CODING_STYLE

   1 - 8ch indent, no tabs, except for files in man/ which are 2ch indent,
   2   and still no tabs
   3
   4 - We prefer /* comments */ over // comments, please. This is not C++, after
   5   all. (Yes we know that C99 supports both kinds of comments, but still,
   6   please!)
   7
   8 - Don't break code lines too eagerly. We do *not* force line breaks at
   9   80ch, all of today's screens should be much larger than that. But
  10   then again, don't overdo it, ~140ch should be enough really.
  11
  12 - Variables and functions *must* be static, unless they have a
  13   prototype, and are supposed to be exported.
  14
  15 - structs in MixedCase (with exceptions, such as public API structs),
  16   variables + functions in lower_case.
  17
  18 - The destructors always unregister the object from the next bigger
  19   object, not the other way around
  20
  21 - To minimize strict aliasing violations, we prefer unions over casting
  22
  23 - For robustness reasons, destructors should be able to destruct
  24   half-initialized objects, too
  25
  26 - Error codes are returned as negative Exxx. e.g. return -EINVAL. There
  27   are some exceptions: for constructors, it is OK to return NULL on
  28   OOM. For lookup functions, NULL is fine too for "not found".
  29
  30   Be strict with this. When you write a function that can fail due to
  31   more than one cause, it *really* should have "int" as return value
  32   for the error code.
  33
  34 - Do not bother with error checking whether writing to stdout/stderr
  35   worked.
  36
  37 - Do not log errors from "library" code, only do so from "main
  38   program" code. (With one exception: it is OK to log with DEBUG level
  39   from any code, with the exception of maybe inner loops).
  40
  41 - Always check OOM. There is no excuse. In program code, you can use
  42   "log_oom()" for then printing a short message, but not in "library" code.
  43
  44 - Do not issue NSS requests (that includes user name and host name
  45   lookups) from PID 1 as this might trigger deadlocks when those
  46   lookups involve synchronously talking to services that we would need
  47   to start up
  48
  49 - Do not synchronously talk to any other service from PID 1, due to
  50   risk of deadlocks
  51
  52 - Avoid fixed-size string buffers, unless you really know the maximum
  53   size and that maximum size is small. They are a source of errors,
  54   since they possibly result in truncated strings. It is often nicer
  55   to use dynamic memory, alloca() or VLAs. If you do allocate fixed-size
  56   strings on the stack, then it is probably only OK if you either
  57   use a maximum size such as LINE_MAX, or count in detail the maximum
  58   size a string can have. (DECIMAL_STR_MAX and DECIMAL_STR_WIDTH
  59   macros are your friends for this!)
  60
  61   Or in other words, if you use "char buf[256]" then you are likely
  62   doing something wrong!
  63
  64 - Stay uniform. For example, always use "usec_t" for time
  65   values. Do not mix usec and msec, and usec and whatnot.
  66
  67 - Make use of _cleanup_free_ and friends. It makes your code much
  68   nicer to read!
  69
  70 - Be exceptionally careful when formatting and parsing floating point
  71   numbers. Their syntax is locale dependent (i.e. "5.000" in en_US is
  72   generally understood as 5, while on de_DE as 5000.).
  73
  74 - Try to use this:
  75
  76       void foo() {
  77       }
  78
  79   instead of this:
  80
  81       void foo()
  82       {
  83       }
  84
  85   But it is OK if you do not.
  86
  87 - Single-line "if" blocks should not be enclosed in {}. Use this:
  88
  89   if (foobar)
  90           waldo();
  91
  92   instead of this:
  93
  94   if (foobar) {
  95           waldo();
  96   }
  97
  98 - Do not write "foo ()", write "foo()".
  99
 100 - Please use streq() and strneq() instead of strcmp(), strncmp() where applicable.
 101
 102 - Please do not allocate variables on the stack in the middle of code,
 103   even if C99 allows it. Wrong:
 104
 105   {
 106           a = 5;
 107           int b;
 108           b = a;
 109   }
 110
 111   Right:
 112
 113   {
 114           int b;
 115           a = 5;
 116           b = a;
 117   }
 118
 119 - Unless you allocate an array, "double" is always the better choice
 120   than "float". Processors speak "double" natively anyway, so this is
 121   no speed benefit, and on calls like printf() "float"s get promoted
 122   to "double"s anyway, so there is no point.
 123
 124 - Do not mix function invocations with variable definitions in one
 125   line. Wrong:
 126
 127   {
 128           int a = foobar();
 129           uint64_t x = 7;
 130   }
 131
 132   Right:
 133
 134   {
 135           int a;
 136           uint64_t x = 7;
 137
 138           a = foobar();
 139   }
 140
 141 - Use "goto" for cleaning up, and only use it for that. i.e. you may
 142   only jump to the end of a function, and little else. Never jump
 143   backwards!
 144
 145 - Think about the types you use. If a value cannot sensibly be
 146   negative, do not use "int", but use "unsigned".
 147
 148 - Use "char" only for actual characters. Use "uint8_t" or "int8_t"
 149   when you actually mean a byte-sized signed or unsigned
 150   integers. When referring to a generic byte, we generally prefer the
 151   unsigned variant "uint8_t". Do not use types based on "short". They
 152   *never* make sense. Use ints, longs, long longs, all in
 153   unsigned+signed fashion, and the fixed size types
 154   uint8_t/uint16_t/uint32_t/uint64_t/int8_t/int16_t/int32_t and so on,
 155   as well as size_t, but nothing else. Do not use kernel types like
 156   u32 and so on, leave that to the kernel.
 157
 158 - Public API calls (i.e. functions exported by our shared libraries)
 159   must be marked "_public_" and need to be prefixed with "sd_". No
 160   other functions should be prefixed like that.
 161
 162 - In public API calls, you *must* validate all your input arguments for
 163   programming error with assert_return() and return a sensible return
 164   code. In all other calls, it is recommended to check for programming
 165   errors with a more brutal assert(). We are more forgiving to public
 166   users then for ourselves! Note that assert() and assert_return()
 167   really only should be used for detecting programming errors, not for
 168   runtime errors. assert() and assert_return() by usage of _likely_()
 169   inform the compiler that he should not expect these checks to fail,
 170   and they inform fellow programmers about the expected validity and
 171   range of parameters.
 172
 173 - Never use strtol(), atoi() and similar calls. Use safe_atoli(),
 174   safe_atou32() and suchlike instead. They are much nicer to use in
 175   most cases and correctly check for parsing errors.
 176
 177 - For every function you add, think about whether it is a "logging"
 178   function or a "non-logging" function. "Logging" functions do logging
 179   on their own, "non-logging" function never log on their own and
 180   expect their callers to log. All functions in "library" code,
 181   i.e. in src/shared/ and suchlike must be "non-logging". Every time a
 182   "logging" function calls a "non-logging" function, it should log
 183   about the resulting errors. If a "logging" function calls another
 184   "logging" function, then it should not generate log messages, so
 185   that log messages are not generated twice for the same errors.
 186
 187 - Avoid static variables, except for caches and very few other
 188   cases. Think about thread-safety! While most of our code is never
 189   used in threaded environments, at least the library code should make
 190   sure it works correctly in them. Instead of doing a lot of locking
 191   for that, we tend to prefer using TLS to do per-thread caching (which
 192   only works for small, fixed-size cache objects), or we disable
 193   caching for any thread that is not the main thread. Use
 194   is_main_thread() to detect whether the calling thread is the main
 195   thread.
 196
 197 - Command line option parsing:
 198   - Do not print full help() on error, be specific about the error.
 199   - Do not print messages to stdout on error.
 200   - Do not POSIX_ME_HARDER unless necessary, i.e. avoid "+" in option string.
 201
 202 - Do not write functions that clobber call-by-reference variables on
 203   failure. Use temporary variables for these cases and change the
 204   passed in variables only on success.
 205
 206 - When you allocate a file descriptor, it should be made O_CLOEXEC
 207   right from the beginning, as none of our files should leak to forked
 208   binaries by default. Hence, whenever you open a file, O_CLOEXEC must
 209   be specified, right from the beginning. This also applies to
 210   sockets. Effectively this means that all invocations to:
 211
 212   a) open() must get O_CLOEXEC passed
 213   b) socket() and socketpair() must get SOCK_CLOEXEC passed
 214   c) recvmsg() must get MSG_CMSG_CLOEXEC set
 215   d) F_DUPFD_CLOEXEC should be used instead of F_DUPFD, and so on
 216
 217 - We never use the POSIX version of basename() (which glibc defines it in
 218   libgen.h), only the GNU version (which glibc defines in string.h).
 219   The only reason to include libgen.h is because dirname()
 220   is needed. Everytime you need that please immediately undefine
 221   basename(), and add a comment about it, so that no code ever ends up
 222   using the POSIX version!
 223
 224 - Use the bool type for booleans, not integers. One exception: in public
 225   headers (i.e those in src/systemd/sd-*.h) use integers after all, as "bool"
 226   is C99 and in our public APIs we try to stick to C89 (with a few extension).
 227
 228 - When you invoke certain calls like unlink(), or mkdir_p() and you
 229   know it is safe to ignore the error it might return (because a later
 230   call would detect the failure anyway, or because the error is in an
 231   error path and you thus couldn't do anything about it anyway), then
 232   make this clear by casting the invocation explicitly to (void). Code
 233   checks like Coverity understand that, and will not complain about
 234   ignored error codes. Hence, please use this:
 235
 236       (void) unlink("/foo/bar/baz");
 237
 238   instead of just this:
 239
 240       unlink("/foo/bar/baz");
 241
 242 - Don't invoke exit(), ever. It is not replacement for proper error
 243   handling. Please escalate errors up your call chain, and use normal
 244   "return" to exit from the main function of a process. If you
 245   fork()ed off a child process, please use _exit() instead of exit(),
 246   so that the exit handlers are not run.
 247
 248 - Please never use dup(). Use fcntl(fd, F_DUPFD_CLOEXEC, 3)
 249   instead. For two reason: first, you want O_CLOEXEC set on the new fd
 250   (see above). Second, dup() will happily duplicate your fd as 0, 1,
 251   2, i.e. stdin, stdout, stderr, should those fds be closed. Given the
 252   special semantics of those fds, it's probably a good idea to avoid
 253   them. F_DUPFD_CLOEXEC with "3" as parameter avoids them.
 254
 255 - When you define a destructor or unref() call for an object, please
 256   accept a NULL object and simply treat this as NOP. This is similar
 257   to how libc free() works, which accepts NULL pointers and becomes a
 258   NOP for them. By following this scheme a lot of if checks can be
 259   removed before invoking your destructor, which makes the code
 260   substantially more readable and robust.
 261
 262 - Related to this: when you define a destructor or unref() call for an
 263   object, please make it return the same type it takes and always
 264   return NULL from it. This allows writing code like this:
 265
 266             p = foobar_unref(p);
 267
 268   which will always work regardless if p is initialized or not, and
 269   guarantees that p is NULL afterwards, all in just one line.
 270
 271 - Use alloca(), but never forget that it is not OK to invoke alloca()
 272   within a loop or within function call parameters. alloca() memory is
 273   released at the end of a function, and not at the end of a {}
 274   block. Thus, if you invoke it in a loop, you keep increasing the
 275   stack pointer without ever releasing memory again. (VLAs have better
 276   behaviour in this case, so consider using them as an alternative.)
 277   Regarding not using alloca() within function parameters, see the
 278   BUGS section of the alloca(3) man page.
 279
 280 - Use memzero() or even better zero() instead of memset(..., 0, ...)
 281
 282 - Instead of using memzero()/memset() to initialize structs allocated
 283   on the stack, please try to use c99 structure initializers. It's
 284   short, prettier and actually even faster at execution. Hence:
 285
 286           struct foobar t = {
 287                   .foo = 7,
 288                   .bar = "bazz",
 289           };
 290
 291   instead of:
 292
 293           struct foobar t;
 294           zero(t);
 295           t.foo = 7;
 296           t.bar = "bazz";
 297
 298 - When returning a return code from main(), please preferably use
 299   EXIT_FAILURE and EXIT_SUCCESS as defined by libc.
 300
 301 - The order in which header files are included doesn't matter too
 302   much. systemd-internal headers must not rely on an include order, so
 303   it is safe to include them in any order possible.
 304   However, to not clutter global includes, and to make sure internal
 305   definitions will not affect global headers, please always include the
 306   headers of external components first (these are all headers enclosed
 307   in <>), followed by our own exported headers (usually everything
 308   that's prefixed by "sd-"), and then followed by internal headers.
 309   Furthermore, in all three groups, order all includes alphabetically
 310   so duplicate includes can easily be detected.
 311
 312 - To implement an endless loop, use "for (;;)" rather than "while
 313   (1)". The latter is a bit ugly anyway, since you probably really
 314   meant "while (true)"... To avoid the discussion what the right
 315   always-true expression for an infinite while() loop is our
 316   recommendation is to simply write it without any such expression by
 317   using "for (;;)".
 318
 319 - Never use the "off_t" type, and particularly avoid it in public
 320   APIs. It's really weirdly defined, as it usually is 64bit and we
 321   don't support it any other way, but it could in theory also be
 322   32bit. Which one it is depends on a compiler switch chosen by the
 323   compiled program, which hence corrupts APIs using it unless they can
 324   also follow the program's choice. Moreover, in systemd we should
 325   parse values the same way on all architectures and cannot expose
 326   off_t values over D-Bus. To avoid any confusion regarding conversion
 327   and ABIs, always use simply uint64_t directly.
 328
 329 - Commit message subject lines should be prefixed with an appropriate
 330   component name of some kind. For example "journal: ", "nspawn: " and
 331   so on.
 332
 333 - Do not use "Signed-Off-By:" in your commit messages. That's a kernel
 334   thing we don't do in the systemd project.
 335
 336 - Avoid leaving long-running child processes around, i.e. fork()s that
 337   are not followed quickly by an execv() in the child. Resource
 338   management is unclear in this case, and memory CoW will result in
 339   unexpected penalties in the parent much much later on.
 340
 341 - Don't block execution for arbitrary amounts of time using usleep()
 342   or a similar call, unless you really know what you do. Just "giving
 343   something some time", or so is a lazy excuse. Always wait for the
 344   proper event, instead of doing time-based poll loops.
 345
 346 - To determine the length of a constant string "foo", don't bother
 347   with sizeof("foo")-1, please use strlen("foo") directly. gcc knows
 348   strlen() anyway and turns it into a constant expression if possible.
 349
 350 - If you want to concatenate two or more strings, consider using
 351   strjoin() rather than asprintf(), as the latter is a lot
 352   slower. This matters particularly in inner loops.
 353
 354 - Please avoid using global variables as much as you can. And if you
 355   do use them make sure they are static at least, instead of
 356   exported. Especially in library-like code it is important to avoid
 357   global variables. Why are global variables bad? They usually hinder
 358   generic reusability of code (since they break in threaded programs,
 359   and usually would require locking there), and as the code using them
 360   has side-effects make programs intransparent. That said, there are
 361   many cases where they explicitly make a lot of sense, and are OK to
 362   use. For example, the log level and target in log.c is stored in a
 363   global variable, and that's OK and probably expected by most. Also
 364   in many cases we cache data in global variables. If you add more
 365   caches like this, please be careful however, and think about
 366   threading. Only use static variables if you are sure that
 367   thread-safety doesn't matter in your case. Alternatively consider
 368   using TLS, which is pretty easy to use with gcc's "thread_local"
 369   concept. It's also OK to store data that is inherently global in
 370   global variables, for example data parsed from command lines, see
 371   below.
 372
 373 - If you parse a command line, and want to store the parsed parameters
 374   in global variables, please consider prefixing their names with
 375   "arg_". We have been following this naming rule in most of our
 376   tools, and we should continue to do so, as it makes it easy to
 377   identify command line parameter variables, and makes it clear why it
 378   is OK that they are global variables.