man: document that nspawn -x, --template= and machinectl clone leave hostname and...

[thirdparty/systemd.git] / CODING_STYLE

- 8ch indent, no tabs, except for files in man/ which are 2ch indent,
  and still no tabs

- We prefer /* comments */ over // comments, please. This is not C++, after
  all. (Yes we know that C99 supports both kinds of comments, but still,
  please!)

- 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, ~140ch should be enough really.

- Variables and functions *must* be static, unless they have a
  prototype, and are supposed to be exported.

- structs in MixedCase (with exceptions, such as public API structs),
  variables + functions in lower_case.

- The destructors always unregister 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 "int" as 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!

- 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 on de_DE as 5000.).

- Try to use this:

      void foo() {
      }

  instead of this:

      void foo()
      {
      }

  But it is OK if you do not.

- Single-line "if" blocks should not be enclosed in {}. Use this:

  if (foobar)
          waldo();

  instead of this:

  if (foobar) {
          waldo();
  }

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

- Please use streq() and strneq() instead of strcmp(), strncmp() where applicable.

- Please do not allocate variables on the stack in the middle of code,
  even if C99 allows it. Wrong:

  {
          a = 5;
          int b;
          b = a;
  }

  Right:

  {
          int b;
          a = 5;
          b = a;
  }

- Unless you allocate an array, "double" is always the better choice
  than "float". Processors speak "double" natively anyway, so this is
  no speed benefit, and on calls like printf() "float"s get promoted
  to "double"s anyway, so there is no point.

- Do not invoke functions when you allocate variables on the stack. Wrong:

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

  Right:

  {
          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!

- Think about the types you use. If a value cannot sensibly be
  negative, do not use "int", but use "unsigned".

- Do not use types like "short". They *never* make sense. Use ints,
  longs, long longs, all in unsigned+signed fashion, and the fixed
  size types uint32_t and so on, as well as size_t, but nothing else.

- 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.

- 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 then 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.

- 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.

- 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.

- 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.

- 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:

  a) open() must get O_CLOEXEC passed
  b) socket() and socketpair() must get SOCK_CLOEXEC passed
  c) recvmsg() must get MSG_CMSG_CLOEXEC set
  d) F_DUPFD_CLOEXEC should be used instead of F_DUPFD, and so on

- We never use the XDG version of basename(). glibc defines it in
  libgen.h. The only reason to include that file is because dirname()
  is needed. Everytime you need that please immediately undefine
  basename(), and add a comment about it, so that no code ever ends up
  using the XDG version!

- 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).

- 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:

      (void) unlink("/foo/bar/baz");

  instead of just this:

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

- 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 fds be closed. Given the
  special semantics of those fds, it's probably a good idea to avoid
  them. F_DUPFD_CLOEXEC with "3" as parameter avoids them.

- 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:

            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.