (int @var{port}, const char *@var{proto}) The @code{getservbyport} function returns information about the service at port @var{port} using protocol @var{proto}. If it can't find such a service, it returns a null pointer. @end deftypefun @noindent You can also scan the services database using @code{setservent}, @code{getservent} and @code{endservent}. Be careful when using these functions because they are not reentrant. @comment netdb.h @comment BSD @deftypefun void setservent (int @var{stayopen}) This function opens the services database to begin scanning it. If the @var{stayopen} argument is nonzero, this sets a flag so that subsequent calls to @code{getservbyname} or @code{getservbyport} will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call. @end deftypefun @comment netdb.h @comment BSD @deftypefun {struct servent *} getservent (void) This function returns the next entry in the services database. If there are no more entries, it returns a null pointer. @end deftypefun @comment netdb.h @comment BSD @deftypefun void endservent (void) This function closes the services database. @end deftypefun @node Byte Order @subsection Byte Order Conversion @cindex byte order conversion, for socket @cindex converting byte order @cindex big-endian @cindex little-endian Different kinds of computers use different conventions for the ordering of bytes within a word. Some computers put the most significant byte within a word first (this is called ``big-endian'' order), and others put it last (``little-endian'' order). @cindex network byte order So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network. This is known as @dfn{network byte order}. When establishing an Internet socket connection, you must make sure that the data in the @code{sin_port} and @code{sin_addr} members of the @code{sockaddr_in} structure are represented in network byte order. If you are encoding integer data in the messages sent through the socket, you should convert this to network byte order too. If you don't do this, your program may fail when running on or talking to other kinds of machines. If you use @code{getservbyname} and @code{gethostbyname} or @code{inet_addr} to get the port number and host address, the values are already in network byte order, and you can copy them directly into the @code{sockaddr_in} structure. Otherwise, you have to convert the values explicitly. Use @code{htons} and @code{ntohs} to convert values for the @code{sin_port} member. Use @code{htonl} and @code{ntohl} to convert IPv4 addresses for the @code{sin_addr} member. (Remember, @code{struct in_addr} is equivalent to @code{uint32_t}.) These functions are declared in @file{netinet/in.h}. @pindex netinet/in.h @comment netinet/in.h @comment BSD @deftypefun {uint16_t} htons (uint16_t @var{hostshort}) This function converts the @code{uint16_t} integer @var{hostshort} from host byte order to network byte order. @end deftypefun @comment netinet/in.h @comment BSD @deftypefun {uint16_t} ntohs (uint16_t @var{netshort}) This function converts the @code{uint16_t} integer @var{netshort} from network byte order to host byte order. @end deftypefun @comment netinet/in.h @comment BSD @deftypefun {uint32_t} htonl (uint32_t @var{hostlong}) This function converts the @code{uint32_t} integer @var{hostlong} from host byte order to network byte order. This is used for IPv4 Internet addresses. @end deftypefun @comment netinet/in.h @comment BSD @deftypefun {uint32_t} ntohl (uint32_t @var{netlong}) This function converts the @code{uint32_t} integer @var{netlong} from network byte order to host byte order. This is used for IPv4 Internet addresses. @end deftypefun @node Protocols Database @subsection Protocols Database @cindex protocols database The communications protocol used with a socket controls low-level details of how data are exchanged. For example, the protocol implements things like checksums to detect errors in transmissions, and routing instructions for messages. Normal user programs have little reason to mess with these details directly. @cindex TCP (Internet protocol) The default communications protocol for the Internet namespace depends on the communication style. For stream communication, the default is TCP (``transmission control protocol''). For datagram communication, the default is UDP (``user datagram protocol''). For reliable datagram communication, the default is RDP (``reliable datagram protocol''). You should nearly always use the default. @pindex /etc/protocols Internet protocols are generally specified by a name instead of a number. The network protocols that a host knows about are stored in a database. This is usually either derived from the file @file{/etc/protocols}, or it may be an equivalent provided by a name server. You look up the protocol number associated with a named protocol in the database using the @code{getprotobyname} function. Here are detailed descriptions of the utilities for accessing the protocols database. These are declared in @file{netdb.h}. @pindex netdb.h @comment netdb.h @comment BSD @deftp {Data Type} {struct protoent} This data type is used to represent entries in the network protocols database. It has the following members: @table @code @item char *p_name This is the official name of the protocol. @item char **p_aliases These are alternate names for the protocol, specified as an array of strings. The last element of the array is a null pointer. @item int p_proto This is the protocol number (in host byte order); use this member as the @var{protocol} argument to @code{socket}. @end table @end deftp You can use @code{getprotobyname} and @code{getprotobynumber} to search the protocols database for a specific protocol. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls. @comment netdb.h @comment BSD @deftypefun {struct protoent *} getprotobyname (const char *@var{name}) The @code{getprotobyname} function returns information about the network protocol named @var{name}. If there is no such protocol, it returns a null pointer. @end deftypefun @comment netdb.h @comment BSD @deftypefun {struct protoent *} getprotobynumber (int @var{protocol}) The @code{getprotobynumber} function returns information about the network protocol with number @var{protocol}. If there is no such protocol, it returns a null pointer. @end deftypefun You can also scan the whole protocols database one protocol at a time by using @code{setprotoent}, @code{getprotoent} and @code{endprotoent}. Be careful when using these functions because they are not reentrant. @comment netdb.h @comment BSD @deftypefun void setprotoent (int @var{stayopen}) This function opens the protocols database to begin scanning it. If the @var{stayopen} argument is nonzero, this sets a flag so that subsequent calls to @code{getprotobyname} or @code{getprotobynumber} will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call. @end deftypefun @comment netdb.h @comment BSD @deftypefun {struct protoent *} getprotoent (void) This function returns the next entry in the protocols database. It returns a null pointer if there are no more entries. @end deftypefun @comment netdb.h @comment BSD @deftypefun void endprotoent (void) This function closes the protocols database. @end deftypefun @node Inet Example @subsection Internet Socket Example Here is an example showing how to create and name a socket in the Internet namespace. The newly created socket exists on the machine that the program is running on. Rather than finding and using the machine's Internet address, this example specifies @code{INADDR_ANY} as the host address; the system replaces that with the machine's actual address. @smallexample @include mkisock.c.texi @end smallexample Here is another example, showing how you can fill in a @code{sockaddr_in} structure, given a host name string and a port number: @smallexample @include isockad.c.texi @end smallexample @node Misc Namespaces @section Other Namespaces @vindex PF_NS @vindex PF_ISO @vindex PF_CCITT @vindex PF_IMPLINK @vindex PF_ROUTE Certain other namespaces and associated protocol families are supported but not documented yet because they are not often used. @code{PF_NS} refers to the Xerox Network Software protocols. @code{PF_ISO} stands for Open Systems Interconnect. @code{PF_CCITT} refers to protocols from CCITT. @file{socket.h} defines these symbols and others naming protocols not actually implemented. @code{PF_IMPLINK} is used for communicating between hosts and Internet Message Processors. For information on this and @code{PF_ROUTE}, an occasionally-used local area routing protocol, see the GNU Hurd Manual (to appear in the future). @node Open/Close Sockets @section Opening and Closing Sockets This section describes the actual library functions for opening and closing sockets. The same functions work for all namespaces and connection styles. @menu * Creating a Socket:: How to open a socket. * Closing a Socket:: How to close a socket. * Socket Pairs:: These are created like pipes. @end menu @node Creating a Socket @subsection Creating a Socket @cindex creating a socket @cindex socket, creating @cindex opening a socket The primitive for creating a socket is the @code{socket} function, declared in @file{sys/socket.h}. @pindex sys/socket.h @comment sys/socket.h @comment BSD @deftypefun int socket (int @var{namespace}, int @var{style}, int @var{protocol}) This function creates a socket and specifies communication style @var{style}, which should be one of the socket styles listed in @ref{Communication Styles}. The @var{namespace} argument specifies the namespace; it must be @code{PF_LOCAL} (@pxref{Local Namespace}) or @code{PF_INET} (@pxref{Internet Namespace}). @var{protocol} designates the specific protocol (@pxref{Socket Concepts}); zero is usually right for @var{protocol}. The return value from @code{socket} is the file descriptor for the new socket, or @code{-1} in case of error. The following @code{errno} error conditions are defined for this function: @table @code @item EPROTONOSUPPORT The @var{protocol} or @var{style} is not supported by the @var{namespace} specified. @item EMFILE The process already has too many file descriptors open. @item ENFILE The system already has too many file descriptors open. @item EACCES The process does not have the privilege to create a socket of the specified @var{style} or @var{protocol}. @item ENOBUFS The system ran out of internal buffer space. @end table The file descriptor returned by the @code{socket} function supports both read and write operations. However, like pipes, sockets do not support file positioning operations. @end deftypefun For examples of how to call the @code{socket} function, see @ref{Local Socket Example}, or @ref{Inet Example}. @node Closing a Socket @subsection Closing a Socket @cindex socket, closing @cindex closing a socket @cindex shutting down a socket @cindex socket shutdown When you have finished using a socket, you can simply close its file descriptor with @code{close}; see @ref{Opening and Closing Files}. If there is still data waiting to be transmitted over the connection, normally @code{close} tries to complete this transmission. You can control this behavior using the @code{SO_LINGER} socket option to specify a timeout period; see @ref{Socket Options}. @pindex sys/socket.h You can also shut down only reception or transmission on a connection by calling @code{shutdown}, which is declared in @file{sys/socket.h}. @comment sys/socket.h @comment BSD @deftypefun int shutdown (int @var{socket}, int @var{how}) The @code{shutdown} function shuts down the connection of socket @var{socket}. The argument @var{how} specifies what action to perform: @table @code @item 0 Stop receiving data for this socket. If further data arrives, reject it. @item 1 Stop trying to transmit data from this socket. Discard any data waiting to be sent. Stop looking for acknowledgement of data already sent; don't retransmit it if it is lost. @item 2 Stop both reception and transmission. @end table The return value is @code{0} on success and @code{-1} on failure. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF @var{socket} is not a valid file descriptor. @item ENOTSOCK @var{socket} is not a socket. @item ENOTCONN @var{socket} is not connected. @end table @end deftypefun @node Socket Pairs @subsection Socket Pairs @cindex creating a socket pair @cindex socket pair @cindex opening a socket pair @pindex sys/socket.h A @dfn{socket pair} consists of a pair of connected (but unnamed) sockets. It is very similar to a pipe and is used in much the same way. Socket pairs are created with the @code{socketpair} function, declared in @file{sys/socket.h}. A socket pair is much like a pipe; the main difference is that the socket pair is bidirectional, whereas the pipe has one input-only end and one output-only end (@pxref{Pipes and FIFOs}). @comment sys/socket.h @comment BSD @deftypefun int socketpair (int @var{namespace}, int @var{style}, int @var{protocol}, int @var{filedes}@t{[2]}) This function creates a socket pair, returning the file descriptors in @code{@var{filedes}[0]} and @code{@var{filedes}[1]}. The socket pair is a full-duplex communications channel, so that both reading and writing may be performed at either end. The @var{namespace}, @var{style} and @var{protocol} arguments are interpreted as for the @code{socket} function. @var{style} should be one of the communication styles listed in @ref{Communication Styles}. The @var{namespace} argument specifies the namespace, which must be @code{AF_LOCAL} (@pxref{Local Namespace}); @var{protocol} specifies the communications protocol, but zero is the only meaningful value. If @var{style} specifies a connectionless communication style, then the two sockets you get are not @emph{connected}, strictly speaking, but each of them knows the other as the default destination address, so they can send packets to each other. The @code{socketpair} function returns @code{0} on success and @code{-1} on failure. The following @code{errno} error conditions are defined for this function: @table @code @item EMFILE The process has too many file descriptors open. @item EAFNOSUPPORT The specified namespace is not supported. @item EPROTONOSUPPORT The specified protocol is not supported. @item EOPNOTSUPP The specified protocol does not support the creation of socket pairs. @end table @end deftypefun @node Connections @section Using Sockets with Connections @cindex connection @cindex client @cindex server The most common communication styles involve making a connection to a particular other socket, and then exchanging data with that socket over and over. Making a connection is asymmetric; one side (the @dfn{client}) acts to request a connection, while the other side (the @dfn{server}) makes a socket and waits for the connection request. @iftex @itemize @bullet @item @ref{Connecting}, describes what the client program must do to initiate a connection with a server. @item @ref{Listening} and @ref{Accepting Connections} describe what the server program must do to wait for and act upon connection requests from clients. @item @ref{Transferring Data}, describes how data are transferred through the connected socket. @end itemize @end iftex @menu * Connecting:: What the client program must do. * Listening:: How a server program waits for requests. * Accepting Connections:: What the server does when it gets a request. * Who is Connected:: Getting the address of the other side of a connection. * Transferring Data:: How to send and receive data. * Byte Stream Example:: An example program: a client for communicating over a byte stream socket in the Internet namespace. * Server Example:: A corresponding server program. * Out-of-Band Data:: This is an advanced feature. @end menu @node Connecting @subsection Making a Connection @cindex connecting a socket @cindex socket, connecting @cindex socket, initiating a connection @cindex socket, client actions In making a connection, the client makes a connection while the server waits for and accepts the connection. Here we discuss what the client program must do with the @code{connect} function, which is declared in @file{sys/socket.h}. @comment sys/socket.h @comment BSD @deftypefun int connect (int @var{socket}, struct sockaddr *@var{addr}, socklen_t @var{length}) The @code{connect} function initiates a connection from the socket with file descriptor @var{socket} to the socket whose address is specified by the @var{addr} and @var{length} arguments. (This socket is typically on another machine, and it must be already set up as a server.) @xref{Socket Addresses}, for information about how these arguments are interpreted. Normally, @code{connect} waits until the server responds to the request before it returns. You can set nonblocking mode on the socket @var{socket} to make @code{connect} return immediately without waiting for the response. @xref{File Status Flags}, for information about nonblocking mode. @c !!! how do you tell when it has finished connecting? I suspect the @c way you do it is select for writing. The normal return value from @code{connect} is @code{0}. If an error occurs, @code{connect} returns @code{-1}. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The socket @var{socket} is not a valid file descriptor. @item ENOTSOCK File descriptor @var{socket} is not a socket. @item EADDRNOTAVAIL The specified address is not available on the remote machine. @item EAFNOSUPPORT The namespace of the @var{addr} is not supported by this socket. @item EISCONN The socket @var{socket} is already connected. @item ETIMEDOUT The attempt to establish the connection timed out. @item ECONNREFUSED The server has actively refused to establish the connection. @item ENETUNREACH The network of the given @var{addr} isn't reachable from this host. @item EADDRINUSE The socket address of the given @var{addr} is already in use. @item EINPROGRESS The socket @var{socket} is non-blocking and the connection could not be established immediately. You can determine when the connection is completely established with @code{select}; @pxref{Waiting for I/O}. Another @code{connect} call on the same socket, before the connection is completely established, will fail with @code{EALREADY}. @item EALREADY The socket @var{socket} is non-blocking and already has a pending connection in progress (see @code{EINPROGRESS} above). @end table This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. @c @xref{pthread_cleanup_push}, for a method how to do this. @end deftypefun @node Listening @subsection Listening for Connections @cindex listening (sockets) @cindex sockets, server actions @cindex sockets, listening Now let us consider what the server process must do to accept connections on a socket. First it must use the @code{listen} function to enable connection requests on the socket, and then accept each incoming connection with a call to @code{accept} (@pxref{Accepting Connections}). Once connection requests are enabled on a server socket, the @code{select} function reports when the socket has a connection ready to be accepted (@pxref{Waiting for I/O}). The @code{listen} function is not allowed for sockets using connectionless communication styles. You can write a network server that does not even start running until a connection to it is requested. @xref{Inetd Servers}. In the Internet namespace, there are no special protection mechanisms for controlling access to a port; any process on any machine can make a connection to your server. If you want to restrict access to your server, make it examine the addresses associated with connection requests or implement some other handshaking or identification protocol. In the local namespace, the ordinary file protection bits control who has access to connect to the socket. @comment sys/socket.h @comment BSD @deftypefun int listen (int @var{socket}, int @var{n}) The @code{listen} function enables the socket @var{socket} to accept connections, thus making it a server socket. The argument @var{n} specifies the length of the queue for pending connections. When the queue fills, new clients attempting to connect fail with @code{ECONNREFUSED} until the server calls @code{accept} to accept a connection from the queue. The @code{listen} function returns @code{0} on success and @code{-1} on failure. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The argument @var{socket} is not a valid file descriptor. @item ENOTSOCK The argument @var{socket} is not a socket. @item EOPNOTSUPP The socket @var{socket} does not support this operation. @end table @end deftypefun @node Accepting Connections @subsection Accepting Connections @cindex sockets, accepting connections @cindex accepting connections When a server receives a connection request, it can complete the connection by accepting the request. Use the function @code{accept} to do this. A socket that has been established as a server can accept connection requests from multiple clients. The server's original socket @emph{does not become part of the connection}; instead, @code{accept} makes a new socket which participates in the connection. @code{accept} returns the descriptor for this socket. The server's original socket remains available for listening for further connection requests. The number of pending connection requests on a server socket is finite. If connection requests arrive from clients faster than the server can act upon them, the queue can fill up and additional requests are refused with an @code{ECONNREFUSED} error. You can specify the maximum length of this queue as an argument to the @code{listen} function, although the system may also impose its own internal limit on the length of this queue. @comment sys/socket.h @comment BSD @deftypefun int accept (int @var{socket}, struct sockaddr *@var{addr}, socklen_t *@var{length_ptr}) This function is used to accept a connection request on the server socket @var{socket}. The @code{accept} function waits if there are no connections pending, unless the socket @var{socket} has nonblocking mode set. (You can use @code{select} to wait for a pending connection, with a nonblocking socket.) @xref{File Status Flags}, for information about nonblocking mode. The @var{addr} and @var{length-ptr} arguments are used to return information about the name of the client socket that initiated the connection. @xref{Socket Addresses}, for information about the format of the information. Accepting a connection does not make @var{socket} part of the connection. Instead, it creates a new socket which becomes connected. The normal return value of @code{accept} is the file descriptor for the new socket. After @code{accept}, the original socket @var{socket} remains open and unconnected, and continues listening until you close it. You can accept further connections with @var{socket} by calling @code{accept} again. If an error occurs, @code{accept} returns @code{-1}. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{socket} argument is not a valid file descriptor. @item ENOTSOCK The descriptor @var{socket} argument is not a socket. @item EOPNOTSUPP The descriptor @var{socket} does not support this operation. @item EWOULDBLOCK @var{socket} has nonblocking mode set, and there are no pending connections immediately available. @end table This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. @c @xref{pthread_cleanup_push}, for a method how to do this. @end deftypefun The @code{accept} function is not allowed for sockets using connectionless communication styles. @node Who is Connected @subsection Who is Connected to Me? @comment sys/socket.h @comment BSD @deftypefun int getpeername (int @var{socket}, struct sockaddr *@var{addr}, socklen_t *@var{length-ptr}) The @code{getpeername} function returns the address of the socket that @var{socket} is connected to; it stores the address in the memory space specified by @var{addr} and @var{length-ptr}. It stores the length of the address in @code{*@var{length-ptr}}. @xref{Socket Addresses}, for information about the format of the address. In some operating systems, @code{getpeername} works only for sockets in the Internet domain. The return value is @code{0} on success and @code{-1} on error. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The argument @var{socket} is not a valid file descriptor. @item ENOTSOCK The descriptor @var{socket} is not a socket. @item ENOTCONN The socket @var{socket} is not connected. @item ENOBUFS There are not enough internal buffers available. @end table @end deftypefun @node Transferring Data @subsection Transferring Data @cindex reading from a socket @cindex writing to a socket Once a socket has been connected to a peer, you can use the ordinary @code{read} and @code{write} operations (@pxref{I/O Primitives}) to transfer data. A socket is a two-way communications channel, so read and write operations can be performed at either end. There are also some I/O modes that are specific to socket operations. In order to specify these modes, you must use the @code{recv} and @code{send} functions instead of the more generic @code{read} and @code{write} functions. The @code{recv} and @code{send} functions take an additional argument which you can use to specify various flags to control special I/O modes. For example, you can specify the @code{MSG_OOB} flag to read or write out-of-band data, the @code{MSG_PEEK} flag to peek at input, or the @code{MSG_DONTROUTE} flag to control inclusion of routing information on output. @menu * Sending Data:: Sending data with @code{send}. * Receiving Data:: Reading data with @code{recv}. * Socket Data Options:: Using @code{send} and @code{recv}. @end menu @node Sending Data @subsubsection Sending Data @pindex sys/socket.h The @code{send} function is declared in the header file @file{sys/socket.h}. If your @var{flags} argument is zero, you can just as well use @code{write} instead of @code{send}; see @ref{I/O Primitives}. If the socket was connected but the connection has broken, you get a @code{SIGPIPE} signal for any use of @code{send} or @code{write} (@pxref{Miscellaneous Signals}). @comment sys/socket.h @comment BSD @deftypefun int send (int @var{socket}, void *@var{buffer}, size_t @var{size}, int @var{flags}) The @code{send} function is like @code{write}, but with the additional flags @var{flags}. The possible values of @var{flags} are described in @ref{Socket Data Options}. This function returns the number of bytes transmitted, or @code{-1} on failure. If the socket is nonblocking, then @code{send} (like @code{write}) can return after sending just part of the data. @xref{File Status Flags}, for information about nonblocking mode. Note, however, that a successful return value merely indicates that the message has been sent without error, not necessarily that it has been received without error. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{socket} argument is not a valid file descriptor. @item EINTR The operation was interrupted by a signal before any data was sent. @xref{Interrupted Primitives}. @item ENOTSOCK The descriptor @var{socket} is not a socket. @item EMSGSIZE The socket type requires that the message be sent atomically, but the message is too large for this to be possible. @item EWOULDBLOCK Nonblocking mode has been set on the socket, and the write operation would block. (Normally @code{send} blocks until the operation can be completed.) @item ENOBUFS There is not enough internal buffer space available. @item ENOTCONN You never connected this socket. @item EPIPE This socket was connected but the connection is now broken. In this case, @code{send} generates a @code{SIGPIPE} signal first; if that signal is ignored or blocked, or if its handler returns, then @code{send} fails with @code{EPIPE}. @end table This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. @c @xref{pthread_cleanup_push}, for a method how to do this. @end deftypefun @node Receiving Data @subsubsection Receiving Data @pindex sys/socket.h The @code{recv} function is declared in the header file @file{sys/socket.h}. If your @var{flags} argument is zero, you can just as well use @code{read} instead of @code{recv}; see @ref{I/O Primitives}. @comment sys/socket.h @comment BSD @deftypefun int recv (int @var{socket}, void *@var{buffer}, size_t @var{size}, int @var{flags}) The @code{recv} function is like @code{read}, but with the additional flags @var{flags}. The possible values of @var{flags} are described in @ref{Socket Data Options}. If nonblocking mode is set for @var{socket}, and no data are available to be read, @code{recv} fails immediately rather than waiting. @xref{File Status Flags}, for information about nonblocking mode. This function returns the number of bytes received, or @code{-1} on failure. The following @code{errno} error conditions are defined for this function: @table @code @item EBADF The @var{socket} argument is not a valid file descriptor. @item ENOTSOCK The descriptor @var{socket} is not a socket. @item EWOULDBLOCK Nonblocking mode has been set on the socket, and the read operation would block. (Normally, @code{recv} blocks until there is input available to be read.) @item EINTR The operation was interrupted by a signal before any data was read. @xref{Interrupted Primitives}. @item ENOTCONN You never connected this socket. @end table This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. @c @xref{pthread_cleanup_push}, for a method how to do this. @end deftypefun @node Socket Data Options @subsubsection Socket Data Options @pindex sys/socket.h The @var{flags} argument to @code{send} and @code{recv} is a bit mask. You can bitwise-OR the values of the following macros together to obtain a value for this argument. All are defined in the header file @file{sys/socket.h}. @comment sys/socket.h @comment BSD @deftypevr Macro int MSG_OOB Send or receive out-of-band data. @xref{Out-of-Band Data}. @end deftypevr @comment sys/socket.h @comment BSD @deftypevr Macro int MSG_PEEK Look at the data but don't remove it from the input queue. This is only meaningful with input functions such as @code{recv}, not with @code{send}. @end deftypevr @comment sys/socket.h @comment BSD @deftypevr Macro int MSG_DONTROUTE Don't include routing information in the message. This is only meaningful with output operations, and is usually only of interest for diagnostic or routing programs. We don't try to explain it here. @end deftypevr @node Byte Stream Example @subsection Byte Stream Socket Example Here is an example client program that makes a connection for a byte stream socket in the Internet namespace. It doesn't do anything particularly interesting once it has connected to the server; it just sends a text string to the server and exits. This program uses @code{init_sockaddr} to set up the socket address; see @ref{Inet Example}. @smallexample @include inetcli.c.texi @end smallexample @node Server Example @subsection Byte Stream Connection Server Example The server end is much more complicated. Since we want to allow multiple clients to be connected to the server at the same time, it would be incorrect to wait for input from a single client by simply calling @code{read} or @code{recv}. Instead, the right thing to do is to use @code{select} (@pxref{Waiting for I/O}) to wait for input on all of the open sockets. This also allows the server to deal with additional connection requests. This particular server doesn't do anything interesting once it has gotten a message from a client. It does close the socket for that client when it detects an end-of-file condition (resulting from the client shutting down its end of the connection). This program uses @code{make_socket} to set up the socket address; see @ref{Inet Example}. @smallexample @include inetsrv.c.texi @end smallexample @node Out-of-Band Data @subsection Out-of-Band Data @cindex out-of-band data @cindex high-priority data Streams with connections permit @dfn{out-of-band} data that is delivered with higher priority than ordinary data. Typically the reason for sending out-of-band data is to send notice of an exceptional condition. To send out-of-band data use @code{send}, specifying the flag @code{MSG_OOB} (@pxref{Sending Data}). Out-of-band data are received with higher priority because the receiving process need not read it in sequence; to read the next available out-of-band data, use @code{recv} with the @code{MSG_OOB} flag (@pxref{Receiving Data}). Ordinary read operations do not read out-of-band data; they read only ordinary data. @cindex urgent socket condition When a socket finds that out-of-band data are on their way, it sends a @code{SIGURG} signal to the owner process or process group of the socket. You can specify the owner using the @code{F_SETOWN} command to the @code{fcntl} function; see @ref{Interrupt Input}. You must also establish a handler for this signal, as described in @ref{Signal Handling}, in order to take appropriate action such as reading the out-of-band data. Alternatively, you can test for pending out-of-band data, or wait until there is out-of-band data, using the @code{select} function; it can wait for an exceptional condition on the socket. @xref{Waiting for I/O}, for more information about @code{select}. Notification of out-of-band data (whether with @code{SIGURG} or with @code{select}) indicates that out-of-band data are on the way; the data may not actually arrive until later. If you try to read the out-of-band data before it arrives, @code{recv} fails with an @code{EWOULDBLOCK} error. Sending out-of-band data automatically places a ``mark'' in the stream of ordinary data, showing where in the sequence the out-of-band data ``would have been''. This is useful when the meaning of out-of-band data is ``cancel everything sent so far''. Here is how you can test, in the receiving process, whether any ordinary data was sent before the mark: @smallexample success = ioctl (socket, SIOCATMARK, &atmark); @end smallexample The @code{integer} variable @var{atmark} is set to a nonzero value if the socket's read pointer has reached the ``mark''. @c Posix 1.g specifies sockatmark for this ioctl. sockatmark is not @c implemented yet. Here's a function to discard any ordinary data preceding the out-of-band mark: @smallexample int discard_until_mark (int socket) @{ while (1) @{ /* @r{This is not an arbitrary limit; any size will do.} */ char buffer[1024]; int atmark, success; /* @r{If we have reached the mark, return.} */ success = ioctl (socket, SIOCATMARK, &atmark); if (success < 0) perror ("ioctl"); if (result) return; /* @r{Otherwise, read a bunch of ordinary data and discard it.} @r{This is guaranteed not to read past the mark} @r{if it starts before the mark.} */ success = read (socket, buffer, sizeof buffer); if (success < 0) perror ("read"); @} @} @end smallexample If you don't want to discard the ordinary data preceding the mark, you may need to read some of it anyway, to make room in internal system buffers for the out-of-band data. If you try to read out-of-band data and get an @code{EWOULDBLOCK} error, try reading some ordinary data (saving it so that you can use it when you want it) and see if that makes room. Here is an example: @smallexample struct buffer @{ char *buf; int size; struct buffer *next; @}; /* @r{Read the out-of-band data from SOCKET and return it} @r{as a `struct buffer', which records the address of the data} @r{and its size.} @r{It may be necessary to read some ordinary data} @r{in order to make room for the out-of-band data.} @r{If so, the ordinary data are saved as a chain of buffers} @r{found in the `next' field of the value.} */ struct buffer * read_oob (int socket) @{ struct buffer *tail = 0; struct buffer *list = 0; while (1) @{ /* @r{This is an arbitrary limit.} @r{Does anyone know how to do this without a limit?} */ #define BUF_SZ 1024 char *buf = (char *) xmalloc (BUF_SZ); int success; int atmark; /* @r{Try again to read the out-of-band data.} */ success = recv (socket, buf, BUF_SZ, MSG_OOB); if (success >= 0) @{ /* @r{We got it, so return it.} */ struct buffer *link = (struct buffer *) xmalloc (sizeof (struct buffer)); link->buf = buf; link->size = success; link->next = list; return link; @} /* @r{If we fail, see if we are at the mark.} */ success = ioctl (socket, SIOCATMARK, &atmark); if (success < 0) perror ("ioctl"); if (atmark) @{ /* @r{At the mark; skipping past more ordinary data cannot help.} @r{So just wait a while.} */ sleep (1); continue; @} /* @r{Otherwise, read a bunch of ordinary data and save it.} @r{This is guaranteed not to read past the mark} @r{if it starts before the mark.} */ success = read (socket, buf, BUF_SZ); if (success < 0) perror ("read"); /* @r{Save this data in the buffer list.} */ @{ struct buffer *link = (struct buffer *) xmalloc (sizeof (struct buffer)); link->buf = buf; link->size = success; /* @r{Add the new link to the end of the list.} */ if (tail) tail->next = link; else list = link; tail = link; @} @} @} @end smallexample @node Datagrams @section Datagram Socket Operations @cindex datagram socket This section describes how to use communication styles that don't use connections (styles @code{SOCK_DGRAM} and @code{SOCK_RDM}). Using these styles, you group data into packets and each packet is an independent communication. You specify the destination for each packet individually. Datagram packets are like letters: you send each one independently with its own destination address, and they may arrive in the wrong order or not at all. The @code{listen} and @code{accept} functions are not allowed for sockets using connectionless communication styles. @menu * Sending Datagrams:: Sending packets on a datagram socket. * Receiving Datagrams:: Receiving packets on a datagram socket. * Datagram Example:: An example program: packets sent over a datagram socket in the local namespace. * Example Receiver:: Another program, that receives those packets. @end menu @node Sending Datagrams @subsection Sending Datagrams @cindex sending a datagram @cindex transmitting datagrams @cindex datagrams, transmitting @pindex sys/socket.h The normal way of sending data on a datagram socket is by using the @code{sendto} function, declared in @file{sys/socket.h}. You can call @code{connect} on a datagram socket, but this only specifies a default destination for further data transmission on the socket. When a socket has a default destination you can use @code{send} (@pxref{Sending Data}) or even @code{write} (@pxref{I/O Primitives}) to send a packet there. You can cancel the default destination by calling @code{connect} using an address format of @code{AF_UNSPEC} in the @var{addr} argument. @xref{Connecting}, for more information about the @code{connect} function. @comment sys/socket.h @comment BSD @deftypefun int sendto (int @var{socket}, void *@var{buffer}, size_t @var{size}, int @var{flags}, struct sockaddr *@var{addr}, socklen_t @var{length}) The @code{sendto} function transmits the data in the @var{buffer} through the socket @var{socket} to the destination address specified by the @var{addr} and @var{length} arguments. The @var{size} argument specifies the number of bytes to be transmitted. The @var{flags} are interpreted the same way as for @code{send}; see @ref{Socket Data Options}. The return value and error conditions are also the same as for @code{send}, but you cannot rely on the system to detect errors and report them; the most common error is that the packet is lost or there is no-one at the specified address to receive it, and the operating system on your machine usually does not know this. It is also possible for one call to @code{sendto} to report an error owing to a problem related to a previous call. This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. @c @xref{pthread_cleanup_push}, for a method how to do this. @end deftypefun @node Receiving Datagrams @subsection Receiving Datagrams @cindex receiving datagrams The @code{recvfrom} function reads a packet from a datagram socket and also tells you where it was sent from. This function is declared in @file{sys/socket.h}. @comment sys/socket.h @comment BSD @deftypefun int recvfrom (int @var{socket}, void *@var{buffer}, size_t @var{size}, int @var{flags}, struct sockaddr *@var{addr}, socklen_t *@var{length-ptr}) The @code{recvfrom} function reads one packet from the socket @var{socket} into the buffer @var{buffer}. The @var{size} argument specifies the maximum number of bytes to be read. If the packet is longer than @var{size} bytes, then you get the first @var{size} bytes of the packet and the rest of the packet is lost. There's no way to read the rest of the packet. Thus, when you use a packet protocol, you must always know how long a packet to expect. The @var{addr} and @var{length-ptr} arguments are used to return the address where the packet came from. @xref{Socket Addresses}. For a socket in the local domain the address information won't be meaningful, since you can't read the address of such a socket (@pxref{Local Namespace}). You can specify a null pointer as the @var{addr} argument if you are not interested in this information. The @var{flags} are interpreted the same way as for @code{recv} (@pxref{Socket Data Options}). The return value and error conditions are also the same as for @code{recv}. This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. @c @xref{pthread_cleanup_push}, for a method how to do this. @end deftypefun You can use plain @code{recv} (@pxref{Receiving Data}) instead of @code{recvfrom} if you don't need to find out who sent the packet (either because you know where it should come from or because you treat all possible senders alike). Even @code{read} can be used if you don't want to specify @var{flags} (@pxref{I/O Primitives}). @ignore @c sendmsg and recvmsg are like readv and writev in that they @c use a series of buffers. It's not clear this is worth @c supporting or that we support them. @c !!! they can do more; it is hairy @comment sys/socket.h @comment BSD @deftp {Data Type} {struct msghdr} @end deftp @comment sys/socket.h @comment BSD @deftypefun int sendmsg (int @var{socket}, const struct msghdr *@var{message}, int @var{flags}) This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is cancel. @c @xref{pthread_cleanup_push}, for a method how to do this. @end deftypefun @comment sys/socket.h @comment BSD @deftypefun int recvmsg (int @var{socket}, struct msghdr *@var{message}, int @var{flags}) This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. @c @xref{pthread_cleanup_push}, for a method how to do this. @end deftypefun @end ignore @node Datagram Example @subsection Datagram Socket Example Here is a set of example programs that send messages over a datagram stream in the local namespace. Both the client and server programs use the @code{make_named_socket} function that was presented in @ref{Local Socket Example}, to create and name their sockets. First, here is the server program. It sits in a loop waiting for messages to arrive, bouncing each message back to the sender. Obviously this isn't a particularly useful program, but it does show the general ideas involved. @smallexample @include filesrv.c.texi @end smallexample @node Example Receiver @subsection Example of Reading Datagrams Here is the client program corresponding to the server above. It sends a datagram to the server and then waits for a reply. Notice that the socket for the client (as well as for the server) in this example has to be given a name. This is so that the server can direct a message back to the client. Since the socket has no associated connection state, the only way the server can do this is by referencing the name of the client. @smallexample @include filecli.c.texi @end smallexample Keep in mind that datagram socket communications are unreliable. In this example, the client program waits indefinitely if the message never reaches the server or if the server's response never comes back. It's up to the user running the program to kill and restart it if desired. A more automatic solution could be to use @code{select} (@pxref{Waiting for I/O}) to establish a timeout period for the reply, and in case of timeout either re-send the message or shut down the socket and exit. @node Inetd @section The @code{inetd} Daemon We've explained above how to write a server program that does its own listening. Such a server must already be running in order for anyone to connect to it. Another way to provide a service on an Internet port is to let the daemon program @code{inetd} do the listening. @code{inetd} is a program that runs all the time and waits (using @code{select}) for messages on a specified set of ports. When it receives a message, it accepts the connection (if the socket style calls for connections) and then forks a child process to run the corresponding server program. You specify the ports and their programs in the file @file{/etc/inetd.conf}. @menu * Inetd Servers:: * Configuring Inetd:: @end menu @node Inetd Servers @subsection @code{inetd} Servers Writing a server program to be run by @code{inetd} is very simple. Each time someone requests a connection to the appropriate port, a new server process starts. The connection already exists at this time; the socket is available as the standard input descriptor and as the standard output descriptor (descriptors 0 and 1) in the server process. Thus the server program can begin reading and writing data right away. Often the program needs only the ordinary I/O facilities; in fact, a general-purpose filter program that knows nothing about sockets can work as a byte stream server run by @code{inetd}. You can also use @code{inetd} for servers that use connectionless communication styles. For these servers, @code{inetd} does not try to accept a connection since no connection is possible. It just starts the server program, which can read the incoming datagram packet from descriptor 0. The server program can handle one request and then exit, or you can choose to write it to keep reading more requests until no more arrive, and then exit. You must specify which of these two techniques the server uses when you configure @code{inetd}. @node Configuring Inetd @subsection Configuring @code{inetd} The file @file{/etc/inetd.conf} tells @code{inetd} which ports to listen to and what server programs to run for them. Normally each entry in the file is one line, but you can split it onto multiple lines provided all but the first line of the entry start with whitespace. Lines that start with @samp{#} are comments. Here are two standard entries in @file{/etc/inetd.conf}: @smallexample ftp stream tcp nowait root /libexec/ftpd ftpd talk dgram udp wait root /libexec/talkd talkd @end smallexample An entry has this format: @smallexample @var{service} @var{style} @var{protocol} @var{wait} @var{username} @var{program} @var{arguments} @end smallexample The @var{service} field says which service this program provides. It should be the name of a service defined in @file{/etc/services}. @code{inetd} uses @var{service} to decide which port to listen on for this entry. The fields @var{style} and @var{protocol} specify the communication style and the protocol to use for the listening socket. The style should be the name of a communication style, converted to lower case and with @samp{SOCK_} deleted---for example, @samp{stream} or @samp{dgram}. @var{protocol} should be one of the protocols listed in @file{/etc/protocols}. The typical protocol names are @samp{tcp} for byte stream connections and @samp{udp} for unreliable datagrams. The @var{wait} field should be either @samp{wait} or @samp{nowait}. Use @samp{wait} if @var{style} is a connectionless style and the server, once started, handles multiple requests as they come in. Use @samp{nowait} if @code{inetd} should start a new process for each message or request that comes in. If @var{style} uses connections, then @var{wait} @strong{must} be @samp{nowait}. @var{user} is the user name that the server should run as. @code{inetd} runs as root, so it can set the user ID of its children arbitrarily. It's best to avoid using @samp{root} for @var{user} if you can; but some servers, such as Telnet and FTP, read a username and password themselves. These servers need to be root initially so they can log in as commanded by the data coming over the network. @var{program} together with @var{arguments} specifies the command to run to start the server. @var{program} should be an absolute file name specifying the executable file to run. @var{arguments} consists of any number of whitespace-separated words, which become the command-line arguments of @var{program}. The first word in @var{arguments} is argument zero, which should by convention be the program name itself (sans directories). If you edit @file{/etc/inetd.conf}, you can tell @code{inetd} to reread the file and obey its new contents by sending the @code{inetd} process the @code{SIGHUP} signal. You'll have to use @code{ps} to determine the process ID of the @code{inetd} process as it is not fixed. @c !!! could document /etc/inetd.sec @node Socket Options @section Socket Options @cindex socket options This section describes how to read or set various options that modify the behavior of sockets and their underlying communications protocols. @cindex level, for socket options @cindex socket option level When you are manipulating a socket option, you must specify which @dfn{level} the option pertains to. This describes whether the option applies to the socket interface, or to a lower-level communications protocol interface. @menu * Socket Option Functions:: The basic functions for setting and getting socket options. * Socket-Level Options:: Details of the options at the socket level. @end menu @node Socket Option Functions @subsection Socket Option Functions @pindex sys/socket.h Here are the functions for examining and modifying socket options. They are declared in @file{sys/socket.h}. @comment sys/socket.h @comment BSD @deftypefun int getsockopt (int @var{socket}, int @var{level}, int @var{optname}, void *@var{optval}, socklen_t *@var{optlen-ptr}) The @code{getsockopt} function gets information about the value of option @var{optname} at level @var{level} for socket @var{socket}. The option value is stored in a buffer that @var{optval} points to. Before the call, you should supply in @code{*@var{optlen-ptr}} the size of this buffer; on return, it contains the number of bytes of information actually stored in the buffer. Most options interpret the @var{optval} buffer as a single @code{int} value. The actual return value of @code{getsockopt} is @code{0} on success and @code{-1} on failure. The following @code{errno} error conditions are defined: @table @code @item EBADF The @var{socket} argument is not a valid file descriptor. @item ENOTSOCK The descriptor @var{socket} is not a socket. @item ENOPROTOOPT The @var{optname} doesn't make sense for the given @var{level}. @end table @end deftypefun @comment sys/socket.h @comment BSD @deftypefun int setsockopt (int @var{socket}, int @var{level}, int @var{optname}, void *@var{optval}, socklen_t @var{optlen}) This function is used to set the socket option @var{optname} at level @var{level} for socket @var{socket}. The value of the option is passed in the buffer @var{optval} of size @var{optlen}. @c Argh. -zw @iftex @hfuzz 6pt The return value and error codes for @code{setsockopt} are the same as for @code{getsockopt}. @end iftex @ifinfo The return value and error codes for @code{setsockopt} are the same as for @code{getsockopt}. @end ifinfo @end deftypefun @node Socket-Level Options @subsection Socket-Level Options @comment sys/socket.h @comment BSD @deftypevr Constant int SOL_SOCKET Use this constant as the @var{level} argument to @code{getsockopt} or @code{setsockopt} to manipulate the socket-level options described in this section. @end deftypevr @pindex sys/socket.h @noindent Here is a table of socket-level option names; all are defined in the header file @file{sys/socket.h}. @table @code @comment sys/socket.h @comment BSD @item SO_DEBUG @c Extra blank line here makes the table look better. This option toggles recording of debugging information in the underlying protocol modules. The value has type @code{int}; a nonzero value means ``yes''. @c !!! should say how this is used @c OK, anyone who knows, please explain. @comment sys/socket.h @comment BSD @item SO_REUSEADDR This option controls whether @code{bind} (@pxref{Setting Address}) should permit reuse of local addresses for this socket. If you enable this option, you can actually have two sockets with the same Internet port number; but the system won't allow you to use the two identically-named sockets in a way that would confuse the Internet. The reason for this option is that some higher-level Internet protocols, including FTP, require you to keep reusing the same port number. The value has type @code{int}; a nonzero value means ``yes''. @comment sys/socket.h @comment BSD @item SO_KEEPALIVE This option controls whether the underlying protocol should periodically transmit messages on a connected socket. If the peer fails to respond to these messages, the connection is considered broken. The value has type @code{int}; a nonzero value means ``yes''. @comment sys/socket.h @comment BSD @item SO_DONTROUTE This option controls whether outgoing messages bypass the normal message routing facilities. If set, messages are sent directly to the network interface instead. The value has type @code{int}; a nonzero value means ``yes''. @comment sys/socket.h @comment BSD @item SO_LINGER This option specifies what should happen when the socket of a type that promises reliable delivery still has untransmitted messages when it is closed; see @ref{Closing a Socket}. The value has type @code{struct linger}. @comment sys/socket.h @comment BSD @deftp {Data Type} {struct linger} This structure type has the following members: @table @code @item int l_onoff This field is interpreted as a boolean. If nonzero, @code{close} blocks until the data are transmitted or the timeout period has expired. @item int l_linger This specifies the timeout period, in seconds. @end table @end deftp @comment sys/socket.h @comment BSD @item SO_BROADCAST This option controls whether datagrams may be broadcast from the socket. The value has type @code{int}; a nonzero value means ``yes''. @comment sys/socket.h @comment BSD @item SO_OOBINLINE If this option is set, out-of-band data received on the socket is placed in the normal input queue. This permits it to be read using @code{read} or @code{recv} without specifying the @code{MSG_OOB} flag. @xref{Out-of-Band Data}. The value has type @code{int}; a nonzero value means ``yes''. @comment sys/socket.h @comment BSD @item SO_SNDBUF This option gets or sets the size of the output buffer. The value is a @code{size_t}, which is the size in bytes. @comment sys/socket.h @comment BSD @item SO_RCVBUF This option gets or sets the size of the input buffer. The value is a @code{size_t}, which is the size in bytes. @comment sys/socket.h @comment GNU @item SO_STYLE @comment sys/socket.h @comment BSD @itemx SO_TYPE This option can be used with @code{getsockopt} only. It is used to get the socket's communication style. @code{SO_TYPE} is the historical name, and @code{SO_STYLE} is the preferred name in GNU. The value has type @code{int} and its value designates a communication style; see @ref{Communication Styles}. @comment sys/socket.h @comment BSD @item SO_ERROR @c Extra blank line here makes the table look better. This option can be used with @code{getsockopt} only. It is used to reset the error status of the socket. The value is an @code{int}, which represents the previous error status. @c !!! what is "socket error status"? this is never defined. @end table @node Networks Database @section Networks Database @cindex networks database @cindex converting network number to network name @cindex converting network name to network number @pindex /etc/networks @pindex netdb.h Many systems come with a database that records a list of networks known to the system developer. This is usually kept either in the file @file{/etc/networks} or in an equivalent from a name server. This data base is useful for routing programs such as @code{route}, but it is not useful for programs that simply communicate over the network. We provide functions to access this database, which are declared in @file{netdb.h}. @comment netdb.h @comment BSD @deftp {Data Type} {struct netent} This data type is used to represent information about entries in the networks database. It has the following members: @table @code @item char *n_name This is the ``official'' name of the network. @item char **n_aliases These are alternative names for the network, represented as a vector of strings. A null pointer terminates the array. @item int n_addrtype This is the type of the network number; this is always equal to @code{AF_INET} for Internet networks. @item unsigned long int n_net This is the network number. Network numbers are returned in host byte order; see @ref{Byte Order}. @end table @end deftp Use the @code{getnetbyname} or @code{getnetbyaddr} functions to search the networks database for information about a specific network. The information is returned in a statically-allocated structure; you must copy the information if you need to save it. @comment netdb.h @comment BSD @deftypefun {struct netent *} getnetbyname (const char *@var{name}) The @code{getnetbyname} function returns information about the network named @var{name}. It returns a null pointer if there is no such network. @end deftypefun @comment netdb.h @comment BSD @deftypefun {struct netent *} getnetbyaddr (unsigned long int @var{net}, int @var{type}) The @code{getnetbyaddr} function returns information about the network of type @var{type} with number @var{net}. You should specify a value of @code{AF_INET} for the @var{type} argument for Internet networks. @code{getnetbyaddr} returns a null pointer if there is no such network. @end deftypefun You can also scan the networks database using @code{setnetent}, @code{getnetent} and @code{endnetent}. Be careful when using these functions because they are not reentrant. @comment netdb.h @comment BSD @deftypefun void setnetent (int @var{stayopen}) This function opens and rewinds the networks database. If the @var{stayopen} argument is nonzero, this sets a flag so that subsequent calls to @code{getnetbyname} or @code{getnetbyaddr} will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call. @end deftypefun @comment netdb.h @comment BSD @deftypefun {struct netent *} getnetent (void) This function returns the next entry in the networks database. It returns a null pointer if there are no more entries. @end deftypefun @comment netdb.h @comment BSD @deftypefun void endnetent (void) This function closes the networks database. @end deftypefun