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- 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".
- 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.
- 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 XDG version of basename(). glibc defines it in
libgen.h. The only reason to include that file 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!
- 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.