CODING_STYLE

- 8ch indent, no tabs

- 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. i.e. 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 usec mix 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.

- 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". Everytime 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.