CODING_STYLE: export section about exporting symbols

[thirdparty/systemd.git] / docs / CODING_STYLE.md

---
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).

- 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
  overdo it, ~109ch should be enough really. The `.editorconfig`, `.vimrc` and
  `.dir-locals.el` files contained in the repository will set this limit up for
  you automatically, if you let them (as well as a few other things). Please
  note that emacs loads `.dir-locals.el` automatically, but vim needs to be
  configured to load `.vimrc`, see that file for instructions.

- Try to write this:

  ```c
  void foo() {
  }
  ```

  instead of this:

  ```c
  void foo()
  {
  }
  ```

- Single-line `if` blocks should not be enclosed in `{}`. Write this:

  ```c
  if (foobar)
          waldo();
  ```

  instead of this:

  ```c
  if (foobar) {
          waldo();
  }
  ```

- Do not write `foo ()`, write `foo()`.

- Preferably allocate local variables on the top of the block:

  ```c
  {
          int a, b;

          a = 5;
          b = a;
  }
  ```

## Other

- structs in `PascalCase` (with exceptions, such as public API structs),
  variables and functions in `snake_case`.

- To minimize strict aliasing violations, we prefer unions over casting.

- 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.

- 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.).

- Do not mix function invocations with variable definitions in one
  line. Wrong:

  ```c
  {
          int a = foobar();
          uint64_t x = 7;
  }
  ```

  Right:

  ```c
  {
          int a;
          uint64_t x = 7;

          a = foobar();
  }
  ```

- 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!

- 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.

- If possible, do a combined log & return operation:

  ```c
  r = operation(...);
  if (r < 0)
          return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m");
  ```

  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
  n = read(..., s, sizeof s);
  if (n != sizeof s)
          return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ...");
  ```

- 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.


- 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:

  ```c
  struct foobar t = {
          .foo = 7,
          .bar = "bazz",
  };
  ```

  instead of:

  ```c
  struct foobar t;
  zero(t);
  t.foo = 7;
  t.bar = "bazz";
  ```

- 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.

- 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 (;;)`.

- 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.

- 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.

- 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.

- 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.

- 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.

- 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.

- 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.

## 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
  p = foobar_unref(p);
  ```

  which will always work regardless if `p` is initialized or not,x and
  guarantees that `p` is `NULL` afterwards, all in just one line.

## Error Handling

- 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).

- 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");
  ```

  instead of just this:

  ```c
  unlink("/foo/bar/baz");
  ```

  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.

- When returning a return code from `main()`, please preferably use
  `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc.

## Memory Allocation

- 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.

- 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!

- Make use of `_cleanup_free_` and friends. It makes your code much nicer to
  read (and shorter)!

- 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).

## 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.

- 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.

- 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).

## File Descriptors

- 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:

  - `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
  '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.

## 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
  setting.

- When referring to a command line option in the documentation and such, please
  always prefix with `--` or `-` (as appropriate), to indicate that it is a
  command line option.

- When referring to a file system path that is a directory, please always
  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.