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379 lines
16 KiB
Plaintext
379 lines
16 KiB
Plaintext
- 8ch indent, no tabs, except for files in man/ which are 2ch indent,
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and still no tabs
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- We prefer /* comments */ over // comments, please. This is not C++, after
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all. (Yes we know that C99 supports both kinds of comments, but still,
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please!)
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- Don't break code lines too eagerly. We do *not* force line breaks at
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80ch, all of today's screens should be much larger than that. But
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then again, don't overdo it, ~119ch should be enough really.
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- Variables and functions *must* be static, unless they have a
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prototype, and are supposed to be exported.
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- structs in MixedCase (with exceptions, such as public API structs),
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variables + functions in lower_case.
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- The destructors always unregister the object from the next bigger
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object, not the other way around
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- To minimize strict aliasing violations, we prefer unions over casting
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- For robustness reasons, destructors should be able to destruct
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half-initialized objects, too
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- Error codes are returned as negative Exxx. e.g. return -EINVAL. There
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are some exceptions: for constructors, it is OK to return NULL on
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OOM. For lookup functions, NULL is fine too for "not found".
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Be strict with this. When you write a function that can fail due to
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more than one cause, it *really* should have "int" as return value
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for the error code.
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- Do not bother with error checking whether writing to stdout/stderr
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worked.
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- Do not log errors from "library" code, only do so from "main
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program" code. (With one exception: it is OK to log with DEBUG level
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from any code, with the exception of maybe inner loops).
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- Always check OOM. There is no excuse. In program code, you can use
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"log_oom()" for then printing a short message, but not in "library" code.
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- Do not issue NSS requests (that includes user name and host name
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lookups) from PID 1 as this might trigger deadlocks when those
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lookups involve synchronously talking to services that we would need
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to start up
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- Do not synchronously talk to any other service from PID 1, due to
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risk of deadlocks
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- Avoid fixed-size string buffers, unless you really know the maximum
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size and that maximum size is small. They are a source of errors,
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since they possibly result in truncated strings. It is often nicer
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to use dynamic memory, alloca() or VLAs. If you do allocate fixed-size
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strings on the stack, then it is probably only OK if you either
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use a maximum size such as LINE_MAX, or count in detail the maximum
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size a string can have. (DECIMAL_STR_MAX and DECIMAL_STR_WIDTH
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macros are your friends for this!)
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Or in other words, if you use "char buf[256]" then you are likely
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doing something wrong!
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- Stay uniform. For example, always use "usec_t" for time
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values. Do not mix usec and msec, and usec and whatnot.
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- Make use of _cleanup_free_ and friends. It makes your code much
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nicer to read!
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- Be exceptionally careful when formatting and parsing floating point
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numbers. Their syntax is locale dependent (i.e. "5.000" in en_US is
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generally understood as 5, while on de_DE as 5000.).
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- Try to use this:
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void foo() {
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}
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instead of this:
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void foo()
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{
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}
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But it is OK if you do not.
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- Single-line "if" blocks should not be enclosed in {}. Use this:
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if (foobar)
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waldo();
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instead of this:
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if (foobar) {
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waldo();
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}
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- Do not write "foo ()", write "foo()".
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- Please use streq() and strneq() instead of strcmp(), strncmp() where applicable.
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- Please do not allocate variables on the stack in the middle of code,
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even if C99 allows it. Wrong:
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{
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a = 5;
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int b;
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b = a;
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}
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Right:
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{
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int b;
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a = 5;
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b = a;
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}
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- Unless you allocate an array, "double" is always the better choice
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than "float". Processors speak "double" natively anyway, so this is
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no speed benefit, and on calls like printf() "float"s get promoted
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to "double"s anyway, so there is no point.
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- Do not mix function invocations with variable definitions in one
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line. Wrong:
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{
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int a = foobar();
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uint64_t x = 7;
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}
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Right:
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{
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int a;
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uint64_t x = 7;
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a = foobar();
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}
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- Use "goto" for cleaning up, and only use it for that. i.e. you may
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only jump to the end of a function, and little else. Never jump
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backwards!
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- Think about the types you use. If a value cannot sensibly be
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negative, do not use "int", but use "unsigned".
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- Use "char" only for actual characters. Use "uint8_t" or "int8_t"
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when you actually mean a byte-sized signed or unsigned
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integers. When referring to a generic byte, we generally prefer the
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unsigned variant "uint8_t". Do not use types based on "short". They
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*never* make sense. Use ints, longs, long longs, all in
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unsigned+signed fashion, and the fixed size types
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uint8_t/uint16_t/uint32_t/uint64_t/int8_t/int16_t/int32_t and so on,
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as well as size_t, but nothing else. Do not use kernel types like
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u32 and so on, leave that to the kernel.
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- Public API calls (i.e. functions exported by our shared libraries)
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must be marked "_public_" and need to be prefixed with "sd_". No
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other functions should be prefixed like that.
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- In public API calls, you *must* validate all your input arguments for
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programming error with assert_return() and return a sensible return
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code. In all other calls, it is recommended to check for programming
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errors with a more brutal assert(). We are more forgiving to public
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users than for ourselves! Note that assert() and assert_return()
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really only should be used for detecting programming errors, not for
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runtime errors. assert() and assert_return() by usage of _likely_()
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inform the compiler that he should not expect these checks to fail,
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and they inform fellow programmers about the expected validity and
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range of parameters.
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- Never use strtol(), atoi() and similar calls. Use safe_atoli(),
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safe_atou32() and suchlike instead. They are much nicer to use in
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most cases and correctly check for parsing errors.
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- For every function you add, think about whether it is a "logging"
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function or a "non-logging" function. "Logging" functions do logging
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on their own, "non-logging" function never log on their own and
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expect their callers to log. All functions in "library" code,
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i.e. in src/shared/ and suchlike must be "non-logging". Every time a
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"logging" function calls a "non-logging" function, it should log
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about the resulting errors. If a "logging" function calls another
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"logging" function, then it should not generate log messages, so
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that log messages are not generated twice for the same errors.
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- Avoid static variables, except for caches and very few other
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cases. Think about thread-safety! While most of our code is never
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used in threaded environments, at least the library code should make
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sure it works correctly in them. Instead of doing a lot of locking
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for that, we tend to prefer using TLS to do per-thread caching (which
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only works for small, fixed-size cache objects), or we disable
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caching for any thread that is not the main thread. Use
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is_main_thread() to detect whether the calling thread is the main
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thread.
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- Command line option parsing:
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- Do not print full help() on error, be specific about the error.
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- Do not print messages to stdout on error.
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- Do not POSIX_ME_HARDER unless necessary, i.e. avoid "+" in option string.
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- Do not write functions that clobber call-by-reference variables on
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failure. Use temporary variables for these cases and change the
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passed in variables only on success.
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- When you allocate a file descriptor, it should be made O_CLOEXEC
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right from the beginning, as none of our files should leak to forked
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binaries by default. Hence, whenever you open a file, O_CLOEXEC must
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be specified, right from the beginning. This also applies to
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sockets. Effectively this means that all invocations to:
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a) open() must get O_CLOEXEC passed
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b) socket() and socketpair() must get SOCK_CLOEXEC passed
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c) recvmsg() must get MSG_CMSG_CLOEXEC set
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d) F_DUPFD_CLOEXEC should be used instead of F_DUPFD, and so on
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- We never use the POSIX version of basename() (which glibc defines it in
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libgen.h), only the GNU version (which glibc defines in string.h).
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The only reason to include libgen.h is because dirname()
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is needed. Everytime you need that please immediately undefine
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basename(), and add a comment about it, so that no code ever ends up
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using the POSIX version!
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- Use the bool type for booleans, not integers. One exception: in public
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headers (i.e those in src/systemd/sd-*.h) use integers after all, as "bool"
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is C99 and in our public APIs we try to stick to C89 (with a few extension).
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- When you invoke certain calls like unlink(), or mkdir_p() and you
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know it is safe to ignore the error it might return (because a later
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call would detect the failure anyway, or because the error is in an
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error path and you thus couldn't do anything about it anyway), then
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make this clear by casting the invocation explicitly to (void). Code
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checks like Coverity understand that, and will not complain about
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ignored error codes. Hence, please use this:
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(void) unlink("/foo/bar/baz");
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instead of just this:
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unlink("/foo/bar/baz");
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- Don't invoke exit(), ever. It is not replacement for proper error
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handling. Please escalate errors up your call chain, and use normal
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"return" to exit from the main function of a process. If you
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fork()ed off a child process, please use _exit() instead of exit(),
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so that the exit handlers are not run.
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- Please never use dup(). Use fcntl(fd, F_DUPFD_CLOEXEC, 3)
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instead. For two reason: first, you want O_CLOEXEC set on the new fd
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(see above). Second, dup() will happily duplicate your fd as 0, 1,
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2, i.e. stdin, stdout, stderr, should those fds be closed. Given the
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special semantics of those fds, it's probably a good idea to avoid
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them. F_DUPFD_CLOEXEC with "3" as parameter avoids them.
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- When you define a destructor or unref() call for an object, please
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accept a NULL object and simply treat this as NOP. This is similar
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to how libc free() works, which accepts NULL pointers and becomes a
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NOP for them. By following this scheme a lot of if checks can be
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removed before invoking your destructor, which makes the code
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substantially more readable and robust.
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- Related to this: when you define a destructor or unref() call for an
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object, please make it return the same type it takes and always
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return NULL from it. This allows writing code like this:
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p = foobar_unref(p);
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which will always work regardless if p is initialized or not, and
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guarantees that p is NULL afterwards, all in just one line.
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- Use alloca(), but never forget that it is not OK to invoke alloca()
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within a loop or within function call parameters. alloca() memory is
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released at the end of a function, and not at the end of a {}
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block. Thus, if you invoke it in a loop, you keep increasing the
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stack pointer without ever releasing memory again. (VLAs have better
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behaviour in this case, so consider using them as an alternative.)
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Regarding not using alloca() within function parameters, see the
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BUGS section of the alloca(3) man page.
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- Use memzero() or even better zero() instead of memset(..., 0, ...)
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- Instead of using memzero()/memset() to initialize structs allocated
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on the stack, please try to use c99 structure initializers. It's
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short, prettier and actually even faster at execution. Hence:
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struct foobar t = {
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.foo = 7,
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.bar = "bazz",
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};
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instead of:
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struct foobar t;
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zero(t);
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t.foo = 7;
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t.bar = "bazz";
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- When returning a return code from main(), please preferably use
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EXIT_FAILURE and EXIT_SUCCESS as defined by libc.
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- The order in which header files are included doesn't matter too
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much. systemd-internal headers must not rely on an include order, so
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it is safe to include them in any order possible.
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However, to not clutter global includes, and to make sure internal
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definitions will not affect global headers, please always include the
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headers of external components first (these are all headers enclosed
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in <>), followed by our own exported headers (usually everything
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that's prefixed by "sd-"), and then followed by internal headers.
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Furthermore, in all three groups, order all includes alphabetically
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so duplicate includes can easily be detected.
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- To implement an endless loop, use "for (;;)" rather than "while
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(1)". The latter is a bit ugly anyway, since you probably really
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meant "while (true)"... To avoid the discussion what the right
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always-true expression for an infinite while() loop is our
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recommendation is to simply write it without any such expression by
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using "for (;;)".
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- Never use the "off_t" type, and particularly avoid it in public
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APIs. It's really weirdly defined, as it usually is 64bit and we
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don't support it any other way, but it could in theory also be
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32bit. Which one it is depends on a compiler switch chosen by the
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compiled program, which hence corrupts APIs using it unless they can
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also follow the program's choice. Moreover, in systemd we should
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parse values the same way on all architectures and cannot expose
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off_t values over D-Bus. To avoid any confusion regarding conversion
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and ABIs, always use simply uint64_t directly.
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- Commit message subject lines should be prefixed with an appropriate
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component name of some kind. For example "journal: ", "nspawn: " and
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so on.
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- Do not use "Signed-Off-By:" in your commit messages. That's a kernel
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thing we don't do in the systemd project.
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- Avoid leaving long-running child processes around, i.e. fork()s that
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are not followed quickly by an execv() in the child. Resource
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management is unclear in this case, and memory CoW will result in
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unexpected penalties in the parent much much later on.
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- Don't block execution for arbitrary amounts of time using usleep()
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or a similar call, unless you really know what you do. Just "giving
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something some time", or so is a lazy excuse. Always wait for the
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proper event, instead of doing time-based poll loops.
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- To determine the length of a constant string "foo", don't bother
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with sizeof("foo")-1, please use strlen("foo") directly. gcc knows
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strlen() anyway and turns it into a constant expression if possible.
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- If you want to concatenate two or more strings, consider using
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strjoin() rather than asprintf(), as the latter is a lot
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slower. This matters particularly in inner loops.
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- Please avoid using global variables as much as you can. And if you
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do use them make sure they are static at least, instead of
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exported. Especially in library-like code it is important to avoid
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global variables. Why are global variables bad? They usually hinder
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generic reusability of code (since they break in threaded programs,
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and usually would require locking there), and as the code using them
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has side-effects make programs intransparent. That said, there are
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many cases where they explicitly make a lot of sense, and are OK to
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use. For example, the log level and target in log.c is stored in a
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global variable, and that's OK and probably expected by most. Also
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in many cases we cache data in global variables. If you add more
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caches like this, please be careful however, and think about
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threading. Only use static variables if you are sure that
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thread-safety doesn't matter in your case. Alternatively consider
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using TLS, which is pretty easy to use with gcc's "thread_local"
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concept. It's also OK to store data that is inherently global in
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global variables, for example data parsed from command lines, see
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below.
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- If you parse a command line, and want to store the parsed parameters
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in global variables, please consider prefixing their names with
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"arg_". We have been following this naming rule in most of our
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tools, and we should continue to do so, as it makes it easy to
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identify command line parameter variables, and makes it clear why it
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is OK that they are global variables.
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