Merge pull request #12296 from poettering/coding-style-sections

split CODING_STYLE document into multiple thematic sections

1 files changed, 373 insertions, 362 deletions

diff --git a/docs/CODING_STYLE.md b/docs/CODING_STYLE.md
index 71642bdf9b..d945f8cdbe 100644
--- a/docs/CODING_STYLE.md
+++ b/docs/CODING_STYLE.md
@@ -4,14 +4,16 @@ title: Coding Style
 
 # Coding Style
 
+## Formatting
+
 - 8ch indent, no tabs, except for files in `man/` which are 2ch indent, and
   still no tabs, and shell scripts, which are 4ch indent, and no tabs either.
 
-- We prefer `/* comments */` over `// comments` in code you commit, please. This
-  way `// comments` are left for developers to use for local, temporary
-  commenting of code for debug purposes (i.e. uncommittable stuff), making such
-  comments easily discernible from explanatory, documenting code comments
-  (i.e. committable stuff).
+- We prefer `/* comments */` over `// comments` in code you commit,
+  please. This way `// comments` are left for developers to use for local,
+  temporary commenting of code for debug purposes (i.e. uncommittable stuff),
+  making such comments easily discernible from explanatory, documenting code
+  comments (i.e. committable stuff).
 
 - Don't break code lines too eagerly. We do **not** force line breaks at 80ch,
   all of today's screens should be much larger than that. But then again, don't
@@ -21,69 +23,7 @@ title: Coding Style
   note that emacs loads `.dir-locals.el` automatically, but vim needs to be
   configured to load `.vimrc`, see that file for instructions.
 
-- Variables and functions **must** be static, unless they have a
-  prototype, and are supposed to be exported.
-
-- structs in `PascalCase` (with exceptions, such as public API structs),
-  variables and functions in `snake_case`.
-
-- The destructors always deregister the object from the next bigger
-  object, not the other way around.
-
-- To minimize strict aliasing violations, we prefer unions over casting.
-
-- For robustness reasons, destructors should be able to destruct
-  half-initialized objects, too.
-
-- Error codes are returned as negative `Exxx`. e.g. `return -EINVAL`. There
-  are some exceptions: for constructors, it is OK to return `NULL` on
-  OOM. For lookup functions, `NULL` is fine too for "not found".
-
-  Be strict with this. When you write a function that can fail due to
-  more than one cause, it *really* should have an `int` as the return value
-  for the error code.
-
-- Do not bother with error checking whether writing to stdout/stderr
-  worked.
-
-- Do not log errors from "library" code, only do so from "main
-  program" code. (With one exception: it is OK to log with DEBUG level
-  from any code, with the exception of maybe inner loops).
-
-- Always check OOM. There is no excuse. In program code, you can use
-  `log_oom()` for then printing a short message, but not in "library" code.
-
-- Do not issue NSS requests (that includes user name and host name
-  lookups) from PID 1 as this might trigger deadlocks when those
-  lookups involve synchronously talking to services that we would need
-  to start up.
-
-- Do not synchronously talk to any other service from PID 1, due to
-  risk of deadlocks.
-
-- Avoid fixed-size string buffers, unless you really know the maximum
-  size and that maximum size is small. They are a source of errors,
-  since they possibly result in truncated strings. It is often nicer
-  to use dynamic memory, `alloca()` or VLAs. If you do allocate fixed-size
-  strings on the stack, then it is probably only OK if you either
-  use a maximum size such as `LINE_MAX`, or count in detail the maximum
-  size a string can have. (`DECIMAL_STR_MAX` and `DECIMAL_STR_WIDTH`
-  macros are your friends for this!)
-
-  Or in other words, if you use `char buf[256]` then you are likely
-  doing something wrong!
-
-- Stay uniform. For example, always use `usec_t` for time
-  values. Do not mix `usec` and `msec`, and `usec` and whatnot.
-
-- Make use of `_cleanup_free_` and friends. It makes your code much
-  nicer to read (and shorter)!
-
-- Be exceptionally careful when formatting and parsing floating point
-  numbers. Their syntax is locale dependent (i.e. `5.000` in en_US is
-  generally understood as 5, while in de_DE as 5000.).
-
-- Try to use this:
+- Try to write this:
 
   ```c
   void foo() {
@@ -98,9 +38,7 @@ title: Coding Style
   }
   ```
 
-  But it is OK if you do not.
-
-- Single-line `if` blocks should not be enclosed in `{}`. Use this:
+- Single-line `if` blocks should not be enclosed in `{}`. Write this:
 
   ```c
   if (foobar)
@@ -117,11 +55,85 @@ title: Coding Style
 
 - Do not write `foo ()`, write `foo()`.
 
-- Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where
-  applicable (i.e. wherever you just care about equality/inequality, not about
-  the sorting order).
+## Code Organization and Semantics
+
+- Please name structures in `PascalCase` (with exceptions, such as public API
+  structs), variables and functions in `snake_case`.
+
+- Avoid static variables, except for caches and very few other cases. Think
+  about thread-safety! While most of our code is never used in threaded
+  environments, at least the library code should make sure it works correctly
+  in them. Instead of doing a lot of locking for that, we tend to prefer using
+  TLS to do per-thread caching (which only works for small, fixed-size cache
+  objects), or we disable caching for any thread that is not the main
+  thread. Use `is_main_thread()` to detect whether the calling thread is the
+  main thread.
+
+- Do not write functions that clobber call-by-reference variables on
+  failure. Use temporary variables for these cases and change the passed in
+  variables only on success.
+
+- The order in which header files are included doesn't matter too
+  much. systemd-internal headers must not rely on an include order, so it is
+  safe to include them in any order possible.  However, to not clutter global
+  includes, and to make sure internal definitions will not affect global
+  headers, please always include the headers of external components first
+  (these are all headers enclosed in <>), followed by our own exported headers
+  (usually everything that's prefixed by `sd-`), and then followed by internal
+  headers.  Furthermore, in all three groups, order all includes alphabetically
+  so duplicate includes can easily be detected.
+
+- Please avoid using global variables as much as you can. And if you do use
+  them make sure they are static at least, instead of exported. Especially in
+  library-like code it is important to avoid global variables. Why are global
+  variables bad? They usually hinder generic reusability of code (since they
+  break in threaded programs, and usually would require locking there), and as
+  the code using them has side-effects make programs non-transparent. That
+  said, there are many cases where they explicitly make a lot of sense, and are
+  OK to use. For example, the log level and target in `log.c` is stored in a
+  global variable, and that's OK and probably expected by most. Also in many
+  cases we cache data in global variables. If you add more caches like this,
+  please be careful however, and think about threading. Only use static
+  variables if you are sure that thread-safety doesn't matter in your
+  case. Alternatively, consider using TLS, which is pretty easy to use with
+  gcc's `thread_local` concept. It's also OK to store data that is inherently
+  global in global variables, for example data parsed from command lines, see
+  below.
+
+- You might wonder what kind of common code belongs in `src/shared/` and what
+  belongs in `src/basic/`. The split is like this: anything that is used to
+  implement the public shared object we provide (sd-bus, sd-login, sd-id128,
+  nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must
+  be located in `src/basic` (those objects are not allowed to link to
+  libsystemd-shared.so). Conversely, anything which is shared between multiple
+  components and does not need to be in `src/basic/`, should be in
+  `src/shared/`.
+
+  To summarize:
+
+  `src/basic/`
+  - may be used by all code in the tree
+  - may not use any code outside of `src/basic/`
+
+  `src/libsystemd/`
+  - may be used by all code in the tree, except for code in `src/basic/`
+  - may not use any code outside of `src/basic/`, `src/libsystemd/`
+
+  `src/shared/`
+  - may be used by all code in the tree, except for code in `src/basic/`,
+    `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
+    `src/journal/` that end up in `libjournal-client.a` convenience library.
+  - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/`
+
+- Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are
+  incompatible with glibc it's on them. However, if there are equivalent POSIX
+  and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there
+  aren't, we are happy to use GNU or Linux APIs, and expect non-GNU
+  implementations of libc to catch up with glibc.
+
+## Using C Constructs
 
-- Preferably allocate stack variables on the top of the block:
+- Preferably allocate local variables on the top of the block:
 
   ```c
   {
@@ -132,13 +144,7 @@ title: Coding Style
   }
   ```
 
-- Unless you allocate an array, `double` is always a better choice
-  than `float`. Processors speak `double` natively anyway, so there is
-  no speed benefit, and on calls like `printf()` `float`s get promoted
-  to `double`s anyway, so there is no point.
-
-- Do not mix function invocations with variable definitions in one
-  line. Wrong:
+- Do not mix function invocations with variable definitions in one line. Wrong:
 
   ```c
   {
@@ -158,119 +164,101 @@ title: Coding Style
   }
   ```
 
-- Use `goto` for cleaning up, and only use it for that. i.e. you may
-  only jump to the end of a function, and little else. Never jump
-  backwards!
-
-- Think about the types you use. If a value cannot sensibly be
-  negative, do not use `int`, but use `unsigned`.
-
-- Use `char` only for actual characters. Use `uint8_t` or `int8_t`
-  when you actually mean a byte-sized signed or unsigned
-  integers. When referring to a generic byte, we generally prefer the
-  unsigned variant `uint8_t`. Do not use types based on `short`. They
-  *never* make sense. Use `int`, `long`, `long long`, all in
-  unsigned and signed fashion, and the fixed-size types
-  `uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`, `int8_t`, `int16_t`, `int32_t` and so on,
-  as well as `size_t`, but nothing else. Do not use kernel types like
-  `u32` and so on, leave that to the kernel.
+- Use `goto` for cleaning up, and only use it for that. i.e. you may only jump
+  to the end of a function, and little else. Never jump backwards!
 
-- Public API calls (i.e. functions exported by our shared libraries)
-  must be marked `_public_` and need to be prefixed with `sd_`. No
-  other functions should be prefixed like that.
+- To minimize strict aliasing violations, we prefer unions over casting.
 
-- In public API calls, you **must** validate all your input arguments for
-  programming error with `assert_return()` and return a sensible return
-  code. In all other calls, it is recommended to check for programming
-  errors with a more brutal `assert()`. We are more forgiving to public
-  users than for ourselves! Note that `assert()` and `assert_return()`
-  really only should be used for detecting programming errors, not for
-  runtime errors. `assert()` and `assert_return()` by usage of `_likely_()`
-  inform the compiler that he should not expect these checks to fail,
-  and they inform fellow programmers about the expected validity and
-  range of parameters.
+- Instead of using `memzero()`/`memset()` to initialize structs allocated on
+  the stack, please try to use c99 structure initializers. It's short, prettier
+  and actually even faster at execution. Hence:
 
-- Never use `strtol()`, `atoi()` and similar calls. Use `safe_atoli()`,
-  `safe_atou32()` and suchlike instead. They are much nicer to use in
-  most cases and correctly check for parsing errors.
-
-- For every function you add, think about whether it is a "logging"
-  function or a "non-logging" function. "Logging" functions do logging
-  on their own, "non-logging" function never log on their own and
-  expect their callers to log. All functions in "library" code,
-  i.e. in `src/shared/` and suchlike must be "non-logging". Every time a
-  "logging" function calls a "non-logging" function, it should log
-  about the resulting errors. If a "logging" function calls another
-  "logging" function, then it should not generate log messages, so
-  that log messages are not generated twice for the same errors.
+  ```c
+  struct foobar t = {
+          .foo = 7,
+          .bar = "bazz",
+  };
+  ```
 
-- If possible, do a combined log & return operation:
+  instead of:
 
   ```c
-  r = operation(...);
-  if (r < 0)
-          return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m");
+  struct foobar t;
+  zero(t);
+  t.foo = 7;
+  t.bar = "bazz";
   ```
 
-  If the error value is "synthetic", i.e. it was not received from
-  the called function, use `SYNTHETIC_ERRNO` wrapper to tell the logging
-  system to not log the errno value, but still return it:
+- To implement an endless loop, use `for (;;)` rather than `while (1)`.  The
+  latter is a bit ugly anyway, since you probably really meant `while
+  (true)`. To avoid the discussion what the right always-true expression for an
+  infinite while loop is, our recommendation is to simply write it without any
+  such expression by using `for (;;)`.
+
+- To determine the length of a constant string `"foo"`, don't bother with
+  `sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize
+  the call away for fixed strings). The only exception is when declaring an
+  array. In that case use STRLEN, which evaluates to a static constant and
+  doesn't force the compiler to create a VLA.
+
+## Destructors
+
+- The destructors always deregister the object from the next bigger object, not
+  the other way around.
+
+- For robustness reasons, destructors should be able to destruct
+  half-initialized objects, too.
+
+- When you define a destructor or `unref()` call for an object, please accept a
+  `NULL` object and simply treat this as NOP. This is similar to how libc
+  `free()` works, which accepts `NULL` pointers and becomes a NOP for them. By
+  following this scheme a lot of `if` checks can be removed before invoking
+  your destructor, which makes the code substantially more readable and robust.
+
+- Related to this: when you define a destructor or `unref()` call for an
+  object, please make it return the same type it takes and always return `NULL`
+  from it. This allows writing code like this:
 
   ```c
-  n = read(..., s, sizeof s);
-  if (n != sizeof s)
-          return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ...");
+  p = foobar_unref(p);
   ```
 
-- Avoid static variables, except for caches and very few other
-  cases. Think about thread-safety! While most of our code is never
-  used in threaded environments, at least the library code should make
-  sure it works correctly in them. Instead of doing a lot of locking
-  for that, we tend to prefer using TLS to do per-thread caching (which
-  only works for small, fixed-size cache objects), or we disable
-  caching for any thread that is not the main thread. Use
-  `is_main_thread()` to detect whether the calling thread is the main
-  thread.
-
-- Command line option parsing:
-  - Do not print full `help()` on error, be specific about the error.
-  - Do not print messages to stdout on error.
-  - Do not POSIX_ME_HARDER unless necessary, i.e. avoid `+` in option string.
+  which will always work regardless if `p` is initialized or not,x and
+  guarantees that `p` is `NULL` afterwards, all in just one line.
 
-- Do not write functions that clobber call-by-reference variables on
-  failure. Use temporary variables for these cases and change the
-  passed in variables only on success.
+## Error Handling
 
-- When you allocate a file descriptor, it should be made `O_CLOEXEC`
-  right from the beginning, as none of our files should leak to forked
-  binaries by default. Hence, whenever you open a file, `O_CLOEXEC` must
-  be specified, right from the beginning. This also applies to
-  sockets. Effectively, this means that all invocations to:
+- Error codes are returned as negative `Exxx`. e.g. `return -EINVAL`. There are
+  some exceptions: for constructors, it is OK to return `NULL` on OOM. For
+  lookup functions, `NULL` is fine too for "not found".
 
-  - `open()` must get `O_CLOEXEC` passed,
-  - `socket()` and `socketpair()` must get `SOCK_CLOEXEC` passed,
-  - `recvmsg()` must get `MSG_CMSG_CLOEXEC` set,
-  - `F_DUPFD_CLOEXEC` should be used instead of `F_DUPFD`, and so on,
-  - invocations of `fopen()` should take `e`.
+  Be strict with this. When you write a function that can fail due to more than
+  one cause, it *really* should have an `int` as the return value for the error
+  code.
 
-- We never use the POSIX version of `basename()` (which glibc defines it in
-  `libgen.h`), only the GNU version (which glibc defines in `string.h`).
-  The only reason to include `libgen.h` is because `dirname()`
-  is needed. Every time you need that please immediately undefine
-  `basename()`, and add a comment about it, so that no code ever ends up
-  using the POSIX version!
+- Do not bother with error checking whether writing to stdout/stderr worked.
 
-- Use the bool type for booleans, not integers. One exception: in public
-  headers (i.e those in `src/systemd/sd-*.h`) use integers after all, as `bool`
-  is C99 and in our public APIs we try to stick to C89 (with a few extension).
+- Do not log errors from "library" code, only do so from "main program"
+  code. (With one exception: it is OK to log with DEBUG level from any code,
+  with the exception of maybe inner loops).
 
-- When you invoke certain calls like `unlink()`, or `mkdir_p()` and you
-  know it is safe to ignore the error it might return (because a later
-  call would detect the failure anyway, or because the error is in an
-  error path and you thus couldn't do anything about it anyway), then
-  make this clear by casting the invocation explicitly to `(void)`. Code
-  checks like Coverity understand that, and will not complain about
-  ignored error codes. Hence, please use this:
+- In public API calls, you **must** validate all your input arguments for
+  programming error with `assert_return()` and return a sensible return
+  code. In all other calls, it is recommended to check for programming errors
+  with a more brutal `assert()`. We are more forgiving to public users than for
+  ourselves! Note that `assert()` and `assert_return()` really only should be
+  used for detecting programming errors, not for runtime errors. `assert()` and
+  `assert_return()` by usage of `_likely_()` inform the compiler that he should
+  not expect these checks to fail, and they inform fellow programmers about the
+  expected validity and range of parameters.
+
+- When you invoke certain calls like `unlink()`, or `mkdir_p()` and you know it
+  is safe to ignore the error it might return (because a later call would
+  detect the failure anyway, or because the error is in an error path and you
+  thus couldn't do anything about it anyway), then make this clear by casting
+  the invocation explicitly to `(void)`. Code checks like Coverity understand
+  that, and will not complain about ignored error codes. Hence, please use
+  this:
 
   ```c
   (void) unlink("/foo/bar/baz");
@@ -283,231 +271,208 @@ title: Coding Style
   ```
 
   Don't cast function calls to `(void)` that return no error
-  conditions. Specifically, the various `xyz_unref()` calls that return a `NULL`
-  object shouldn't be cast to `(void)`, since not using the return value does not
-  hide any errors.
-
-- Don't invoke `exit()`, ever. It is not replacement for proper error
-  handling. Please escalate errors up your call chain, and use normal
-  `return` to exit from the main function of a process. If you
-  `fork()`ed off a child process, please use `_exit()` instead of `exit()`,
-  so that the exit handlers are not run.
-
-- Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)`
-  instead. For two reason: first, you want `O_CLOEXEC` set on the new `fd`
-  (see above). Second, `dup()` will happily duplicate your `fd` as 0, 1,
-  2, i.e. stdin, stdout, stderr, should those `fd`s be closed. Given the
-  special semantics of those `fd`s, it's probably a good idea to avoid
-  them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them.
+  conditions. Specifically, the various `xyz_unref()` calls that return a
+  `NULL` object shouldn't be cast to `(void)`, since not using the return value
+  does not hide any errors.
 
-- When you define a destructor or `unref()` call for an object, please
-  accept a `NULL` object and simply treat this as NOP. This is similar
-  to how libc `free()` works, which accepts `NULL` pointers and becomes a
-  NOP for them. By following this scheme a lot of `if` checks can be
-  removed before invoking your destructor, which makes the code
-  substantially more readable and robust.
-
-- Related to this: when you define a destructor or `unref()` call for an
-  object, please make it return the same type it takes and always
-  return `NULL` from it. This allows writing code like this:
-
-  ```c
-  p = foobar_unref(p);
-  ```
-
-  which will always work regardless if `p` is initialized or not, and
-  guarantees that `p` is `NULL` afterwards, all in just one line.
+- When returning a return code from `main()`, please preferably use
+  `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc.
 
-- Use `alloca()`, but never forget that it is not OK to invoke `alloca()`
-  within a loop or within function call parameters. `alloca()` memory is
-  released at the end of a function, and not at the end of a `{}`
-  block. Thus, if you invoke it in a loop, you keep increasing the
-  stack pointer without ever releasing memory again. (VLAs have better
-  behavior in this case, so consider using them as an alternative.)
-  Regarding not using `alloca()` within function parameters, see the
-  BUGS section of the `alloca(3)` man page.
+## Logging
 
-- Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)`
+- For every function you add, think about whether it is a "logging" function or
+  a "non-logging" function. "Logging" functions do logging on their own,
+  "non-logging" function never log on their own and expect their callers to
+  log. All functions in "library" code, i.e. in `src/shared/` and suchlike must
+  be "non-logging". Every time a "logging" function calls a "non-logging"
+  function, it should log about the resulting errors. If a "logging" function
+  calls another "logging" function, then it should not generate log messages,
+  so that log messages are not generated twice for the same errors.
 
-- Instead of using `memzero()`/`memset()` to initialize structs allocated
-  on the stack, please try to use c99 structure initializers. It's
-  short, prettier and actually even faster at execution. Hence:
+- If possible, do a combined log & return operation:
 
   ```c
-  struct foobar t = {
-          .foo = 7,
-          .bar = "bazz",
-  };
+  r = operation(...);
+  if (r < 0)
+          return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m");
   ```
 
-  instead of:
+  If the error value is "synthetic", i.e. it was not received from
+  the called function, use `SYNTHETIC_ERRNO` wrapper to tell the logging
+  system to not log the errno value, but still return it:
 
   ```c
-  struct foobar t;
-  zero(t);
-  t.foo = 7;
-  t.bar = "bazz";
+  n = read(..., s, sizeof s);
+  if (n != sizeof s)
+          return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ...");
   ```
 
-- When returning a return code from `main()`, please preferably use
-  `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc.
-
-- The order in which header files are included doesn't matter too
-  much. systemd-internal headers must not rely on an include order, so
-  it is safe to include them in any order possible.
-  However, to not clutter global includes, and to make sure internal
-  definitions will not affect global headers, please always include the
-  headers of external components first (these are all headers enclosed
-  in <>), followed by our own exported headers (usually everything
-  that's prefixed by `sd-`), and then followed by internal headers.
-  Furthermore, in all three groups, order all includes alphabetically
-  so duplicate includes can easily be detected.
+## Memory Allocation
 
-- To implement an endless loop, use `for (;;)` rather than `while (1)`.
-  The latter is a bit ugly anyway, since you probably really
-  meant `while (true)`. To avoid the discussion what the right
-  always-true expression for an infinite while loop is, our
-  recommendation is to simply write it without any such expression by
-  using `for (;;)`.
-
-- Never use the `off_t` type, and particularly avoid it in public
-  APIs. It's really weirdly defined, as it usually is 64-bit and we
-  don't support it any other way, but it could in theory also be
-  32-bit. Which one it is depends on a compiler switch chosen by the
-  compiled program, which hence corrupts APIs using it unless they can
-  also follow the program's choice. Moreover, in systemd we should
-  parse values the same way on all architectures and cannot expose
-  `off_t` values over D-Bus. To avoid any confusion regarding conversion
-  and ABIs, always use simply `uint64_t` directly.
-
-- Commit message subject lines should be prefixed with an appropriate
-  component name of some kind. For example "journal: ", "nspawn: " and
-  so on.
+- Always check OOM. There is no excuse. In program code, you can use
+  `log_oom()` for then printing a short message, but not in "library" code.
 
-- Do not use "Signed-Off-By:" in your commit messages. That's a kernel
-  thing we don't do in the systemd project.
+- Avoid fixed-size string buffers, unless you really know the maximum size and
+  that maximum size is small. They are a source of errors, since they possibly
+  result in truncated strings. It is often nicer to use dynamic memory,
+  `alloca()` or VLAs. If you do allocate fixed-size strings on the stack, then
+  it is probably only OK if you either use a maximum size such as `LINE_MAX`,
+  or count in detail the maximum size a string can have. (`DECIMAL_STR_MAX` and
+  `DECIMAL_STR_WIDTH` macros are your friends for this!)
 
-- Avoid leaving long-running child processes around, i.e. `fork()`s that
-  are not followed quickly by an `execv()` in the child. Resource
-  management is unclear in this case, and memory CoW will result in
-  unexpected penalties in the parent much, much later on.
+  Or in other words, if you use `char buf[256]` then you are likely doing
+  something wrong!
 
-- Don't block execution for arbitrary amounts of time using `usleep()`
-  or a similar call, unless you really know what you do. Just "giving
-  something some time", or so is a lazy excuse. Always wait for the
-  proper event, instead of doing time-based poll loops.
+- Make use of `_cleanup_free_` and friends. It makes your code much nicer to
+  read (and shorter)!
 
-- To determine the length of a constant string `"foo"`, don't bother with
-  `sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize
-  the call away for fixed strings). The only exception is when declaring an
-  array. In that case use STRLEN, which evaluates to a static constant and
-  doesn't force the compiler to create a VLA.
+- Use `alloca()`, but never forget that it is not OK to invoke `alloca()`
+  within a loop or within function call parameters. `alloca()` memory is
+  released at the end of a function, and not at the end of a `{}` block. Thus,
+  if you invoke it in a loop, you keep increasing the stack pointer without
+  ever releasing memory again. (VLAs have better behavior in this case, so
+  consider using them as an alternative.)  Regarding not using `alloca()`
+  within function parameters, see the BUGS section of the `alloca(3)` man page.
 
 - If you want to concatenate two or more strings, consider using `strjoina()`
   or `strjoin()` rather than `asprintf()`, as the latter is a lot slower. This
   matters particularly in inner loops (but note that `strjoina()` cannot be
   used there).
 
-- Please avoid using global variables as much as you can. And if you
-  do use them make sure they are static at least, instead of
-  exported. Especially in library-like code it is important to avoid
-  global variables. Why are global variables bad? They usually hinder
-  generic reusability of code (since they break in threaded programs,
-  and usually would require locking there), and as the code using them
-  has side-effects make programs non-transparent. That said, there are
-  many cases where they explicitly make a lot of sense, and are OK to
-  use. For example, the log level and target in `log.c` is stored in a
-  global variable, and that's OK and probably expected by most. Also
-  in many cases we cache data in global variables. If you add more
-  caches like this, please be careful however, and think about
-  threading. Only use static variables if you are sure that
-  thread-safety doesn't matter in your case. Alternatively, consider
-  using TLS, which is pretty easy to use with gcc's `thread_local`
-  concept. It's also OK to store data that is inherently global in
-  global variables, for example data parsed from command lines, see
-  below.
+## Runtime Behaviour
 
-- If you parse a command line, and want to store the parsed parameters
-  in global variables, please consider prefixing their names with
-  `arg_`. We have been following this naming rule in most of our
-  tools, and we should continue to do so, as it makes it easy to
-  identify command line parameter variables, and makes it clear why it
-  is OK that they are global variables.
+- Avoid leaving long-running child processes around, i.e. `fork()`s that are
+  not followed quickly by an `execv()` in the child. Resource management is
+  unclear in this case, and memory CoW will result in unexpected penalties in
+  the parent much, much later on.
 
-- When exposing public C APIs, be careful what function parameters you make
-  `const`. For example, a parameter taking a context object should probably not
-  be `const`, even if you are writing an otherwise read-only accessor function
-  for it. The reason is that making it `const` fixates the contract that your
-  call won't alter the object ever, as part of the API. However, that's often
-  quite a promise, given that this even prohibits object-internal caching or
-  lazy initialization of object variables. Moreover, it's usually not too useful
-  for client applications. Hence, please be careful and avoid `const` on object
-  parameters, unless you are very sure `const` is appropriate.
+- Don't block execution for arbitrary amounts of time using `usleep()` or a
+  similar call, unless you really know what you do. Just "giving something some
+  time", or so is a lazy excuse. Always wait for the proper event, instead of
+  doing time-based poll loops.
+
+- Whenever installing a signal handler, make sure to set `SA_RESTART` for it,
+  so that interrupted system calls are automatically restarted, and we minimize
+  hassles with handling `EINTR` (in particular as `EINTR` handling is pretty
+  broken on Linux).
+
+- When applying C-style unescaping as well as specifier expansion on the same
+  string, always apply the C-style unescaping fist, followed by the specifier
+  expansion. When doing the reverse, make sure to escape `%` in specifier-style
+  first (i.e. `%` → `%%`), and then do C-style escaping where necessary.
+
+- Be exceptionally careful when formatting and parsing floating point
+  numbers. Their syntax is locale dependent (i.e. `5.000` in en_US is generally
+  understood as 5, while in de_DE as 5000.).
 
 - Make sure to enforce limits on every user controllable resource. If the user
   can allocate resources in your code, your code must enforce some form of
-  limits after which it will refuse operation. It's fine if it is hard-coded (at
-  least initially), but it needs to be there. This is particularly important
-  for objects that unprivileged users may allocate, but also matters for
-  everything else any user may allocated.
-
-- `htonl()`/`ntohl()` and `htons()`/`ntohs()` are weird. Please use `htobe32()` and
-  `htobe16()` instead, it's much more descriptive, and actually says what really
-  is happening, after all `htonl()` and `htons()` don't operate on `long`s and
-  `short`s as their name would suggest, but on `uint32_t` and `uint16_t`. Also,
-  "network byte order" is just a weird name for "big endian", hence we might
-  want to call it "big endian" right-away.
+  limits after which it will refuse operation. It's fine if it is hard-coded
+  (at least initially), but it needs to be there. This is particularly
+  important for objects that unprivileged users may allocate, but also matters
+  for everything else any user may allocated.
+
+## Types
+
+- Think about the types you use. If a value cannot sensibly be negative, do not
+  use `int`, but use `unsigned`.
+
+- Use `char` only for actual characters. Use `uint8_t` or `int8_t` when you
+  actually mean a byte-sized signed or unsigned integers. When referring to a
+  generic byte, we generally prefer the unsigned variant `uint8_t`. Do not use
+  types based on `short`. They *never* make sense. Use `int`, `long`, `long
+  long`, all in unsigned and signed fashion, and the fixed-size types
+  `uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`, `int8_t`, `int16_t`, `int32_t`
+  and so on, as well as `size_t`, but nothing else. Do not use kernel types
+  like `u32` and so on, leave that to the kernel.
+
+- Stay uniform. For example, always use `usec_t` for time values. Do not mix
+  `usec` and `msec`, and `usec` and whatnot.
+
+- Never use the `off_t` type, and particularly avoid it in public APIs. It's
+  really weirdly defined, as it usually is 64-bit and we don't support it any
+  other way, but it could in theory also be 32-bit. Which one it is depends on
+  a compiler switch chosen by the compiled program, which hence corrupts APIs
+  using it unless they can also follow the program's choice. Moreover, in
+  systemd we should parse values the same way on all architectures and cannot
+  expose `off_t` values over D-Bus. To avoid any confusion regarding conversion
+  and ABIs, always use simply `uint64_t` directly.
 
-- You might wonder what kind of common code belongs in `src/shared/` and what
-  belongs in `src/basic/`. The split is like this: anything that is used to
-  implement the public shared object we provide (sd-bus, sd-login, sd-id128,
-  nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must
-  be located in `src/basic` (those objects are not allowed to link to
-  libsystemd-shared.so). Conversely, anything which is shared between multiple
-  components and does not need to be in `src/basic/`, should be in
-  `src/shared/`.
+- Unless you allocate an array, `double` is always a better choice than
+  `float`. Processors speak `double` natively anyway, so there is no speed
+  benefit, and on calls like `printf()` `float`s get promoted to `double`s
+  anyway, so there is no point.
 
-  To summarize:
+- Use the bool type for booleans, not integers. One exception: in public
+  headers (i.e those in `src/systemd/sd-*.h`) use integers after all, as `bool`
+  is C99 and in our public APIs we try to stick to C89 (with a few extension).
 
-  `src/basic/`
-  - may be used by all code in the tree
-  - may not use any code outside of `src/basic/`
+## Deadlocks
 
-  `src/libsystemd/`
-  - may be used by all code in the tree, except for code in `src/basic/`
-  - may not use any code outside of `src/basic/`, `src/libsystemd/`
+- Do not issue NSS requests (that includes user name and host name lookups)
+  from PID 1 as this might trigger deadlocks when those lookups involve
+  synchronously talking to services that we would need to start up.
 
-  `src/shared/`
-  - may be used by all code in the tree, except for code in `src/basic/`,
-    `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
-    `src/journal/` that end up in `libjournal-client.a` convenience library.
-  - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/`
+- Do not synchronously talk to any other service from PID 1, due to risk of
+  deadlocks.
 
-- Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are
-  incompatible with glibc it's on them. However, if there are equivalent POSIX
-  and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there
-  aren't, we are happy to use GNU or Linux APIs, and expect non-GNU
-  implementations of libc to catch up with glibc.
+## File Descriptors
 
-- Whenever installing a signal handler, make sure to set `SA_RESTART` for it, so
-  that interrupted system calls are automatically restarted, and we minimize
-  hassles with handling `EINTR` (in particular as `EINTR` handling is pretty broken
-  on Linux).
+- When you allocate a file descriptor, it should be made `O_CLOEXEC` right from
+  the beginning, as none of our files should leak to forked binaries by
+  default. Hence, whenever you open a file, `O_CLOEXEC` must be specified,
+  right from the beginning. This also applies to sockets. Effectively, this
+  means that all invocations to:
 
-- When applying C-style unescaping as well as specifier expansion on the same
-  string, always apply the C-style unescaping fist, followed by the specifier
-  expansion. When doing the reverse, make sure to escape `%` in specifier-style
-  first (i.e. `%` → `%%`), and then do C-style escaping where necessary.
+  - `open()` must get `O_CLOEXEC` passed,
+  - `socket()` and `socketpair()` must get `SOCK_CLOEXEC` passed,
+  - `recvmsg()` must get `MSG_CMSG_CLOEXEC` set,
+  - `F_DUPFD_CLOEXEC` should be used instead of `F_DUPFD`, and so on,
+  - invocations of `fopen()` should take `e`.
 
-- It's a good idea to use `O_NONBLOCK` when opening 'foreign' regular files, i.e.
-  file system objects that are supposed to be regular files whose paths where
-  specified by the user and hence might actually refer to other types of file
-  system objects. This is a good idea so that we don't end up blocking on
+- It's a good idea to use `O_NONBLOCK` when opening 'foreign' regular files,
+  i.e.  file system objects that are supposed to be regular files whose paths
+  where specified by the user and hence might actually refer to other types of
+  file system objects. This is a good idea so that we don't end up blocking on
   'strange' file nodes, for example if the user pointed us to a FIFO or device
   node which may block when opening. Moreover even for actual regular files
   `O_NONBLOCK` has a benefit: it bypasses any mandatory lock that might be in
-  effect on the regular file. If in doubt consider turning off `O_NONBLOCK` again
-  after opening.
+  effect on the regular file. If in doubt consider turning off `O_NONBLOCK`
+  again after opening.
+
+## Command Line
+
+- If you parse a command line, and want to store the parsed parameters in
+  global variables, please consider prefixing their names with `arg_`. We have
+  been following this naming rule in most of our tools, and we should continue
+  to do so, as it makes it easy to identify command line parameter variables,
+  and makes it clear why it is OK that they are global variables.
+
+- Command line option parsing:
+  - Do not print full `help()` on error, be specific about the error.
+  - Do not print messages to stdout on error.
+  - Do not POSIX_ME_HARDER unless necessary, i.e. avoid `+` in option string.
+
+## Exporting Symbols
+
+- Variables and functions **must** be static, unless they have a prototype, and
+  are supposed to be exported.
+
+- Public API calls (i.e. functions exported by our shared libraries)
+  must be marked `_public_` and need to be prefixed with `sd_`. No
+  other functions should be prefixed like that.
+
+- When exposing public C APIs, be careful what function parameters you make
+  `const`. For example, a parameter taking a context object should probably not
+  be `const`, even if you are writing an otherwise read-only accessor function
+  for it. The reason is that making it `const` fixates the contract that your
+  call won't alter the object ever, as part of the API. However, that's often
+  quite a promise, given that this even prohibits object-internal caching or
+  lazy initialization of object variables. Moreover, it's usually not too
+  useful for client applications. Hence, please be careful and avoid `const` on
+  object parameters, unless you are very sure `const` is appropriate.
+
+## Referencing Concepts
 
 - When referring to a configuration file option in the documentation and such,
   please always suffix it with `=`, to indicate that it is a configuration file
@@ -521,6 +486,52 @@ title: Coding Style
   suffix it with `/`, to indicate that it is a directory, not a regular file
   (or other file system object).
 
+## Functions to Avoid
+
+- Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)`
+
+- Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where
+  applicable (i.e. wherever you just care about equality/inequality, not about
+  the sorting order).
+
+- Never use `strtol()`, `atoi()` and similar calls. Use `safe_atoli()`,
+  `safe_atou32()` and suchlike instead. They are much nicer to use in most
+  cases and correctly check for parsing errors.
+
+- `htonl()`/`ntohl()` and `htons()`/`ntohs()` are weird. Please use `htobe32()`
+  and `htobe16()` instead, it's much more descriptive, and actually says what
+  really is happening, after all `htonl()` and `htons()` don't operate on
+  `long`s and `short`s as their name would suggest, but on `uint32_t` and
+  `uint16_t`. Also, "network byte order" is just a weird name for "big endian",
+  hence we might want to call it "big endian" right-away.
+
+- Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)` instead. For
+  two reason: first, you want `O_CLOEXEC` set on the new `fd` (see
+  above). Second, `dup()` will happily duplicate your `fd` as 0, 1, 2,
+  i.e. stdin, stdout, stderr, should those `fd`s be closed. Given the special
+  semantics of those `fd`s, it's probably a good idea to avoid
+  them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them.
+
 - Don't use `fgets()`, it's too hard to properly handle errors such as overly
   long lines. Use `read_line()` instead, which is our own function that handles
   this much nicer.
+
+- Don't invoke `exit()`, ever. It is not replacement for proper error
+  handling. Please escalate errors up your call chain, and use normal `return`
+  to exit from the main function of a process. If you `fork()`ed off a child
+  process, please use `_exit()` instead of `exit()`, so that the exit handlers
+  are not run.
+
+- We never use the POSIX version of `basename()` (which glibc defines it in
+  `libgen.h`), only the GNU version (which glibc defines in `string.h`).  The
+  only reason to include `libgen.h` is because `dirname()` is needed. Every
+  time you need that please immediately undefine `basename()`, and add a
+  comment about it, so that no code ever ends up using the POSIX version!
+
+# Committing to git
+
+- Commit message subject lines should be prefixed with an appropriate component
+  name of some kind. For example "journal: ", "nspawn: " and so on.
+
+- Do not use "Signed-Off-By:" in your commit messages. That's a kernel thing we
+  don't do in the systemd project.