- 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.
+ then again, don't overdo it, ~119ch should be enough really.
- Variables and functions *must* be static, unless they have a
prototype, and are supposed to be exported.
- 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. Do not use kernel types like u32 and so on, leave that to the
- kernel.
+- 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
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()
+ users than 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,
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
+ f) invocations of fopen() should take "e"
-- We never use the XDG version of basename(). glibc defines it in
- libgen.h. The only reason to include that file is because dirname()
+- 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 XDG version!
+ 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"
unlink("/foo/bar/baz");
+ Don't cast function calls to (void) that return no error
+ conditions. Specifically, the various xyz_unref() calls that return a NULL
+ object shouldn't be cast to (void), since not using the return value does not
+ hide any errors.
+
- 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
- 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.
+
+- When exposing public C APIs, be careful what function parameters you make
+ "const". For example, a parameter taking a context object should probably not
+ be "const", even if you are writing an other-wise read-only accessor function
+ for it. The reason is that making it "const" fixates the contract that your
+ call won't alter the object ever, as part of the API. However, that's often
+ quite a promise, given that this even prohibits object-internal caching or
+ lazy initialization of object variables. Moreover it's usually not too useful
+ for client applications. Hence: please be careful and avoid "const" on object
+ parameters, unless you are very sure "const" is appropriate.
+
+- Make sure to enforce limits on every user controllable resource. If the user
+ can allocate resources in your code, your code must enforce some form of
+ limits after which it will refuse operation. It's fine if it is hardcoded (at
+ least initially), but it needs to be there. This is particularly important
+ for objects that unprivileged users may allocate, but also matters for
+ everything else any user may allocated.
+
+- htonl()/ntohl() and htons()/ntohs() are weird. Please use htobe32() and
+ htobe16() instead, it's much more descriptive, and actually says what really
+ is happening, after all htonl() and htons() don't operation on longs and
+ shorts as their name would suggest, but on uint32_t and uint16_t. Also,
+ "network byte order" is just a weird name for "big endian", hence we might
+ want to call it "big endian" right-away.
+
+- You might wonder what kind of common code belongs in src/shared/ and what
+ belongs in src/basic/. The split is like this: anything that uses public APIs
+ we expose (i.e. any of the sd-bus, sd-login, sd-id128, ... APIs) must be
+ located in src/shared/. All stuff that only uses external libraries from
+ other projects (such as glibc's APIs), or APIs from src/basic/ itself should
+ be placed in src/basic/. Conversely, src/libsystemd/ may only use symbols
+ from src/basic, but not from src/shared/. To summarize:
+
+ src/basic/ → may be used by all code in the tree
+ → may not use any code outside of src/basic/
+
+ src/libsystemd/ → may be used by all code in the tree, except for code in src/basic/
+ → may not use any code outside of src/basic/, src/libsystemd/
+
+ src/shared/ → may be used by all code in the tree, except for code in src/basic/, src/libsystemd/
+ → may not use any code outside of src/basic/, src/libsystemd/, src/shared/
+
+- Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are
+ incompatible with glibc it's on them. However, if there are equivalent POSIX
+ and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there
+ aren't, we are happy to use GNU or Linux APIs, and expect non-GNU
+ implementations of libc to catch up with glibc.
+
+- Whenever installing a signal handler, make sure to set SA_RESTART for it, so
+ that interrupted system calls are automatically restarted, and we minimize
+ hassles with handling EINTR (in particular as EINTR handling is pretty broken
+ on Linux).
~ianmdlvl / elogind.git / blobdiff