- 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 mix function invocations with variable definitions in one line. 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". - 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 ints, longs, long longs, all in unsigned+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. - 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 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. Everytime you need that please immediately undefine basename(), and add a comment about it, so that no code ever ends up using the POSIX 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. - 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 behaviour 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. - Use memzero() or even better zero() instead of memset(..., 0, ...) - 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: struct foobar t = { .foo = 7, .bar = "bazz", }; instead of: struct foobar t; zero(t); t.foo = 7; t.bar = "bazz"; - When returning a return code from main(), please preferably use EXIT_FAILURE and EXIT_SUCCESS as defined by libc. - 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 (;;)". - Never use the "off_t" type, and particularly avoid it in public APIs. It's really weirdly defined, as it usually is 64bit and we don't support it any other way, but it could in theory also be 32bit. 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. - 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. - 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("foo") directly. gcc knows strlen() anyway and turns it into a constant expression if possible. - If you want to concatenate two or more strings, consider using strjoin() rather than asprintf(), as the latter is a lot slower. This matters particularly in inner loops. - 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 intransparent. 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. - 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.