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, ~119ch 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 than 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   f) invocations of fopen() should take "e"
 217
 218 - We never use the POSIX version of basename() (which glibc defines it in
 219   libgen.h), only the GNU version (which glibc defines in string.h).
 220   The only reason to include libgen.h is because dirname()
 221   is needed. Every time you need that please immediately undefine
 222   basename(), and add a comment about it, so that no code ever ends up
 223   using the POSIX version!
 224
 225 - Use the bool type for booleans, not integers. One exception: in public
 226   headers (i.e those in src/systemd/sd-*.h) use integers after all, as "bool"
 227   is C99 and in our public APIs we try to stick to C89 (with a few extension).
 228
 229 - When you invoke certain calls like unlink(), or mkdir_p() and you
 230   know it is safe to ignore the error it might return (because a later
 231   call would detect the failure anyway, or because the error is in an
 232   error path and you thus couldn't do anything about it anyway), then
 233   make this clear by casting the invocation explicitly to (void). Code
 234   checks like Coverity understand that, and will not complain about
 235   ignored error codes. Hence, please use this:
 236
 237       (void) unlink("/foo/bar/baz");
 238
 239   instead of just this:
 240
 241       unlink("/foo/bar/baz");
 242
 243   Don't cast function calls to (void) that return no error
 244   conditions. Specifically, the various xyz_unref() calls that return a NULL
 245   object shouldn't be cast to (void), since not using the return value does not
 246   hide any errors.
 247
 248 - Don't invoke exit(), ever. It is not replacement for proper error
 249   handling. Please escalate errors up your call chain, and use normal
 250   "return" to exit from the main function of a process. If you
 251   fork()ed off a child process, please use _exit() instead of exit(),
 252   so that the exit handlers are not run.
 253
 254 - Please never use dup(). Use fcntl(fd, F_DUPFD_CLOEXEC, 3)
 255   instead. For two reason: first, you want O_CLOEXEC set on the new fd
 256   (see above). Second, dup() will happily duplicate your fd as 0, 1,
 257   2, i.e. stdin, stdout, stderr, should those fds be closed. Given the
 258   special semantics of those fds, it's probably a good idea to avoid
 259   them. F_DUPFD_CLOEXEC with "3" as parameter avoids them.
 260
 261 - When you define a destructor or unref() call for an object, please
 262   accept a NULL object and simply treat this as NOP. This is similar
 263   to how libc free() works, which accepts NULL pointers and becomes a
 264   NOP for them. By following this scheme a lot of if checks can be
 265   removed before invoking your destructor, which makes the code
 266   substantially more readable and robust.
 267
 268 - Related to this: when you define a destructor or unref() call for an
 269   object, please make it return the same type it takes and always
 270   return NULL from it. This allows writing code like this:
 271
 272             p = foobar_unref(p);
 273
 274   which will always work regardless if p is initialized or not, and
 275   guarantees that p is NULL afterwards, all in just one line.
 276
 277 - Use alloca(), but never forget that it is not OK to invoke alloca()
 278   within a loop or within function call parameters. alloca() memory is
 279   released at the end of a function, and not at the end of a {}
 280   block. Thus, if you invoke it in a loop, you keep increasing the
 281   stack pointer without ever releasing memory again. (VLAs have better
 282   behaviour in this case, so consider using them as an alternative.)
 283   Regarding not using alloca() within function parameters, see the
 284   BUGS section of the alloca(3) man page.
 285
 286 - Use memzero() or even better zero() instead of memset(..., 0, ...)
 287
 288 - Instead of using memzero()/memset() to initialize structs allocated
 289   on the stack, please try to use c99 structure initializers. It's
 290   short, prettier and actually even faster at execution. Hence:
 291
 292           struct foobar t = {
 293                   .foo = 7,
 294                   .bar = "bazz",
 295           };
 296
 297   instead of:
 298
 299           struct foobar t;
 300           zero(t);
 301           t.foo = 7;
 302           t.bar = "bazz";
 303
 304 - When returning a return code from main(), please preferably use
 305   EXIT_FAILURE and EXIT_SUCCESS as defined by libc.
 306
 307 - The order in which header files are included doesn't matter too
 308   much. systemd-internal headers must not rely on an include order, so
 309   it is safe to include them in any order possible.
 310   However, to not clutter global includes, and to make sure internal
 311   definitions will not affect global headers, please always include the
 312   headers of external components first (these are all headers enclosed
 313   in <>), followed by our own exported headers (usually everything
 314   that's prefixed by "sd-"), and then followed by internal headers.
 315   Furthermore, in all three groups, order all includes alphabetically
 316   so duplicate includes can easily be detected.
 317
 318 - To implement an endless loop, use "for (;;)" rather than "while
 319   (1)". The latter is a bit ugly anyway, since you probably really
 320   meant "while (true)"... To avoid the discussion what the right
 321   always-true expression for an infinite while() loop is our
 322   recommendation is to simply write it without any such expression by
 323   using "for (;;)".
 324
 325 - Never use the "off_t" type, and particularly avoid it in public
 326   APIs. It's really weirdly defined, as it usually is 64bit and we
 327   don't support it any other way, but it could in theory also be
 328   32bit. Which one it is depends on a compiler switch chosen by the
 329   compiled program, which hence corrupts APIs using it unless they can
 330   also follow the program's choice. Moreover, in systemd we should
 331   parse values the same way on all architectures and cannot expose
 332   off_t values over D-Bus. To avoid any confusion regarding conversion
 333   and ABIs, always use simply uint64_t directly.
 334
 335 - Commit message subject lines should be prefixed with an appropriate
 336   component name of some kind. For example "journal: ", "nspawn: " and
 337   so on.
 338
 339 - Do not use "Signed-Off-By:" in your commit messages. That's a kernel
 340   thing we don't do in the systemd project.
 341
 342 - Avoid leaving long-running child processes around, i.e. fork()s that
 343   are not followed quickly by an execv() in the child. Resource
 344   management is unclear in this case, and memory CoW will result in
 345   unexpected penalties in the parent much much later on.
 346
 347 - Don't block execution for arbitrary amounts of time using usleep()
 348   or a similar call, unless you really know what you do. Just "giving
 349   something some time", or so is a lazy excuse. Always wait for the
 350   proper event, instead of doing time-based poll loops.
 351
 352 - To determine the length of a constant string "foo", don't bother
 353   with sizeof("foo")-1, please use STRLEN() instead.
 354
 355 - If you want to concatenate two or more strings, consider using
 356   strjoin() rather than asprintf(), as the latter is a lot
 357   slower. This matters particularly in inner loops.
 358
 359 - Please avoid using global variables as much as you can. And if you
 360   do use them make sure they are static at least, instead of
 361   exported. Especially in library-like code it is important to avoid
 362   global variables. Why are global variables bad? They usually hinder
 363   generic reusability of code (since they break in threaded programs,
 364   and usually would require locking there), and as the code using them
 365   has side-effects make programs non-transparent. That said, there are
 366   many cases where they explicitly make a lot of sense, and are OK to
 367   use. For example, the log level and target in log.c is stored in a
 368   global variable, and that's OK and probably expected by most. Also
 369   in many cases we cache data in global variables. If you add more
 370   caches like this, please be careful however, and think about
 371   threading. Only use static variables if you are sure that
 372   thread-safety doesn't matter in your case. Alternatively consider
 373   using TLS, which is pretty easy to use with gcc's "thread_local"
 374   concept. It's also OK to store data that is inherently global in
 375   global variables, for example data parsed from command lines, see
 376   below.
 377
 378 - If you parse a command line, and want to store the parsed parameters
 379   in global variables, please consider prefixing their names with
 380   "arg_". We have been following this naming rule in most of our
 381   tools, and we should continue to do so, as it makes it easy to
 382   identify command line parameter variables, and makes it clear why it
 383   is OK that they are global variables.
 384
 385 - When exposing public C APIs, be careful what function parameters you make
 386   "const". For example, a parameter taking a context object should probably not
 387   be "const", even if you are writing an otherwise read-only accessor function
 388   for it. The reason is that making it "const" fixates the contract that your
 389   call won't alter the object ever, as part of the API. However, that's often
 390   quite a promise, given that this even prohibits object-internal caching or
 391   lazy initialization of object variables. Moreover it's usually not too useful
 392   for client applications. Hence: please be careful and avoid "const" on object
 393   parameters, unless you are very sure "const" is appropriate.
 394
 395 - Make sure to enforce limits on every user controllable resource. If the user
 396   can allocate resources in your code, your code must enforce some form of
 397   limits after which it will refuse operation. It's fine if it is hard-coded (at
 398   least initially), but it needs to be there. This is particularly important
 399   for objects that unprivileged users may allocate, but also matters for
 400   everything else any user may allocated.
 401
 402 - htonl()/ntohl() and htons()/ntohs() are weird. Please use htobe32() and
 403   htobe16() instead, it's much more descriptive, and actually says what really
 404   is happening, after all htonl() and htons() don't operate on longs and
 405   shorts as their name would suggest, but on uint32_t and uint16_t. Also,
 406   "network byte order" is just a weird name for "big endian", hence we might
 407   want to call it "big endian" right-away.
 408
 409 - You might wonder what kind of common code belongs in src/shared/ and what
 410   belongs in src/basic/. The split is like this: anything that uses public APIs
 411   we expose (i.e. any of the sd-bus, sd-login, sd-id128, ... APIs) must be
 412   located in src/shared/. All stuff that only uses external libraries from
 413   other projects (such as glibc's APIs), or APIs from src/basic/ itself should
 414   be placed in src/basic/. Conversely, src/libsystemd/ may only use symbols
 415   from src/basic, but not from src/shared/. To summarize:
 416
 417   src/basic/      → may be used by all code in the tree
 418                   → may not use any code outside of src/basic/
 419
 420   src/libsystemd/ → may be used by all code in the tree, except for code in src/basic/
 421                   → may not use any code outside of src/basic/, src/libsystemd/
 422
 423   src/shared/     → may be used by all code in the tree, except for code in src/basic/, src/libsystemd/
 424                   → may not use any code outside of src/basic/, src/libsystemd/, src/shared/
 425
 426 - Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are
 427   incompatible with glibc it's on them. However, if there are equivalent POSIX
 428   and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there
 429   aren't, we are happy to use GNU or Linux APIs, and expect non-GNU
 430   implementations of libc to catch up with glibc.
 431
 432 - Whenever installing a signal handler, make sure to set SA_RESTART for it, so
 433   that interrupted system calls are automatically restarted, and we minimize
 434   hassles with handling EINTR (in particular as EINTR handling is pretty broken
 435   on Linux).
 436
 437 - When applying C-style unescaping as well as specifier expansion on the same
 438   string, always apply the C-style unescaping fist, followed by the specifier
 439   expansion. When doing the reverse, make sure to escape '%' in specifier-style
 440   first (i.e. '%' → '%%'), and then do C-style escaping where necessary.