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diff --git a/docs/CODING_STYLE.md b/docs/CODING_STYLE.md new file mode 100644 index 0000000..6d6e549 --- /dev/null +++ b/docs/CODING_STYLE.md @@ -0,0 +1,782 @@ +--- +title: Coding Style +category: Contributing +layout: default +SPDX-License-Identifier: LGPL-2.1-or-later +--- + +# Coding Style + +## Formatting + +- 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). Please + note that emacs loads `.dir-locals.el` automatically, but vim needs to be + configured to load `.vimrc`, see that file for instructions. + +- If you break a function declaration over multiple lines, do it like this: + + ```c + void some_function( + int foo, + bool bar, + char baz) { + + int a, b, c; + ``` + + (i.e. use double indentation — 16 spaces — for the parameter list.) + +- Try to write this: + + ```c + void foo() { + } + ``` + + instead of this: + + ```c + void foo() + { + } + ``` + +- Single-line `if` blocks should not be enclosed in `{}`. Write this: + + ```c + if (foobar) + waldo(); + ``` + + instead of this: + + ```c + if (foobar) { + waldo(); + } + ``` + +- Do not write `foo ()`, write `foo()`. + +- `else` blocks should generally start on the same line as the closing `}`: + ```c + if (foobar) { + find(); + waldo(); + } else + dont_find_waldo(); + ``` + +- Please define flags types like this: + + ```c + typedef enum FoobarFlags { + FOOBAR_QUUX = 1 << 0, + FOOBAR_WALDO = 1 << 1, + FOOBAR_XOXO = 1 << 2, + … + } FoobarFlags; + ``` + + i.e. use an enum for it, if possible. Indicate bit values via `1 <<` + expressions, and align them vertically. Define both an enum and a type for + it. + +- If you define (non-flags) enums, follow this template: + + ```c + typedef enum FoobarMode { + FOOBAR_AAA, + FOOBAR_BBB, + FOOBAR_CCC, + … + _FOOBAR_MAX, + _FOOBAR_INVALID = -EINVAL, + } FoobarMode; + ``` + + i.e. define a `_MAX` enum for the largest defined enum value, plus one. Since + this is not a regular enum value, prefix it with `_`. Also, define a special + "invalid" enum value, and set it to `-EINVAL`. That way the enum type can + safely be used to propagate conversion errors. + +- If you define an enum in a public API, be extra careful, as the size of the + enum might change when new values are added, which would break ABI + compatibility. Since we typically want to allow adding new enum values to an + existing enum type with later API versions, please use the + `_SD_ENUM_FORCE_S64()` macro in the enum definition, which forces the size of + the enum to be signed 64-bit wide. + +- Empty lines to separate code blocks are a good thing, please add them + abundantly. However, please stick to one at a time, i.e. multiple empty lines + immediately following each other are not OK. Also, we try to keep function + calls and their immediate error handling together. Hence: + + ```c + /* → empty line here is good */ + r = some_function(…); + /* → empty line here would be bad */ + if (r < 0) + return log_error_errno(r, "Some function failed: %m"); + /* → empty line here is good */ + ``` + +- In shell scripts, do not use whitespace after the redirection operator + (`>some/file` instead of `> some/file`, `<<EOF` instead of `<< EOF`). + +## Code Organization and Semantics + +- For our codebase we intend to use ISO C11 *with* GNU extensions (aka + "gnu11"). Public APIs (i.e. those we expose via `libsystemd.so` + i.e. `systemd/sd-*.h`) should only use ISO C89 however (with a very limited + set of conservative and common extensions, such as fixed size integer types + from `<inttypes.h>`), so that we don't force consuming programs into C11 + mode. (This discrepancy in particular means one thing: internally we use C99 + `bool` booleans, externally C89-compatible `int` booleans which generally + have different size in memory and slightly different semantics, also see + below.) Both for internal and external code it's OK to use even newer + features and GCC extension than "gnu11", as long as there's reasonable + fallback #ifdeffery in place to ensure compatibility is retained with older + compilers. + +- 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 rule is: never clobber return parameters on + failure, always initialize return parameters on success. + +- Typically, function parameters fit into three categories: input parameters, + mutable objects, and call-by-reference return parameters. Input parameters + should always carry suitable "const" declarators if they are pointers, to + indicate they are input-only and not changed by the function. Return + parameters are best prefixed with "ret_", to clarify they are return + parameters. (Conversely, please do not prefix parameters that aren't + output-only with "ret_", in particular not mutable parameters that are both + input as well as output). Example: + + ```c + static int foobar_frobnicate( + Foobar* object, /* the associated mutable object */ + const char *input, /* immutable input parameter */ + char **ret_frobnicated) { /* return parameter */ + … + return 0; + } + ``` + +- 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. + +- 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 + +- Allocate local variables where it makes sense: at the top of the block, or at + the point where they can be initialized. Avoid huge variable declaration + lists at the top of the function. + + As an exception, `int r` is typically used for a local state variable, but + should almost always be declared as the last variable at the top of the + function. + + ```c + { + uint64_t a; + int r; + + r = frobnicate(&a); + if (r < 0) + … + + uint64_t b = a + 1, c; + + r = foobarify(a, b, &c); + if (r < 0) + … + + const char *pretty = prettify(a, b, c); + … + } + ``` + +- Do not mix multiple variable definitions with function invocations or + complicated expressions: + + ```c + { + uint64_t x = 7; + int a; + + a = foobar(); + } + ``` + + instead of: + + ```c + { + int a = foobar(); + uint64_t x = 7; + } + ``` + +- 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! + +- To minimize strict aliasing violations, we prefer unions over casting. + +- 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: + + ```c + struct foobar t = { + .foo = 7, + .bar = "bazz", + }; + ``` + + instead of: + + ```c + struct foobar t; + zero(t); + t.foo = 7; + t.bar = "bazz"; + ``` + +- 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. + +- Please use C's downgrade-to-bool feature only for expressions that are + actually booleans (or "boolean-like"), and not for variables that are really + numeric. Specifically, if you have an `int b` and it's only used in a boolean + sense, by all means check its state with `if (b) …` — but if `b` can actually + have more than two semantic values, and you want to compare for non-zero, + then please write that explicitly with `if (b != 0) …`. This helps readability + as the value range and semantical behaviour is directly clear from the + condition check. As a special addition: when dealing with pointers which you + want to check for non-NULL-ness, you may also use downgrade-to-bool feature. + +- Please do not use yoda comparisons, i.e. please prefer the more readable `if + (a == 7)` over the less readable `if (7 == a)`. + +## 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 + 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. + +## Common Function Naming + +- Name destructor functions that destroy an object in full freeing all its + memory and associated resources (and thus invalidating the pointer to it) + `xyz_free()`. Example: `strv_free()`. + +- Name destructor functions that destroy only the referenced content of an + object but leave the object itself allocated `xyz_done()`. If it resets all + fields so that the object can be reused later call it `xyz_clear()`. + +- Functions that decrease the reference counter of an object by one should be + called `xyz_unref()`. Example: `json_variant_unref()`. Functions that + increase the reference counter by one should be called `xyz_ref()`. Example: + `json_variant_ref()` + +## Error Handling + +- 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. + +- libc system calls typically return -1 on error (with the error code in + `errno`), and >= 0 on success. Use the RET_NERRNO() helper if you are looking + for a simple way to convert this libc style error returning into systemd + style error returning. e.g. + + ```c + … + r = RET_NERRNO(unlink(t)); + … + ``` + + or + + ```c + … + r = RET_NERRNO(open("/some/file", O_RDONLY|O_CLOEXEC)); + … + ``` + +- 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). + +- 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 it 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"); + ``` + + instead of just this: + + ```c + unlink("/foo/bar/baz"); + ``` + + When returning from a `void` function, you may also want to shorten the error + path boilerplate by returning a function invocation cast to `(void)` like so: + + ```c + if (condition_not_met) + return (void) log_tests_skipped("Cannot run ..."); + ``` + + 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. + +- When returning a return code from `main()`, please preferably use + `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc. + +## Logging + +- For every function you add, think about whether it is a "logging" function or + a "non-logging" function. "Logging" functions do (non-debug) logging on their + own, "non-logging" functions never log on their own (except at debug level) + 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. (Note that debug level logging — at syslog level + `LOG_DEBUG` — is not considered logging in this context, debug logging is + generally always fine and welcome.) + +- If possible, do a combined log & return operation: + + ```c + r = operation(...); + if (r < 0) + return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m"); + ``` + + 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 + n = read(..., s, sizeof s); + if (n != sizeof s) + return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ..."); + ``` + +## Memory Allocation + +- 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. + +- Avoid fixed-size string buffers, unless you really know the maximum size and + that maximum size is small. It is often nicer to use dynamic memory, + `alloca_safe()` 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! + +- Make use of `_cleanup_free_` and friends. It makes your code much nicer to + read (and shorter)! + +- Do not use `alloca()`, `strdupa()` or `strndupa()` directly. Use + `alloca_safe()`, `strdupa_safe()` or `strndupa_safe()` instead. (The + difference is that the latter include an assertion that the specified size is + below a safety threshold, so that the program rather aborts than runs into + possible stack overruns.) + +- Use `alloca_safe()`, but never forget that it is not OK to invoke + `alloca_safe()` within a loop or within function call + parameters. `alloca_safe()` 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_safe()` 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). + +## Runtime Behaviour + +- 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. + +- 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 first, 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 allocate. + +## Types + +- Think about the types you use. If a value cannot sensibly be negative, do not + use `int`, but use `unsigned`. We prefer `unsigned` form to `unsigned int`. + +- 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. + +- 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. + +- 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 extensions; + also see above). + +## Deadlocks + +- Do not issue NSS requests (that includes user name and hostname 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. + +## File Descriptors + +- 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: + + - `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 + were 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. + +- These days we generally prefer `openat()`-style file APIs, i.e. APIs that + accept a combination of file descriptor and path string, and where the path + (if not absolute) is considered relative to the specified file + descriptor. When implementing library calls in similar style, please make + sure to imply `AT_EMPTY_PATH` if an empty or `NULL` path argument is + specified (and convert that latter to an empty string). This differs from the + underlying kernel semantics, where `AT_EMPTY_PATH` must always be specified + explicitly, and `NULL` is not accepted as path. + +## 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 + setting. + +- When referring to a command line option in the documentation and such, please + always prefix with `--` or `-` (as appropriate), to indicate that it is a + command line option. + +- When referring to a file system path that is a directory, please always + 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. + +- Use `typesafe_inet_ntop()`, `typesafe_inet_ntop4()`, and + `typesafe_inet_ntop6()` instead of `inet_ntop()`. But better yet, use the + `IN_ADDR_TO_STRING()`, `IN4_ADDR_TO_STRING()`, and `IN6_ADDR_TO_STRING()` + macros which allocate an anonymous buffer internally. + +- Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)` instead. For + two reasons: 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 more nicely. + +- 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. + +- Do not use `basename()` or `dirname()`. The semantics in corner cases are + full of pitfalls, and the fact that there are two quite different versions of + `basename()` (one POSIX and one GNU, of which the latter is much more useful) + doesn't make it better either. Use path_extract_filename() and + path_extract_directory() instead. + +- Never use `FILENAME_MAX`. Use `PATH_MAX` instead (for checking maximum size + of paths) and `NAME_MAX` (for checking maximum size of filenames). + `FILENAME_MAX` is not POSIX, and is a confusingly named alias for `PATH_MAX` + on Linux. Note that `NAME_MAX` does not include space for a trailing `NUL`, + but `PATH_MAX` does. UNIX FTW! + +## 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. + +## Commenting + +- The best place for code comments and explanations is in the code itself. Only + the second best is in git commit messages. The worst place is in the GitHub + PR cover letter. Hence, whenever you type a commit message consider for a + moment if what you are typing there wouldn't be a better fit for an in-code + comment. And if you type the cover letter of a PR, think hard if this + wouldn't be better as a commit message or even code comment. Comments are + supposed to be useful for somebody who reviews the code, and hence hiding + comments in git commits or PR cover letters makes reviews unnecessarily + hard. Moreover, while we rely heavily on GitHub's project management + infrastructure we'd like to keep everything that can reasonably be kept in + the git repository itself in the git repository, so that we can theoretically + move things elsewhere with the least effort possible. + +- It's OK to reference GitHub PRs, GitHub issues and git commits from code + comments. Cross-referencing code, issues, and documentation is a good thing. + +- Reasonable use of non-ASCII Unicode UTF-8 characters in code comments is + welcome. If your code comment contains an emoji or two this will certainly + brighten the day of the occasional reviewer of your code. Really! 😊 + +## Threading + +- We generally avoid using threads, to the level this is possible. In + particular in the service manager/PID 1 threads are not OK to use. This is + because you cannot mix memory allocation in threads with use of glibc's + `clone()` call, or manual `clone()`/`clone3()` system call wrappers. Only + glibc's own `fork()` call will properly synchronize the memory allocation + locks around the process clone operation. This means that if a process is + cloned via `clone()`/`clone3()` and another thread currently has the + `malloc()` lock taken, it will be cloned in locked state to the child, and + thus can never be acquired in the child, leading to deadlocks. Hence, when + using `clone()`/`clone3()` there are only two ways out: never use threads in the + parent, or never do memory allocation in the child. For our uses we need + `clone()`/`clone3()` and hence decided to avoid threads. Of course, sometimes the + concurrency threads allow is beneficial, however we suggest forking off + worker *processes* rather than worker *threads* for this purpose, ideally + even with an `execve()` to remove the CoW trap situation `fork()` easily + triggers. + +- A corollary of the above is: never use `clone()` where a `fork()` would do + too. Also consider using `posix_spawn()` which combines `clone()` + + `execve()` into one and has nice properties since it avoids becoming a CoW + trap by using `CLONE_VFORK` and `CLONE_VM` together. + +- While we avoid forking off threads on our own, writing thread-safe code is a + good idea where it might end up running inside of libsystemd.so or + similar. Hence, use TLS (i.e. `thread_local`) where appropriate, and maybe + the occasional `pthread_once()`. |