Adding upstream version 247.3.upstream/247.3upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+---
+title: Coding Style
+category: Contributing
+layout: default
+---
+
+# 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()`.
+
+## 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 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.
+
+- 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 local variables on the top of the block:
+
+  ```c
+  {
+          int a, b;
+
+          a = 5;
+          b = a;
+  }
+  ```
+
+- Do not mix function invocations with variable definitions in one line. Wrong:
+
+  ```c
+  {
+          int a = foobar();
+          uint64_t x = 7;
+  }
+  ```
+
+  Right:
+
+  ```c
+  {
+          int a;
+          uint64_t x = 7;
+
+          a = foobar();
+  }
+  ```
+
+- Use `goto` for cleaning up, and only use it for that. i.e. you may only jump
+  to the end of a function, and little else. Never jump backwards!
+
+- 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.
+
+## 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.
+
+- 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 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");
+  ```
+
+  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" function 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. 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!
+
+- Make use of `_cleanup_free_` and friends. It makes your code much nicer to
+  read (and shorter)!
+
+- 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).
+
+## 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 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.
+
+- 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 extension).
+
+## 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
+  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.
+
+## 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.
+
+- 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.
+
+- 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.