X-Git-Url: http://git.ipfire.org/?a=blobdiff_plain;f=docs%2FCODING_STYLE.md;h=d945f8cdbe7f3fb3d59c1e29ed2d3f9586216520;hb=71ec74d19332378dca8a6a7eb30411e08978f9a6;hp=7bad3f5d2ecd5b7a66a8a069d19348f4263535b8;hpb=88a56c767044b06529dd80180fb492d65aae121e;p=thirdparty%2Fsystemd.git diff --git a/docs/CODING_STYLE.md b/docs/CODING_STYLE.md index 7bad3f5d2ec..d945f8cdbe7 100644 --- a/docs/CODING_STYLE.md +++ b/docs/CODING_STYLE.md @@ -4,84 +4,26 @@ title: Coding Style # Coding Style -- 8ch indent, no tabs, except for files in `man/` which are 2ch indent, - and still no tabs. +## Formatting -- We prefer `/* comments */` over `// comments` in code you commit, please. This - way `// comments` are left for developers to use for local, temporary - commenting of code for debug purposes (i.e. uncommittable stuff), making such - comments easily discernible from explanatory, documenting code comments - (i.e. committable stuff). +- 8ch indent, no tabs, except for files in `man/` which are 2ch indent, and + still no tabs, and shell scripts, which are 4ch indent, and no tabs either. + +- We prefer `/* comments */` over `// comments` in code you commit, + please. This way `// comments` are left for developers to use for local, + temporary commenting of code for debug purposes (i.e. uncommittable stuff), + making such comments easily discernible from explanatory, documenting code + comments (i.e. committable stuff). - 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, ~109ch should be enough really. The `.editorconfig`, `.vimrc` and `.dir-locals.el` files contained in the repository will set this limit up for - you automatically, if you let them (as well as a few other things). - -- Variables and functions **must** be static, unless they have a - prototype, and are supposed to be exported. - -- structs in `PascalCase` (with exceptions, such as public API structs), - variables and functions in `snake_case`. - -- The destructors always deregister 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 an `int` as the 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. + you automatically, if you let them (as well as a few other things). Please + note that emacs loads `.dir-locals.el` automatically, but vim needs to be + configured to load `.vimrc`, see that file for instructions. -- 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 (and shorter)! - -- 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 in de_DE as 5000.). - -- Try to use this: +- Try to write this: ```c void foo() { @@ -96,9 +38,7 @@ title: Coding Style } ``` - But it is OK if you do not. - -- Single-line `if` blocks should not be enclosed in `{}`. Use this: +- Single-line `if` blocks should not be enclosed in `{}`. Write this: ```c if (foobar) @@ -115,11 +55,85 @@ title: Coding Style - Do not write `foo ()`, write `foo()`. -- Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where - applicable (i.e. wherever you just care about equality/inequality, not about - the sorting order). +## Code Organization and Semantics + +- Please name structures in `PascalCase` (with exceptions, such as public API + structs), variables and functions in `snake_case`. + +- 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. + +- 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. + +- 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. + +- 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 non-transparent. 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. + +- 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 is used to + implement the public shared object we provide (sd-bus, sd-login, sd-id128, + nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must + be located in `src/basic` (those objects are not allowed to link to + libsystemd-shared.so). Conversely, anything which is shared between multiple + components and does not need to be in `src/basic/`, should be in + `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/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under + `src/journal/` that end up in `libjournal-client.a` convenience library. + - 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. + +## Using C Constructs -- Preferably allocate stack variables on the top of the block: +- Preferably allocate local variables on the top of the block: ```c { @@ -130,13 +144,7 @@ title: Coding Style } ``` -- Unless you allocate an array, `double` is always a better choice - than `float`. Processors speak `double` natively anyway, so there 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: +- Do not mix function invocations with variable definitions in one line. Wrong: ```c { @@ -156,119 +164,101 @@ title: Coding Style } ``` -- 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 `int`, `long`, `long long`, all in - unsigned and 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. +- 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! -- 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. +- To minimize strict aliasing violations, we prefer unions over casting. -- 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 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, - and they inform fellow programmers about the expected validity and - range of parameters. +- 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: -- 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. + ```c + struct foobar t = { + .foo = 7, + .bar = "bazz", + }; + ``` -- If possible, do a combined log & return operation: + instead of: ```c - r = operation(...); - if (r < 0) - return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m"); + struct foobar t; + zero(t); + t.foo = 7; + t.bar = "bazz"; ``` - If the error value is "synthetic", i.e. it was not received from - the called function, use `SYNTHETIC_ERRNO` wrapper to tell the logging - system to not log the errno value, but still return it: +- 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 (;;)`. + +- To determine the length of a constant string `"foo"`, don't bother with + `sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize + the call away for fixed strings). The only exception is when declaring an + array. In that case use STRLEN, which evaluates to a static constant and + doesn't force the compiler to create a VLA. + +## Destructors + +- The destructors always deregister the object from the next bigger object, not + the other way around. + +- For robustness reasons, destructors should be able to destruct + half-initialized objects, too. + +- 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: ```c - n = read(..., s, sizeof s); - if (n != sizeof s) - return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ..."); + p = foobar_unref(p); ``` -- 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. + which will always work regardless if `p` is initialized or not,x and + guarantees that `p` is `NULL` afterwards, all in just one line. -- 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. +## Error Handling -- 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: +- 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". - - `open()` must get `O_CLOEXEC` passed, - - `socket()` and `socketpair()` must get `SOCK_CLOEXEC` passed, - - `recvmsg()` must get `MSG_CMSG_CLOEXEC` set, - - `F_DUPFD_CLOEXEC` should be used instead of `F_DUPFD`, and so on, - - invocations of `fopen()` should take `e`. + Be strict with this. When you write a function that can fail due to more than + one cause, it *really* should have an `int` as the return value for the error + code. -- 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. Every time you need that please immediately undefine - `basename()`, and add a comment about it, so that no code ever ends up - using the POSIX version! +- Do not bother with error checking whether writing to stdout/stderr worked. -- 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). +- 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). -- 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: +- 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 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, and they inform fellow programmers about the + expected validity and range of parameters. + +- 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: ```c (void) unlink("/foo/bar/baz"); @@ -281,231 +271,208 @@ title: Coding Style ``` 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 - `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 `fd`s be closed. Given the - special semantics of those `fd`s, it's probably a good idea to avoid - them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them. + 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. -- 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: - - ```c - 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. +- When returning a return code from `main()`, please preferably use + `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc. -- 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 - behavior 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. +## Logging -- Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)` +- 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. -- 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: +- If possible, do a combined log & return operation: ```c - struct foobar t = { - .foo = 7, - .bar = "bazz", - }; + r = operation(...); + if (r < 0) + return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m"); ``` - instead of: + If the error value is "synthetic", i.e. it was not received from + the called function, use `SYNTHETIC_ERRNO` wrapper to tell the logging + system to not log the errno value, but still return it: ```c - struct foobar t; - zero(t); - t.foo = 7; - t.bar = "bazz"; + n = read(..., s, sizeof s); + if (n != sizeof s) + return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ..."); ``` -- 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. +## Memory Allocation -- 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 64-bit and we - don't support it any other way, but it could in theory also be - 32-bit. 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. +- 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 use "Signed-Off-By:" in your commit messages. That's a kernel - thing we don't do in the systemd project. +- 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!) -- 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. + Or in other words, if you use `char buf[256]` then you are likely doing + something wrong! -- 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. +- Make use of `_cleanup_free_` and friends. It makes your code much nicer to + read (and shorter)! -- To determine the length of a constant string `"foo"`, don't bother with - `sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize - the call away for fixed strings). The only exception is when declaring an - array. In that case use STRLEN, which evaluates to a static constant and - doesn't force the compiler to create a VLA. +- 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 behavior 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. - If you want to concatenate two or more strings, consider using `strjoina()` or `strjoin()` rather than `asprintf()`, as the latter is a lot slower. This matters particularly in inner loops (but note that `strjoina()` cannot be used there). -- 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 non-transparent. 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. +## Runtime Behaviour -- 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. +- 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. -- 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 otherwise 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. +- 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. + +- 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). + +- When applying C-style unescaping as well as specifier expansion on the same + string, always apply the C-style unescaping fist, followed by the specifier + expansion. When doing the reverse, make sure to escape `%` in specifier-style + first (i.e. `%` → `%%`), and then do C-style escaping where necessary. + +- 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 in de_DE as 5000.). - 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 hard-coded (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 operate on `long`s and - `short`s 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. + limits after which it will refuse operation. It's fine if it is hard-coded + (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. + +## Types + +- 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 `int`, `long`, `long + long`, all in unsigned and 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. + +- Stay uniform. For example, always use `usec_t` for time values. Do not mix + `usec` and `msec`, and `usec` and whatnot. + +- Never use the `off_t` type, and particularly avoid it in public APIs. It's + really weirdly defined, as it usually is 64-bit and we don't support it any + other way, but it could in theory also be 32-bit. 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. -- 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 is used to - implement the public shared object we provide (sd-bus, sd-login, sd-id128, - nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must - be located in `src/basic` (those objects are not allowed to link to - libsystemd-shared.so). Conversely, anything which is shared between multiple - components and does not need to be in `src/basic/`, should be in - `src/shared/`. +- Unless you allocate an array, `double` is always a better choice than + `float`. Processors speak `double` natively anyway, so there is no speed + benefit, and on calls like `printf()` `float`s get promoted to `double`s + anyway, so there is no point. - To summarize: +- 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). - `src/basic/` - - may be used by all code in the tree - - may not use any code outside of `src/basic/` +## Deadlocks - `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/` +- 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. - `src/shared/` - - may be used by all code in the tree, except for code in `src/basic/`, - `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under - `src/journal/` that end up in `libjournal-client.a` convenience library. - - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/` +- Do not synchronously talk to any other service from PID 1, due to risk of + deadlocks. -- 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. +## File Descriptors -- 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). +- 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: -- When applying C-style unescaping as well as specifier expansion on the same - string, always apply the C-style unescaping fist, followed by the specifier - expansion. When doing the reverse, make sure to escape `%` in specifier-style - first (i.e. `%` → `%%`), and then do C-style escaping where necessary. + - `open()` must get `O_CLOEXEC` passed, + - `socket()` and `socketpair()` must get `SOCK_CLOEXEC` passed, + - `recvmsg()` must get `MSG_CMSG_CLOEXEC` set, + - `F_DUPFD_CLOEXEC` should be used instead of `F_DUPFD`, and so on, + - invocations of `fopen()` should take `e`. -- It's a good idea to use `O_NONBLOCK` when opening 'foreign' regular files, i.e. - file system objects that are supposed to be regular files whose paths where - specified by the user and hence might actually refer to other types of file - system objects. This is a good idea so that we don't end up blocking on +- It's a good idea to use `O_NONBLOCK` when opening 'foreign' regular files, + i.e. file system objects that are supposed to be regular files whose paths + where specified by the user and hence might actually refer to other types of + file system objects. This is a good idea so that we don't end up blocking on 'strange' file nodes, for example if the user pointed us to a FIFO or device node which may block when opening. Moreover even for actual regular files `O_NONBLOCK` has a benefit: it bypasses any mandatory lock that might be in - effect on the regular file. If in doubt consider turning off `O_NONBLOCK` again - after opening. + effect on the regular file. If in doubt consider turning off `O_NONBLOCK` + again after opening. + +## Command Line + +- 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. + +- 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. + +## Exporting Symbols + +- Variables and functions **must** be static, unless they have a prototype, and + are supposed to be exported. + +- 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. + +- 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 otherwise 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. + +## Referencing Concepts - When referring to a configuration file option in the documentation and such, please always suffix it with `=`, to indicate that it is a configuration file @@ -519,6 +486,52 @@ title: Coding Style suffix it with `/`, to indicate that it is a directory, not a regular file (or other file system object). +## Functions to Avoid + +- Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)` + +- Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where + applicable (i.e. wherever you just care about equality/inequality, not about + the sorting order). + +- 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. + +- `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 operate on + `long`s and `short`s 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. + +- 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 `fd`s be closed. Given the special + semantics of those `fd`s, it's probably a good idea to avoid + them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them. + - Don't use `fgets()`, it's too hard to properly handle errors such as overly long lines. Use `read_line()` instead, which is our own function that handles this much nicer. + +- 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. + +- 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. Every + time you need that please immediately undefine `basename()`, and add a + comment about it, so that no code ever ends up using the POSIX version! + +# Committing to git + +- 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.