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+Table of Contents
+=================
+
+ - [Intro](#intro)
+ - [git](#git)
+ - [Portability](#Portability)
+ - [Windows vs Unix](#winvsunix)
+ - [Library](#Library)
+   - [`Curl_connect`](#Curl_connect)
+   - [`Curl_do`](#Curl_do)
+   - [`Curl_readwrite`](#Curl_readwrite)
+   - [`Curl_done`](#Curl_done)
+   - [`Curl_disconnect`](#Curl_disconnect)
+ - [HTTP(S)](#http)
+ - [FTP](#ftp)
+   - [Kerberos](#kerberos)
+ - [TELNET](#telnet)
+ - [FILE](#file)
+ - [SMB](#smb)
+ - [LDAP](#ldap)
+ - [E-mail](#email)
+ - [General](#general)
+ - [Persistent Connections](#persistent)
+ - [multi interface/non-blocking](#multi)
+ - [SSL libraries](#ssl)
+ - [Library Symbols](#symbols)
+ - [Return Codes and Informationals](#returncodes)
+ - [AP/ABI](#abi)
+ - [Client](#client)
+ - [Memory Debugging](#memorydebug)
+ - [Test Suite](#test)
+ - [Asynchronous name resolves](#asyncdns)
+   - [c-ares](#cares)
+ - [`curl_off_t`](#curl_off_t)
+ - [curlx](#curlx)
+ - [Content Encoding](#contentencoding)
+ - [hostip.c explained](#hostip)
+ - [Track Down Memory Leaks](#memoryleak)
+ - [`multi_socket`](#multi_socket)
+ - [Structs in libcurl](#structs)
+
+<a name="intro"></a>
+curl internals
+==============
+
+ This project is split in two. The library and the client. The client part
+ uses the library, but the library is designed to allow other applications to
+ use it.
+
+ The largest amount of code and complexity is in the library part.
+
+
+<a name="git"></a>
+git
+===
+
+ All changes to the sources are committed to the git repository as soon as
+ they're somewhat verified to work. Changes shall be committed as independently
+ as possible so that individual changes can be easier spotted and tracked
+ afterwards.
+
+ Tagging shall be used extensively, and by the time we release new archives we
+ should tag the sources with a name similar to the released version number.
+
+<a name="Portability"></a>
+Portability
+===========
+
+ We write curl and libcurl to compile with C89 compilers.  On 32bit and up
+ machines. Most of libcurl assumes more or less POSIX compliance but that's
+ not a requirement.
+
+ We write libcurl to build and work with lots of third party tools, and we
+ want it to remain functional and buildable with these and later versions
+ (older versions may still work but is not what we work hard to maintain):
+
+Dependencies
+------------
+
+ - OpenSSL      0.9.7
+ - GnuTLS       1.2
+ - zlib         1.1.4
+ - libssh2      0.16
+ - c-ares       1.6.0
+ - libidn       0.4.1
+ - cyassl       2.0.0
+ - openldap     2.0
+ - MIT Kerberos 1.2.4
+ - GSKit        V5R3M0
+ - NSS          3.14.x
+ - axTLS        1.2.7
+ - PolarSSL     1.3.0
+ - Heimdal      ?
+ - nghttp2      1.0.0
+
+Operating Systems
+-----------------
+
+ On systems where configure runs, we aim at working on them all - if they have
+ a suitable C compiler. On systems that don't run configure, we strive to keep
+ curl running fine on:
+
+ - Windows      98
+ - AS/400       V5R3M0
+ - Symbian      9.1
+ - Windows CE   ?
+ - TPF          ?
+
+Build tools
+-----------
+
+ When writing code (mostly for generating stuff included in release tarballs)
+ we use a few "build tools" and we make sure that we remain functional with
+ these versions:
+
+ - GNU Libtool  1.4.2
+ - GNU Autoconf 2.57
+ - GNU Automake 1.7
+ - GNU M4       1.4
+ - perl         5.004
+ - roffit       0.5
+ - groff        ? (any version that supports "groff -Tps -man [in] [out]")
+ - ps2pdf (gs)  ?
+
+<a name="winvsunix"></a>
+Windows vs Unix
+===============
+
+ There are a few differences in how to program curl the unix way compared to
+ the Windows way. The four perhaps most notable details are:
+
+ 1. Different function names for socket operations.
+
+   In curl, this is solved with defines and macros, so that the source looks
+   the same at all places except for the header file that defines them. The
+   macros in use are sclose(), sread() and swrite().
+
+ 2. Windows requires a couple of init calls for the socket stuff.
+
+   That's taken care of by the `curl_global_init()` call, but if other libs
+   also do it etc there might be reasons for applications to alter that
+   behaviour.
+
+ 3. The file descriptors for network communication and file operations are
+    not easily interchangeable as in unix.
+
+   We avoid this by not trying any funny tricks on file descriptors.
+
+ 4. When writing data to stdout, Windows makes end-of-lines the DOS way, thus
+    destroying binary data, although you do want that conversion if it is
+    text coming through... (sigh)
+
+   We set stdout to binary under windows
+
+ Inside the source code, We make an effort to avoid `#ifdef [Your OS]`. All
+ conditionals that deal with features *should* instead be in the format
+ `#ifdef HAVE_THAT_WEIRD_FUNCTION`. Since Windows can't run configure scripts,
+ we maintain a `curl_config-win32.h` file in lib directory that is supposed to
+ look exactly as a `curl_config.h` file would have looked like on a Windows
+ machine!
+
+ Generally speaking: always remember that this will be compiled on dozens of
+ operating systems. Don't walk on the edge.
+
+<a name="Library"></a>
+Library
+=======
+
+ (See [Structs in libcurl](#structs) for the separate section describing all
+ major internal structs and their purposes.)
+
+ There are plenty of entry points to the library, namely each publicly defined
+ function that libcurl offers to applications. All of those functions are
+ rather small and easy-to-follow. All the ones prefixed with `curl_easy` are
+ put in the lib/easy.c file.
+
+ `curl_global_init_()` and `curl_global_cleanup()` should be called by the
+ application to initialize and clean up global stuff in the library. As of
+ today, it can handle the global SSL initing if SSL is enabled and it can init
+ the socket layer on windows machines. libcurl itself has no "global" scope.
+
+ All printf()-style functions use the supplied clones in lib/mprintf.c. This
+ makes sure we stay absolutely platform independent.
+
+ [ `curl_easy_init()`][2] allocates an internal struct and makes some
+ initializations.  The returned handle does not reveal internals. This is the
+ 'Curl_easy' struct which works as an "anchor" struct for all `curl_easy`
+ functions. All connections performed will get connect-specific data allocated
+ that should be used for things related to particular connections/requests.
+
+ [`curl_easy_setopt()`][1] takes three arguments, where the option stuff must
+ be passed in pairs: the parameter-ID and the parameter-value. The list of
+ options is documented in the man page. This function mainly sets things in
+ the 'Curl_easy' struct.
+
+ `curl_easy_perform()` is just a wrapper function that makes use of the multi
+ API.  It basically calls `curl_multi_init()`, `curl_multi_add_handle()`,
+ `curl_multi_wait()`, and `curl_multi_perform()` until the transfer is done
+ and then returns.
+
+ Some of the most important key functions in url.c are called from multi.c
+ when certain key steps are to be made in the transfer operation.
+
+<a name="Curl_connect"></a>
+Curl_connect()
+--------------
+
+   Analyzes the URL, it separates the different components and connects to the
+   remote host. This may involve using a proxy and/or using SSL. The
+   `Curl_resolv()` function in lib/hostip.c is used for looking up host names
+   (it does then use the proper underlying method, which may vary between
+   platforms and builds).
+
+   When `Curl_connect` is done, we are connected to the remote site. Then it
+   is time to tell the server to get a document/file. `Curl_do()` arranges
+   this.
+
+   This function makes sure there's an allocated and initiated 'connectdata'
+   struct that is used for this particular connection only (although there may
+   be several requests performed on the same connect). A bunch of things are
+   inited/inherited from the Curl_easy struct.
+
+<a name="Curl_do"></a>
+Curl_do()
+---------
+
+   `Curl_do()` makes sure the proper protocol-specific function is called. The
+   functions are named after the protocols they handle.
+
+   The protocol-specific functions of course deal with protocol-specific
+   negotiations and setup. They have access to the `Curl_sendf()` (from
+   lib/sendf.c) function to send printf-style formatted data to the remote
+   host and when they're ready to make the actual file transfer they call the
+   `Curl_Transfer()` function (in lib/transfer.c) to setup the transfer and
+   returns.
+
+   If this DO function fails and the connection is being re-used, libcurl will
+   then close this connection, setup a new connection and re-issue the DO
+   request on that. This is because there is no way to be perfectly sure that
+   we have discovered a dead connection before the DO function and thus we
+   might wrongly be re-using a connection that was closed by the remote peer.
+
+   Some time during the DO function, the `Curl_setup_transfer()` function must
+   be called with some basic info about the upcoming transfer: what socket(s)
+   to read/write and the expected file transfer sizes (if known).
+
+<a name="Curl_readwrite"></a>
+Curl_readwrite()
+----------------
+
+   Called during the transfer of the actual protocol payload.
+
+   During transfer, the progress functions in lib/progress.c are called at a
+   frequent interval (or at the user's choice, a specified callback might get
+   called). The speedcheck functions in lib/speedcheck.c are also used to
+   verify that the transfer is as fast as required.
+
+<a name="Curl_done"></a>
+Curl_done()
+-----------
+
+   Called after a transfer is done. This function takes care of everything
+   that has to be done after a transfer. This function attempts to leave
+   matters in a state so that `Curl_do()` should be possible to call again on
+   the same connection (in a persistent connection case). It might also soon
+   be closed with `Curl_disconnect()`.
+
+<a name="Curl_disconnect"></a>
+Curl_disconnect()
+-----------------
+
+   When doing normal connections and transfers, no one ever tries to close any
+   connections so this is not normally called when `curl_easy_perform()` is
+   used. This function is only used when we are certain that no more transfers
+   is going to be made on the connection. It can be also closed by force, or
+   it can be called to make sure that libcurl doesn't keep too many
+   connections alive at the same time.
+
+   This function cleans up all resources that are associated with a single
+   connection.
+
+<a name="http"></a>
+HTTP(S)
+=======
+
+ HTTP offers a lot and is the protocol in curl that uses the most lines of
+ code. There is a special file (lib/formdata.c) that offers all the multipart
+ post functions.
+
+ base64-functions for user+password stuff (and more) is in (lib/base64.c) and
+ all functions for parsing and sending cookies are found in (lib/cookie.c).
+
+ HTTPS uses in almost every means the same procedure as HTTP, with only two
+ exceptions: the connect procedure is different and the function used to read
+ or write from the socket is different, although the latter fact is hidden in
+ the source by the use of `Curl_read()` for reading and `Curl_write()` for
+ writing data to the remote server.
+
+ `http_chunks.c` contains functions that understands HTTP 1.1 chunked transfer
+ encoding.
+
+ An interesting detail with the HTTP(S) request, is the `Curl_add_buffer()`
+ series of functions we use. They append data to one single buffer, and when
+ the building is done the entire request is sent off in one single write. This
+ is done this way to overcome problems with flawed firewalls and lame servers.
+
+<a name="ftp"></a>
+FTP
+===
+
+ The `Curl_if2ip()` function can be used for getting the IP number of a
+ specified network interface, and it resides in lib/if2ip.c.
+
+ `Curl_ftpsendf()` is used for sending FTP commands to the remote server. It
+ was made a separate function to prevent us programmers from forgetting that
+ they must be CRLF terminated. They must also be sent in one single write() to
+ make firewalls and similar happy.
+
+<a name="kerberos"></a>
+Kerberos
+--------
+
+ Kerberos support is mainly in lib/krb5.c and lib/security.c but also
+ `curl_sasl_sspi.c` and `curl_sasl_gssapi.c` for the email protocols and
+ `socks_gssapi.c` and `socks_sspi.c` for SOCKS5 proxy specifics.
+
+<a name="telnet"></a>
+TELNET
+======
+
+ Telnet is implemented in lib/telnet.c.
+
+<a name="file"></a>
+FILE
+====
+
+ The file:// protocol is dealt with in lib/file.c.
+
+<a name="smb"></a>
+SMB
+===
+
+ The smb:// protocol is dealt with in lib/smb.c.
+
+<a name="ldap"></a>
+LDAP
+====
+
+ Everything LDAP is in lib/ldap.c and lib/openldap.c
+
+<a name="email"></a>
+E-mail
+======
+
+ The e-mail related source code is in lib/imap.c, lib/pop3.c and lib/smtp.c.
+
+<a name="general"></a>
+General
+=======
+
+ URL encoding and decoding, called escaping and unescaping in the source code,
+ is found in lib/escape.c.
+
+ While transferring data in Transfer() a few functions might get used.
+ `curl_getdate()` in lib/parsedate.c is for HTTP date comparisons (and more).
+
+ lib/getenv.c offers `curl_getenv()` which is for reading environment
+ variables in a neat platform independent way. That's used in the client, but
+ also in lib/url.c when checking the proxy environment variables. Note that
+ contrary to the normal unix getenv(), this returns an allocated buffer that
+ must be free()ed after use.
+
+ lib/netrc.c holds the .netrc parser
+
+ lib/timeval.c features replacement functions for systems that don't have
+ gettimeofday() and a few support functions for timeval conversions.
+
+ A function named `curl_version()` that returns the full curl version string
+ is found in lib/version.c.
+
+<a name="persistent"></a>
+Persistent Connections
+======================
+
+ The persistent connection support in libcurl requires some considerations on
+ how to do things inside of the library.
+
+ - The 'Curl_easy' struct returned in the [`curl_easy_init()`][2] call
+   must never hold connection-oriented data. It is meant to hold the root data
+   as well as all the options etc that the library-user may choose.
+
+ - The 'Curl_easy' struct holds the "connection cache" (an array of
+   pointers to 'connectdata' structs).
+
+ - This enables the 'curl handle' to be reused on subsequent transfers.
+
+ - When libcurl is told to perform a transfer, it first checks for an already
+   existing connection in the cache that we can use. Otherwise it creates a
+   new one and adds that the cache. If the cache is full already when a new
+   connection is added added, it will first close the oldest unused one.
+
+ - When the transfer operation is complete, the connection is left
+   open. Particular options may tell libcurl not to, and protocols may signal
+   closure on connections and then they won't be kept open of course.
+
+ - When `curl_easy_cleanup()` is called, we close all still opened connections,
+   unless of course the multi interface "owns" the connections.
+
+ The curl handle must be re-used in order for the persistent connections to
+ work.
+
+<a name="multi"></a>
+multi interface/non-blocking
+============================
+
+ The multi interface is a non-blocking interface to the library. To make that
+ interface work as good as possible, no low-level functions within libcurl
+ must be written to work in a blocking manner. (There are still a few spots
+ violating this rule.)
+
+ One of the primary reasons we introduced c-ares support was to allow the name
+ resolve phase to be perfectly non-blocking as well.
+
+ The FTP and the SFTP/SCP protocols are examples of how we adapt and adjust
+ the code to allow non-blocking operations even on multi-stage command-
+ response protocols. They are built around state machines that return when
+ they would otherwise block waiting for data.  The DICT, LDAP and TELNET
+ protocols are crappy examples and they are subject for rewrite in the future
+ to better fit the libcurl protocol family.
+
+<a name="ssl"></a>
+SSL libraries
+=============
+
+ Originally libcurl supported SSLeay for SSL/TLS transports, but that was then
+ extended to its successor OpenSSL but has since also been extended to several
+ other SSL/TLS libraries and we expect and hope to further extend the support
+ in future libcurl versions.
+
+ To deal with this internally in the best way possible, we have a generic SSL
+ function API as provided by the vtls/vtls.[ch] system, and they are the only
+ SSL functions we must use from within libcurl. vtls is then crafted to use
+ the appropriate lower-level function calls to whatever SSL library that is in
+ use. For example vtls/openssl.[ch] for the OpenSSL library.
+
+<a name="symbols"></a>
+Library Symbols
+===============
+
+ All symbols used internally in libcurl must use a `Curl_` prefix if they're
+ used in more than a single file. Single-file symbols must be made static.
+ Public ("exported") symbols must use a `curl_` prefix. (There are exceptions,
+ but they are to be changed to follow this pattern in future versions.) Public
+ API functions are marked with `CURL_EXTERN` in the public header files so
+ that all others can be hidden on platforms where this is possible.
+
+<a name="returncodes"></a>
+Return Codes and Informationals
+===============================
+
+ I've made things simple. Almost every function in libcurl returns a CURLcode,
+ that must be `CURLE_OK` if everything is OK or otherwise a suitable error
+ code as the curl/curl.h include file defines. The very spot that detects an
+ error must use the `Curl_failf()` function to set the human-readable error
+ description.
+
+ In aiding the user to understand what's happening and to debug curl usage, we
+ must supply a fair amount of informational messages by using the
+ `Curl_infof()` function. Those messages are only displayed when the user
+ explicitly asks for them. They are best used when revealing information that
+ isn't otherwise obvious.
+
+<a name="abi"></a>
+API/ABI
+=======
+
+ We make an effort to not export or show internals or how internals work, as
+ that makes it easier to keep a solid API/ABI over time. See docs/libcurl/ABI
+ for our promise to users.
+
+<a name="client"></a>
+Client
+======
+
+ main() resides in `src/tool_main.c`.
+
+ `src/tool_hugehelp.c` is automatically generated by the mkhelp.pl perl script
+ to display the complete "manual" and the src/tool_urlglob.c file holds the
+ functions used for the URL-"globbing" support. Globbing in the sense that the
+ {} and [] expansion stuff is there.
+
+ The client mostly messes around to setup its 'config' struct properly, then
+ it calls the `curl_easy_*()` functions of the library and when it gets back
+ control after the `curl_easy_perform()` it cleans up the library, checks
+ status and exits.
+
+ When the operation is done, the ourWriteOut() function in src/writeout.c may
+ be called to report about the operation. That function is using the
+ `curl_easy_getinfo()` function to extract useful information from the curl
+ session.
+
+ It may loop and do all this several times if many URLs were specified on the
+ command line or config file.
+
+<a name="memorydebug"></a>
+Memory Debugging
+================
+
+ The file lib/memdebug.c contains debug-versions of a few functions. Functions
+ such as malloc, free, fopen, fclose, etc that somehow deal with resources
+ that might give us problems if we "leak" them. The functions in the memdebug
+ system do nothing fancy, they do their normal function and then log
+ information about what they just did. The logged data can then be analyzed
+ after a complete session,
+
+ memanalyze.pl is the perl script present in tests/ that analyzes a log file
+ generated by the memory tracking system. It detects if resources are
+ allocated but never freed and other kinds of errors related to resource
+ management.
+
+ Internally, definition of preprocessor symbol DEBUGBUILD restricts code which
+ is only compiled for debug enabled builds. And symbol CURLDEBUG is used to
+ differentiate code which is _only_ used for memory tracking/debugging.
+
+ Use -DCURLDEBUG when compiling to enable memory debugging, this is also
+ switched on by running configure with --enable-curldebug. Use -DDEBUGBUILD
+ when compiling to enable a debug build or run configure with --enable-debug.
+
+ curl --version will list 'Debug' feature for debug enabled builds, and
+ will list 'TrackMemory' feature for curl debug memory tracking capable
+ builds. These features are independent and can be controlled when running
+ the configure script. When --enable-debug is given both features will be
+ enabled, unless some restriction prevents memory tracking from being used.
+
+<a name="test"></a>
+Test Suite
+==========
+
+ The test suite is placed in its own subdirectory directly off the root in the
+ curl archive tree, and it contains a bunch of scripts and a lot of test case
+ data.
+
+ The main test script is runtests.pl that will invoke test servers like
+ httpserver.pl and ftpserver.pl before all the test cases are performed. The
+ test suite currently only runs on unix-like platforms.
+
+ You'll find a description of the test suite in the tests/README file, and the
+ test case data files in the tests/FILEFORMAT file.
+
+ The test suite automatically detects if curl was built with the memory
+ debugging enabled, and if it was it will detect memory leaks, too.
+
+<a name="asyncdns"></a>
+Asynchronous name resolves
+==========================
+
+ libcurl can be built to do name resolves asynchronously, using either the
+ normal resolver in a threaded manner or by using c-ares.
+
+<a name="cares"></a>
+[c-ares][3]
+------
+
+### Build libcurl to use a c-ares
+
+1. ./configure --enable-ares=/path/to/ares/install
+2. make
+
+### c-ares on win32
+
+ First I compiled c-ares. I changed the default C runtime library to be the
+ single-threaded rather than the multi-threaded (this seems to be required to
+ prevent linking errors later on). Then I simply build the areslib project
+ (the other projects adig/ahost seem to fail under MSVC).
+
+ Next was libcurl. I opened lib/config-win32.h and I added a:
+ `#define USE_ARES 1`
+
+ Next thing I did was I added the path for the ares includes to the include
+ path, and the libares.lib to the libraries.
+
+ Lastly, I also changed libcurl to be single-threaded rather than
+ multi-threaded, again this was to prevent some duplicate symbol errors. I'm
+ not sure why I needed to change everything to single-threaded, but when I
+ didn't I got redefinition errors for several CRT functions (malloc, stricmp,
+ etc.)
+
+<a name="curl_off_t"></a>
+`curl_off_t`
+==========
+
+ curl_off_t is a data type provided by the external libcurl include
+ headers. It is the type meant to be used for the [`curl_easy_setopt()`][1]
+ options that end with LARGE. The type is 64bit large on most modern
+ platforms.
+
+curlx
+=====
+
+ The libcurl source code offers a few functions by source only. They are not
+ part of the official libcurl API, but the source files might be useful for
+ others so apps can optionally compile/build with these sources to gain
+ additional functions.
+
+ We provide them through a single header file for easy access for apps:
+ "curlx.h"
+
+`curlx_strtoofft()`
+-------------------
+   A macro that converts a string containing a number to a curl_off_t number.
+   This might use the curlx_strtoll() function which is provided as source
+   code in strtoofft.c. Note that the function is only provided if no
+   strtoll() (or equivalent) function exist on your platform. If curl_off_t
+   is only a 32 bit number on your platform, this macro uses strtol().
+
+`curlx_tvnow()`
+---------------
+   returns a struct timeval for the current time.
+
+`curlx_tvdiff()`
+--------------
+   returns the difference between two timeval structs, in number of
+   milliseconds.
+
+`curlx_tvdiff_secs()`
+---------------------
+   returns the same as curlx_tvdiff but with full usec resolution (as a
+   double)
+
+Future
+------
+
+ Several functions will be removed from the public curl_ name space in a
+ future libcurl release. They will then only become available as curlx_
+ functions instead. To make the transition easier, we already today provide
+ these functions with the curlx_ prefix to allow sources to get built properly
+ with the new function names. The functions this concerns are:
+
+ - `curlx_getenv`
+ - `curlx_strequal`
+ - `curlx_strnequal`
+ - `curlx_mvsnprintf`
+ - `curlx_msnprintf`
+ - `curlx_maprintf`
+ - `curlx_mvaprintf`
+ - `curlx_msprintf`
+ - `curlx_mprintf`
+ - `curlx_mfprintf`
+ - `curlx_mvsprintf`
+ - `curlx_mvprintf`
+ - `curlx_mvfprintf`
+
+<a name="contentencoding"></a>
+Content Encoding
+================
+
+## About content encodings
+
+ [HTTP/1.1][4] specifies that a client may request that a server encode its
+ response. This is usually used to compress a response using one of a set of
+ commonly available compression techniques. These schemes are 'deflate' (the
+ zlib algorithm), 'gzip' and 'compress'. A client requests that the sever
+ perform an encoding by including an Accept-Encoding header in the request
+ document. The value of the header should be one of the recognized tokens
+ 'deflate', ... (there's a way to register new schemes/tokens, see sec 3.5 of
+ the spec). A server MAY honor the client's encoding request. When a response
+ is encoded, the server includes a Content-Encoding header in the
+ response. The value of the Content-Encoding header indicates which scheme was
+ used to encode the data.
+
+ A client may tell a server that it can understand several different encoding
+ schemes. In this case the server may choose any one of those and use it to
+ encode the response (indicating which one using the Content-Encoding header).
+ It's also possible for a client to attach priorities to different schemes so
+ that the server knows which it prefers. See sec 14.3 of RFC 2616 for more
+ information on the Accept-Encoding header.
+
+## Supported content encodings
+
+ The 'deflate' and 'gzip' content encoding are supported by libcurl. Both
+ regular and chunked transfers work fine.  The zlib library is required for
+ this feature.
+
+## The libcurl interface
+
+ To cause libcurl to request a content encoding use:
+
+  [`curl_easy_setopt`][1](curl, [`CURLOPT_ACCEPT_ENCODING`][5], string)
+
+ where string is the intended value of the Accept-Encoding header.
+
+ Currently, libcurl only understands how to process responses that use the
+ "deflate" or "gzip" Content-Encoding, so the only values for
+ [`CURLOPT_ACCEPT_ENCODING`][5] that will work (besides "identity," which does
+ nothing) are "deflate" and "gzip" If a response is encoded using the
+ "compress" or methods, libcurl will return an error indicating that the
+ response could not be decoded.  If <string> is NULL no Accept-Encoding header
+ is generated.  If <string> is a zero-length string, then an Accept-Encoding
+ header containing all supported encodings will be generated.
+
+ The [`CURLOPT_ACCEPT_ENCODING`][5] must be set to any non-NULL value for
+ content to be automatically decoded.  If it is not set and the server still
+ sends encoded content (despite not having been asked), the data is returned
+ in its raw form and the Content-Encoding type is not checked.
+
+## The curl interface
+
+ Use the [--compressed][6] option with curl to cause it to ask servers to
+ compress responses using any format supported by curl.
+
+<a name="hostip"></a>
+hostip.c explained
+==================
+
+ The main compile-time defines to keep in mind when reading the host*.c source
+ file are these:
+
+## `CURLRES_IPV6`
+
+ this host has getaddrinfo() and family, and thus we use that. The host may
+ not be able to resolve IPv6, but we don't really have to take that into
+ account. Hosts that aren't IPv6-enabled have CURLRES_IPV4 defined.
+
+## `CURLRES_ARES`
+
+ is defined if libcurl is built to use c-ares for asynchronous name
+ resolves. This can be Windows or *nix.
+
+## `CURLRES_THREADED`
+
+ is defined if libcurl is built to use threading for asynchronous name
+ resolves. The name resolve will be done in a new thread, and the supported
+ asynch API will be the same as for ares-builds. This is the default under
+ (native) Windows.
+
+ If any of the two previous are defined, `CURLRES_ASYNCH` is defined too. If
+ libcurl is not built to use an asynchronous resolver, `CURLRES_SYNCH` is
+ defined.
+
+## host*.c sources
+
+ The host*.c sources files are split up like this:
+
+ - hostip.c      - method-independent resolver functions and utility functions
+ - hostasyn.c    - functions for asynchronous name resolves
+ - hostsyn.c     - functions for synchronous name resolves
+ - asyn-ares.c   - functions for asynchronous name resolves using c-ares
+ - asyn-thread.c - functions for asynchronous name resolves using threads
+ - hostip4.c     - IPv4 specific functions
+ - hostip6.c     - IPv6 specific functions
+
+ The hostip.h is the single united header file for all this. It defines the
+ `CURLRES_*` defines based on the config*.h and curl_setup.h defines.
+
+<a name="memoryleak"></a>
+Track Down Memory Leaks
+=======================
+
+## Single-threaded
+
+  Please note that this memory leak system is not adjusted to work in more
+  than one thread. If you want/need to use it in a multi-threaded app. Please
+  adjust accordingly.
+
+
+## Build
+
+  Rebuild libcurl with -DCURLDEBUG (usually, rerunning configure with
+  --enable-debug fixes this). 'make clean' first, then 'make' so that all
+  files actually are rebuilt properly. It will also make sense to build
+  libcurl with the debug option (usually -g to the compiler) so that debugging
+  it will be easier if you actually do find a leak in the library.
+
+  This will create a library that has memory debugging enabled.
+
+## Modify Your Application
+
+  Add a line in your application code:
+
+       `curl_memdebug("dump");`
+
+  This will make the malloc debug system output a full trace of all resource
+  using functions to the given file name. Make sure you rebuild your program
+  and that you link with the same libcurl you built for this purpose as
+  described above.
+
+## Run Your Application
+
+  Run your program as usual. Watch the specified memory trace file grow.
+
+  Make your program exit and use the proper libcurl cleanup functions etc. So
+  that all non-leaks are returned/freed properly.
+
+## Analyze the Flow
+
+  Use the tests/memanalyze.pl perl script to analyze the dump file:
+
+    tests/memanalyze.pl dump
+
+  This now outputs a report on what resources that were allocated but never
+  freed etc. This report is very fine for posting to the list!
+
+  If this doesn't produce any output, no leak was detected in libcurl. Then
+  the leak is mostly likely to be in your code.
+
+<a name="multi_socket"></a>
+`multi_socket`
+==============
+
+ Implementation of the `curl_multi_socket` API
+
+  The main ideas of this API are simply:
+
+   1 - The application can use whatever event system it likes as it gets info
+       from libcurl about what file descriptors libcurl waits for what action
+       on. (The previous API returns `fd_sets` which is very select()-centric).
+
+   2 - When the application discovers action on a single socket, it calls
+       libcurl and informs that there was action on this particular socket and
+       libcurl can then act on that socket/transfer only and not care about
+       any other transfers. (The previous API always had to scan through all
+       the existing transfers.)
+
+  The idea is that [`curl_multi_socket_action()`][7] calls a given callback
+  with information about what socket to wait for what action on, and the
+  callback only gets called if the status of that socket has changed.
+
+  We also added a timer callback that makes libcurl call the application when
+  the timeout value changes, and you set that with [`curl_multi_setopt()`][9]
+  and the [`CURLMOPT_TIMERFUNCTION`][10] option. To get this to work,
+  Internally, there's an added a struct to each easy handle in which we store
+  an "expire time" (if any). The structs are then "splay sorted" so that we
+  can add and remove times from the linked list and yet somewhat swiftly
+  figure out both how long time there is until the next nearest timer expires
+  and which timer (handle) we should take care of now. Of course, the upside
+  of all this is that we get a [`curl_multi_timeout()`][8] that should also
+  work with old-style applications that use [`curl_multi_perform()`][11].
+
+  We created an internal "socket to easy handles" hash table that given
+  a socket (file descriptor) return the easy handle that waits for action on
+  that socket.  This hash is made using the already existing hash code
+  (previously only used for the DNS cache).
+
+  To make libcurl able to report plain sockets in the socket callback, we had
+  to re-organize the internals of the [`curl_multi_fdset()`][12] etc so that
+  the conversion from sockets to `fd_sets` for that function is only done in
+  the last step before the data is returned. I also had to extend c-ares to
+  get a function that can return plain sockets, as that library too returned
+  only `fd_sets` and that is no longer good enough. The changes done to c-ares
+  are available in c-ares 1.3.1 and later.
+
+<a name="structs"></a>
+Structs in libcurl
+==================
+
+This section should cover 7.32.0 pretty accurately, but will make sense even
+for older and later versions as things don't change drastically that often.
+
+## Curl_easy
+
+  The Curl_easy struct is the one returned to the outside in the external API
+  as a "CURL *". This is usually known as an easy handle in API documentations
+  and examples.
+
+  Information and state that is related to the actual connection is in the
+  'connectdata' struct. When a transfer is about to be made, libcurl will
+  either create a new connection or re-use an existing one. The particular
+  connectdata that is used by this handle is pointed out by
+  Curl_easy->easy_conn.
+
+  Data and information that regard this particular single transfer is put in
+  the SingleRequest sub-struct.
+
+  When the Curl_easy struct is added to a multi handle, as it must be in order
+  to do any transfer, the ->multi member will point to the `Curl_multi` struct
+  it belongs to. The ->prev and ->next members will then be used by the multi
+  code to keep a linked list of Curl_easy structs that are added to that same
+  multi handle. libcurl always uses multi so ->multi *will* point to a
+  `Curl_multi` when a transfer is in progress.
+
+  ->mstate is the multi state of this particular Curl_easy. When
+  `multi_runsingle()` is called, it will act on this handle according to which
+  state it is in. The mstate is also what tells which sockets to return for a
+  specific Curl_easy when [`curl_multi_fdset()`][12] is called etc.
+
+  The libcurl source code generally use the name 'data' for the variable that
+  points to the Curl_easy.
+
+  When doing multiplexed HTTP/2 transfers, each Curl_easy is associated with
+  an individual stream, sharing the same connectdata struct. Multiplexing
+  makes it even more important to keep things associated with the right thing!
+
+## connectdata
+
+  A general idea in libcurl is to keep connections around in a connection
+  "cache" after they have been used in case they will be used again and then
+  re-use an existing one instead of creating a new as it creates a significant
+  performance boost.
+
+  Each 'connectdata' identifies a single physical connection to a server. If
+  the connection can't be kept alive, the connection will be closed after use
+  and then this struct can be removed from the cache and freed.
+
+  Thus, the same Curl_easy can be used multiple times and each time select
+  another connectdata struct to use for the connection. Keep this in mind, as
+  it is then important to consider if options or choices are based on the
+  connection or the Curl_easy.
+
+  Functions in libcurl will assume that connectdata->data points to the
+  Curl_easy that uses this connection (for the moment).
+
+  As a special complexity, some protocols supported by libcurl require a
+  special disconnect procedure that is more than just shutting down the
+  socket. It can involve sending one or more commands to the server before
+  doing so. Since connections are kept in the connection cache after use, the
+  original Curl_easy may no longer be around when the time comes to shut down
+  a particular connection. For this purpose, libcurl holds a special dummy
+  `closure_handle` Curl_easy in the `Curl_multi` struct to use when needed.
+
+  FTP uses two TCP connections for a typical transfer but it keeps both in
+  this single struct and thus can be considered a single connection for most
+  internal concerns.
+
+  The libcurl source code generally use the name 'conn' for the variable that
+  points to the connectdata.
+
+## Curl_multi
+
+  Internally, the easy interface is implemented as a wrapper around multi
+  interface functions. This makes everything multi interface.
+
+  `Curl_multi` is the multi handle struct exposed as "CURLM *" in external APIs.
+
+  This struct holds a list of Curl_easy structs that have been added to this
+  handle with [`curl_multi_add_handle()`][13]. The start of the list is
+  ->easyp and ->num_easy is a counter of added Curl_easys.
+
+  ->msglist is a linked list of messages to send back when
+  [`curl_multi_info_read()`][14] is called. Basically a node is added to that
+  list when an individual Curl_easy's transfer has completed.
+
+  ->hostcache points to the name cache. It is a hash table for looking up name
+  to IP. The nodes have a limited life time in there and this cache is meant
+  to reduce the time for when the same name is wanted within a short period of
+  time.
+
+  ->timetree points to a tree of Curl_easys, sorted by the remaining time
+  until it should be checked - normally some sort of timeout. Each Curl_easy
+  has one node in the tree.
+
+  ->sockhash is a hash table to allow fast lookups of socket descriptor to
+  which Curl_easy that uses that descriptor. This is necessary for the
+  `multi_socket` API.
+
+  ->conn_cache points to the connection cache. It keeps track of all
+  connections that are kept after use. The cache has a maximum size.
+
+  ->closure_handle is described in the 'connectdata' section.
+
+  The libcurl source code generally use the name 'multi' for the variable that
+  points to the Curl_multi struct.
+
+## Curl_handler
+
+  Each unique protocol that is supported by libcurl needs to provide at least
+  one `Curl_handler` struct. It defines what the protocol is called and what
+  functions the main code should call to deal with protocol specific issues.
+  In general, there's a source file named [protocol].c in which there's a
+  "struct `Curl_handler` `Curl_handler_[protocol]`" declared. In url.c there's
+  then the main array with all individual `Curl_handler` structs pointed to
+  from a single array which is scanned through when a URL is given to libcurl
+  to work with.
+
+  ->scheme is the URL scheme name, usually spelled out in uppercase. That's
+  "HTTP" or "FTP" etc. SSL versions of the protcol need its own `Curl_handler`
+  setup so HTTPS separate from HTTP.
+
+  ->setup_connection is called to allow the protocol code to allocate protocol
+  specific data that then gets associated with that Curl_easy for the rest of
+  this transfer. It gets freed again at the end of the transfer. It will be
+  called before the 'connectdata' for the transfer has been selected/created.
+  Most protocols will allocate its private 'struct [PROTOCOL]' here and assign
+  Curl_easy->req.protop to point to it.
+
+  ->connect_it allows a protocol to do some specific actions after the TCP
+  connect is done, that can still be considered part of the connection phase.
+
+  Some protocols will alter the connectdata->recv[] and connectdata->send[]
+  function pointers in this function.
+
+  ->connecting is similarly a function that keeps getting called as long as the
+  protocol considers itself still in the connecting phase.
+
+  ->do_it is the function called to issue the transfer request. What we call
+  the DO action internally. If the DO is not enough and things need to be kept
+  getting done for the entire DO sequence to complete, ->doing is then usually
+  also provided. Each protocol that needs to do multiple commands or similar
+  for do/doing need to implement their own state machines (see SCP, SFTP,
+  FTP). Some protocols (only FTP and only due to historical reasons) has a
+  separate piece of the DO state called `DO_MORE`.
+
+  ->doing keeps getting called while issuing the transfer request command(s)
+
+  ->done gets called when the transfer is complete and DONE. That's after the
+  main data has been transferred.
+
+  ->do_more gets called during the `DO_MORE` state. The FTP protocol uses this
+  state when setting up the second connection.
+
+  ->`proto_getsock`
+  ->`doing_getsock`
+  ->`domore_getsock`
+  ->`perform_getsock`
+  Functions that return socket information. Which socket(s) to wait for which
+  action(s) during the particular multi state.
+
+  ->disconnect is called immediately before the TCP connection is shutdown.
+
+  ->readwrite gets called during transfer to allow the protocol to do extra
+  reads/writes
+
+  ->defport is the default report TCP or UDP port this protocol uses
+
+  ->protocol is one or more bits in the `CURLPROTO_*` set. The SSL versions
+  have their "base" protocol set and then the SSL variation. Like
+  "HTTP|HTTPS".
+
+  ->flags is a bitmask with additional information about the protocol that will
+  make it get treated differently by the generic engine:
+
+  - `PROTOPT_SSL` - will make it connect and negotiate SSL
+
+  - `PROTOPT_DUAL` - this protocol uses two connections
+
+  - `PROTOPT_CLOSEACTION` - this protocol has actions to do before closing the
+    connection. This flag is no longer used by code, yet still set for a bunch
+    protocol handlers.
+
+  - `PROTOPT_DIRLOCK` - "direction lock". The SSH protocols set this bit to
+    limit which "direction" of socket actions that the main engine will
+    concern itself about.
+
+  - `PROTOPT_NONETWORK` - a protocol that doesn't use network (read file:)
+
+  - `PROTOPT_NEEDSPWD` - this protocol needs a password and will use a default
+    one unless one is provided
+
+  - `PROTOPT_NOURLQUERY` - this protocol can't handle a query part on the URL
+    (?foo=bar)
+
+## conncache
+
+  Is a hash table with connections for later re-use. Each Curl_easy has a
+  pointer to its connection cache. Each multi handle sets up a connection
+  cache that all added Curl_easys share by default.
+
+## Curl_share
+
+  The libcurl share API allocates a `Curl_share` struct, exposed to the
+  external API as "CURLSH *".
+
+  The idea is that the struct can have a set of own versions of caches and
+  pools and then by providing this struct in the `CURLOPT_SHARE` option, those
+  specific Curl_easys will use the caches/pools that this share handle
+  holds.
+
+  Then individual Curl_easy structs can be made to share specific things
+  that they otherwise wouldn't, such as cookies.
+
+  The `Curl_share` struct can currently hold cookies, DNS cache and the SSL
+  session cache.
+
+## CookieInfo
+
+  This is the main cookie struct. It holds all known cookies and related
+  information. Each Curl_easy has its own private CookieInfo even when
+  they are added to a multi handle. They can be made to share cookies by using
+  the share API.
+
+
+[1]: https://curl.haxx.se/libcurl/c/curl_easy_setopt.html
+[2]: https://curl.haxx.se/libcurl/c/curl_easy_init.html
+[3]: http://c-ares.haxx.se/
+[4]: https://tools.ietf.org/html/rfc7230 "RFC 7230"
+[5]: https://curl.haxx.se/libcurl/c/CURLOPT_ACCEPT_ENCODING.html
+[6]: https://curl.haxx.se/docs/manpage.html#--compressed
+[7]: https://curl.haxx.se/libcurl/c/curl_multi_socket_action.html
+[8]: https://curl.haxx.se/libcurl/c/curl_multi_timeout.html
+[9]: https://curl.haxx.se/libcurl/c/curl_multi_setopt.html
+[10]: https://curl.haxx.se/libcurl/c/CURLMOPT_TIMERFUNCTION.html
+[11]: https://curl.haxx.se/libcurl/c/curl_multi_perform.html
+[12]: https://curl.haxx.se/libcurl/c/curl_multi_fdset.html
+[13]: https://curl.haxx.se/libcurl/c/curl_multi_add_handle.html
+[14]: https://curl.haxx.se/libcurl/c/curl_multi_info_read.html