docs/CODING_STYLE.md

   1 ---
   2 title: Coding Style
   3 ---
   4
   5 # Coding Style
   6
   7 ## Formatting
   8
   9 - 8ch indent, no tabs, except for files in `man/` which are 2ch indent, and
  10   still no tabs, and shell scripts, which are 4ch indent, and no tabs either.
  11
  12 - We prefer `/* comments */` over `// comments` in code you commit,
  13   please. This way `// comments` are left for developers to use for local,
  14   temporary commenting of code for debug purposes (i.e. uncommittable stuff),
  15   making such comments easily discernible from explanatory, documenting code
  16   comments (i.e. committable stuff).
  17
  18 - Don't break code lines too eagerly. We do **not** force line breaks at 80ch,
  19   all of today's screens should be much larger than that. But then again, don't
  20   overdo it, ~109ch should be enough really. The `.editorconfig`, `.vimrc` and
  21   `.dir-locals.el` files contained in the repository will set this limit up for
  22   you automatically, if you let them (as well as a few other things). Please
  23   note that emacs loads `.dir-locals.el` automatically, but vim needs to be
  24   configured to load `.vimrc`, see that file for instructions.
  25
  26 - Try to write this:
  27
  28   ```c
  29   void foo() {
  30   }
  31   ```
  32
  33   instead of this:
  34
  35   ```c
  36   void foo()
  37   {
  38   }
  39   ```
  40
  41 - Single-line `if` blocks should not be enclosed in `{}`. Write this:
  42
  43   ```c
  44   if (foobar)
  45           waldo();
  46   ```
  47
  48   instead of this:
  49
  50   ```c
  51   if (foobar) {
  52           waldo();
  53   }
  54   ```
  55
  56 - Do not write `foo ()`, write `foo()`.
  57
  58 ## Code Organization and Semantics
  59
  60 - Please name structures in `PascalCase` (with exceptions, such as public API
  61   structs), variables and functions in `snake_case`.
  62
  63 - Avoid static variables, except for caches and very few other cases. Think
  64   about thread-safety! While most of our code is never used in threaded
  65   environments, at least the library code should make sure it works correctly
  66   in them. Instead of doing a lot of locking for that, we tend to prefer using
  67   TLS to do per-thread caching (which only works for small, fixed-size cache
  68   objects), or we disable caching for any thread that is not the main
  69   thread. Use `is_main_thread()` to detect whether the calling thread is the
  70   main thread.
  71
  72 - Do not write functions that clobber call-by-reference variables on
  73   failure. Use temporary variables for these cases and change the passed in
  74   variables only on success.
  75
  76 - The order in which header files are included doesn't matter too
  77   much. systemd-internal headers must not rely on an include order, so it is
  78   safe to include them in any order possible.  However, to not clutter global
  79   includes, and to make sure internal definitions will not affect global
  80   headers, please always include the headers of external components first
  81   (these are all headers enclosed in <>), followed by our own exported headers
  82   (usually everything that's prefixed by `sd-`), and then followed by internal
  83   headers.  Furthermore, in all three groups, order all includes alphabetically
  84   so duplicate includes can easily be detected.
  85
  86 - Please avoid using global variables as much as you can. And if you do use
  87   them make sure they are static at least, instead of exported. Especially in
  88   library-like code it is important to avoid global variables. Why are global
  89   variables bad? They usually hinder generic reusability of code (since they
  90   break in threaded programs, and usually would require locking there), and as
  91   the code using them has side-effects make programs non-transparent. That
  92   said, there are many cases where they explicitly make a lot of sense, and are
  93   OK to use. For example, the log level and target in `log.c` is stored in a
  94   global variable, and that's OK and probably expected by most. Also in many
  95   cases we cache data in global variables. If you add more caches like this,
  96   please be careful however, and think about threading. Only use static
  97   variables if you are sure that thread-safety doesn't matter in your
  98   case. Alternatively, consider using TLS, which is pretty easy to use with
  99   gcc's `thread_local` concept. It's also OK to store data that is inherently
 100   global in global variables, for example data parsed from command lines, see
 101   below.
 102
 103 - You might wonder what kind of common code belongs in `src/shared/` and what
 104   belongs in `src/basic/`. The split is like this: anything that is used to
 105   implement the public shared object we provide (sd-bus, sd-login, sd-id128,
 106   nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must
 107   be located in `src/basic` (those objects are not allowed to link to
 108   libsystemd-shared.so). Conversely, anything which is shared between multiple
 109   components and does not need to be in `src/basic/`, should be in
 110   `src/shared/`.
 111
 112   To summarize:
 113
 114   `src/basic/`
 115   - may be used by all code in the tree
 116   - may not use any code outside of `src/basic/`
 117
 118   `src/libsystemd/`
 119   - may be used by all code in the tree, except for code in `src/basic/`
 120   - may not use any code outside of `src/basic/`, `src/libsystemd/`
 121
 122   `src/shared/`
 123   - may be used by all code in the tree, except for code in `src/basic/`,
 124     `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
 125     `src/journal/` that end up in `libjournal-client.a` convenience library.
 126   - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/`
 127
 128 - Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are
 129   incompatible with glibc it's on them. However, if there are equivalent POSIX
 130   and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there
 131   aren't, we are happy to use GNU or Linux APIs, and expect non-GNU
 132   implementations of libc to catch up with glibc.
 133
 134 ## Using C Constructs
 135
 136 - Preferably allocate local variables on the top of the block:
 137
 138   ```c
 139   {
 140           int a, b;
 141
 142           a = 5;
 143           b = a;
 144   }
 145   ```
 146
 147 - Do not mix function invocations with variable definitions in one line. Wrong:
 148
 149   ```c
 150   {
 151           int a = foobar();
 152           uint64_t x = 7;
 153   }
 154   ```
 155
 156   Right:
 157
 158   ```c
 159   {
 160           int a;
 161           uint64_t x = 7;
 162
 163           a = foobar();
 164   }
 165   ```
 166
 167 - Use `goto` for cleaning up, and only use it for that. i.e. you may only jump
 168   to the end of a function, and little else. Never jump backwards!
 169
 170 - To minimize strict aliasing violations, we prefer unions over casting.
 171
 172 - Instead of using `memzero()`/`memset()` to initialize structs allocated on
 173   the stack, please try to use c99 structure initializers. It's short, prettier
 174   and actually even faster at execution. Hence:
 175
 176   ```c
 177   struct foobar t = {
 178           .foo = 7,
 179           .bar = "bazz",
 180   };
 181   ```
 182
 183   instead of:
 184
 185   ```c
 186   struct foobar t;
 187   zero(t);
 188   t.foo = 7;
 189   t.bar = "bazz";
 190   ```
 191
 192 - To implement an endless loop, use `for (;;)` rather than `while (1)`.  The
 193   latter is a bit ugly anyway, since you probably really meant `while
 194   (true)`. To avoid the discussion what the right always-true expression for an
 195   infinite while loop is, our recommendation is to simply write it without any
 196   such expression by using `for (;;)`.
 197
 198 - To determine the length of a constant string `"foo"`, don't bother with
 199   `sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize
 200   the call away for fixed strings). The only exception is when declaring an
 201   array. In that case use STRLEN, which evaluates to a static constant and
 202   doesn't force the compiler to create a VLA.
 203
 204 - Please use C's downgrade-to-bool feature only for expressions that are
 205   actually booleans (or "boolean-like"), and not for variables that are really
 206   numeric. Specifically, if you have an `int b` and it's only used in a boolean
 207   sense, by all means check its state with `if (b) …` — but if `b` can actually
 208   have more than two semantic values, and you want to compare for non-zero,
 209   then please write that explicitly with `if (b != 0) …`. This helps readability
 210   as the value range and semantical behaviour is directly clear from the
 211   condition check. As a special addition: when dealing with pointers which you
 212   want to check for non-NULL-ness, you may also use downgrade-to-bool feature.
 213
 214 - Please do not use yoda comparisons, i.e. please prefer the more readable `if
 215   (a == 7)` over the less readable `if (7 == a)`.
 216
 217 ## Destructors
 218
 219 - The destructors always deregister the object from the next bigger object, not
 220   the other way around.
 221
 222 - For robustness reasons, destructors should be able to destruct
 223   half-initialized objects, too.
 224
 225 - When you define a destructor or `unref()` call for an object, please accept a
 226   `NULL` object and simply treat this as NOP. This is similar to how libc
 227   `free()` works, which accepts `NULL` pointers and becomes a NOP for them. By
 228   following this scheme a lot of `if` checks can be removed before invoking
 229   your destructor, which makes the code substantially more readable and robust.
 230
 231 - Related to this: when you define a destructor or `unref()` call for an
 232   object, please make it return the same type it takes and always return `NULL`
 233   from it. This allows writing code like this:
 234
 235   ```c
 236   p = foobar_unref(p);
 237   ```
 238
 239   which will always work regardless if `p` is initialized or not, and
 240   guarantees that `p` is `NULL` afterwards, all in just one line.
 241
 242 ## Error Handling
 243
 244 - Error codes are returned as negative `Exxx`. e.g. `return -EINVAL`. There are
 245   some exceptions: for constructors, it is OK to return `NULL` on OOM. For
 246   lookup functions, `NULL` is fine too for "not found".
 247
 248   Be strict with this. When you write a function that can fail due to more than
 249   one cause, it *really* should have an `int` as the return value for the error
 250   code.
 251
 252 - Do not bother with error checking whether writing to stdout/stderr worked.
 253
 254 - Do not log errors from "library" code, only do so from "main program"
 255   code. (With one exception: it is OK to log with DEBUG level from any code,
 256   with the exception of maybe inner loops).
 257
 258 - In public API calls, you **must** validate all your input arguments for
 259   programming error with `assert_return()` and return a sensible return
 260   code. In all other calls, it is recommended to check for programming errors
 261   with a more brutal `assert()`. We are more forgiving to public users than for
 262   ourselves! Note that `assert()` and `assert_return()` really only should be
 263   used for detecting programming errors, not for runtime errors. `assert()` and
 264   `assert_return()` by usage of `_likely_()` inform the compiler that he should
 265   not expect these checks to fail, and they inform fellow programmers about the
 266   expected validity and range of parameters.
 267
 268 - When you invoke certain calls like `unlink()`, or `mkdir_p()` and you know it
 269   is safe to ignore the error it might return (because a later call would
 270   detect the failure anyway, or because the error is in an error path and you
 271   thus couldn't do anything about it anyway), then make this clear by casting
 272   the invocation explicitly to `(void)`. Code checks like Coverity understand
 273   that, and will not complain about ignored error codes. Hence, please use
 274   this:
 275
 276   ```c
 277   (void) unlink("/foo/bar/baz");
 278   ```
 279
 280   instead of just this:
 281
 282   ```c
 283   unlink("/foo/bar/baz");
 284   ```
 285
 286   Don't cast function calls to `(void)` that return no error
 287   conditions. Specifically, the various `xyz_unref()` calls that return a
 288   `NULL` object shouldn't be cast to `(void)`, since not using the return value
 289   does not hide any errors.
 290
 291 - When returning a return code from `main()`, please preferably use
 292   `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc.
 293
 294 ## Logging
 295
 296 - For every function you add, think about whether it is a "logging" function or
 297   a "non-logging" function. "Logging" functions do logging on their own,
 298   "non-logging" function never log on their own and expect their callers to
 299   log. All functions in "library" code, i.e. in `src/shared/` and suchlike must
 300   be "non-logging". Every time a "logging" function calls a "non-logging"
 301   function, it should log about the resulting errors. If a "logging" function
 302   calls another "logging" function, then it should not generate log messages,
 303   so that log messages are not generated twice for the same errors.
 304
 305 - If possible, do a combined log & return operation:
 306
 307   ```c
 308   r = operation(...);
 309   if (r < 0)
 310           return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m");
 311   ```
 312
 313   If the error value is "synthetic", i.e. it was not received from
 314   the called function, use `SYNTHETIC_ERRNO` wrapper to tell the logging
 315   system to not log the errno value, but still return it:
 316
 317   ```c
 318   n = read(..., s, sizeof s);
 319   if (n != sizeof s)
 320           return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ...");
 321   ```
 322
 323 ## Memory Allocation
 324
 325 - Always check OOM. There is no excuse. In program code, you can use
 326   `log_oom()` for then printing a short message, but not in "library" code.
 327
 328 - Avoid fixed-size string buffers, unless you really know the maximum size and
 329   that maximum size is small. They are a source of errors, since they possibly
 330   result in truncated strings. It is often nicer to use dynamic memory,
 331   `alloca()` or VLAs. If you do allocate fixed-size strings on the stack, then
 332   it is probably only OK if you either use a maximum size such as `LINE_MAX`,
 333   or count in detail the maximum size a string can have. (`DECIMAL_STR_MAX` and
 334   `DECIMAL_STR_WIDTH` macros are your friends for this!)
 335
 336   Or in other words, if you use `char buf[256]` then you are likely doing
 337   something wrong!
 338
 339 - Make use of `_cleanup_free_` and friends. It makes your code much nicer to
 340   read (and shorter)!
 341
 342 - Use `alloca()`, but never forget that it is not OK to invoke `alloca()`
 343   within a loop or within function call parameters. `alloca()` memory is
 344   released at the end of a function, and not at the end of a `{}` block. Thus,
 345   if you invoke it in a loop, you keep increasing the stack pointer without
 346   ever releasing memory again. (VLAs have better behavior in this case, so
 347   consider using them as an alternative.)  Regarding not using `alloca()`
 348   within function parameters, see the BUGS section of the `alloca(3)` man page.
 349
 350 - If you want to concatenate two or more strings, consider using `strjoina()`
 351   or `strjoin()` rather than `asprintf()`, as the latter is a lot slower. This
 352   matters particularly in inner loops (but note that `strjoina()` cannot be
 353   used there).
 354
 355 ## Runtime Behaviour
 356
 357 - Avoid leaving long-running child processes around, i.e. `fork()`s that are
 358   not followed quickly by an `execv()` in the child. Resource management is
 359   unclear in this case, and memory CoW will result in unexpected penalties in
 360   the parent much, much later on.
 361
 362 - Don't block execution for arbitrary amounts of time using `usleep()` or a
 363   similar call, unless you really know what you do. Just "giving something some
 364   time", or so is a lazy excuse. Always wait for the proper event, instead of
 365   doing time-based poll loops.
 366
 367 - Whenever installing a signal handler, make sure to set `SA_RESTART` for it,
 368   so that interrupted system calls are automatically restarted, and we minimize
 369   hassles with handling `EINTR` (in particular as `EINTR` handling is pretty
 370   broken on Linux).
 371
 372 - When applying C-style unescaping as well as specifier expansion on the same
 373   string, always apply the C-style unescaping fist, followed by the specifier
 374   expansion. When doing the reverse, make sure to escape `%` in specifier-style
 375   first (i.e. `%` → `%%`), and then do C-style escaping where necessary.
 376
 377 - Be exceptionally careful when formatting and parsing floating point
 378   numbers. Their syntax is locale dependent (i.e. `5.000` in en_US is generally
 379   understood as 5, while in de_DE as 5000.).
 380
 381 - Make sure to enforce limits on every user controllable resource. If the user
 382   can allocate resources in your code, your code must enforce some form of
 383   limits after which it will refuse operation. It's fine if it is hard-coded
 384   (at least initially), but it needs to be there. This is particularly
 385   important for objects that unprivileged users may allocate, but also matters
 386   for everything else any user may allocated.
 387
 388 ## Types
 389
 390 - Think about the types you use. If a value cannot sensibly be negative, do not
 391   use `int`, but use `unsigned`.
 392
 393 - Use `char` only for actual characters. Use `uint8_t` or `int8_t` when you
 394   actually mean a byte-sized signed or unsigned integers. When referring to a
 395   generic byte, we generally prefer the unsigned variant `uint8_t`. Do not use
 396   types based on `short`. They *never* make sense. Use `int`, `long`, `long
 397   long`, all in unsigned and signed fashion, and the fixed-size types
 398   `uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`, `int8_t`, `int16_t`, `int32_t`
 399   and so on, as well as `size_t`, but nothing else. Do not use kernel types
 400   like `u32` and so on, leave that to the kernel.
 401
 402 - Stay uniform. For example, always use `usec_t` for time values. Do not mix
 403   `usec` and `msec`, and `usec` and whatnot.
 404
 405 - Never use the `off_t` type, and particularly avoid it in public APIs. It's
 406   really weirdly defined, as it usually is 64-bit and we don't support it any
 407   other way, but it could in theory also be 32-bit. Which one it is depends on
 408   a compiler switch chosen by the compiled program, which hence corrupts APIs
 409   using it unless they can also follow the program's choice. Moreover, in
 410   systemd we should parse values the same way on all architectures and cannot
 411   expose `off_t` values over D-Bus. To avoid any confusion regarding conversion
 412   and ABIs, always use simply `uint64_t` directly.
 413
 414 - Unless you allocate an array, `double` is always a better choice than
 415   `float`. Processors speak `double` natively anyway, so there is no speed
 416   benefit, and on calls like `printf()` `float`s get promoted to `double`s
 417   anyway, so there is no point.
 418
 419 - Use the bool type for booleans, not integers. One exception: in public
 420   headers (i.e those in `src/systemd/sd-*.h`) use integers after all, as `bool`
 421   is C99 and in our public APIs we try to stick to C89 (with a few extension).
 422
 423 ## Deadlocks
 424
 425 - Do not issue NSS requests (that includes user name and host name lookups)
 426   from PID 1 as this might trigger deadlocks when those lookups involve
 427   synchronously talking to services that we would need to start up.
 428
 429 - Do not synchronously talk to any other service from PID 1, due to risk of
 430   deadlocks.
 431
 432 ## File Descriptors
 433
 434 - When you allocate a file descriptor, it should be made `O_CLOEXEC` right from
 435   the beginning, as none of our files should leak to forked binaries by
 436   default. Hence, whenever you open a file, `O_CLOEXEC` must be specified,
 437   right from the beginning. This also applies to sockets. Effectively, this
 438   means that all invocations to:
 439
 440   - `open()` must get `O_CLOEXEC` passed,
 441   - `socket()` and `socketpair()` must get `SOCK_CLOEXEC` passed,
 442   - `recvmsg()` must get `MSG_CMSG_CLOEXEC` set,
 443   - `F_DUPFD_CLOEXEC` should be used instead of `F_DUPFD`, and so on,
 444   - invocations of `fopen()` should take `e`.
 445
 446 - It's a good idea to use `O_NONBLOCK` when opening 'foreign' regular files,
 447   i.e.  file system objects that are supposed to be regular files whose paths
 448   where specified by the user and hence might actually refer to other types of
 449   file system objects. This is a good idea so that we don't end up blocking on
 450   'strange' file nodes, for example if the user pointed us to a FIFO or device
 451   node which may block when opening. Moreover even for actual regular files
 452   `O_NONBLOCK` has a benefit: it bypasses any mandatory lock that might be in
 453   effect on the regular file. If in doubt consider turning off `O_NONBLOCK`
 454   again after opening.
 455
 456 ## Command Line
 457
 458 - If you parse a command line, and want to store the parsed parameters in
 459   global variables, please consider prefixing their names with `arg_`. We have
 460   been following this naming rule in most of our tools, and we should continue
 461   to do so, as it makes it easy to identify command line parameter variables,
 462   and makes it clear why it is OK that they are global variables.
 463
 464 - Command line option parsing:
 465   - Do not print full `help()` on error, be specific about the error.
 466   - Do not print messages to stdout on error.
 467   - Do not POSIX_ME_HARDER unless necessary, i.e. avoid `+` in option string.
 468
 469 ## Exporting Symbols
 470
 471 - Variables and functions **must** be static, unless they have a prototype, and
 472   are supposed to be exported.
 473
 474 - Public API calls (i.e. functions exported by our shared libraries)
 475   must be marked `_public_` and need to be prefixed with `sd_`. No
 476   other functions should be prefixed like that.
 477
 478 - When exposing public C APIs, be careful what function parameters you make
 479   `const`. For example, a parameter taking a context object should probably not
 480   be `const`, even if you are writing an otherwise read-only accessor function
 481   for it. The reason is that making it `const` fixates the contract that your
 482   call won't alter the object ever, as part of the API. However, that's often
 483   quite a promise, given that this even prohibits object-internal caching or
 484   lazy initialization of object variables. Moreover, it's usually not too
 485   useful for client applications. Hence, please be careful and avoid `const` on
 486   object parameters, unless you are very sure `const` is appropriate.
 487
 488 ## Referencing Concepts
 489
 490 - When referring to a configuration file option in the documentation and such,
 491   please always suffix it with `=`, to indicate that it is a configuration file
 492   setting.
 493
 494 - When referring to a command line option in the documentation and such, please
 495   always prefix with `--` or `-` (as appropriate), to indicate that it is a
 496   command line option.
 497
 498 - When referring to a file system path that is a directory, please always
 499   suffix it with `/`, to indicate that it is a directory, not a regular file
 500   (or other file system object).
 501
 502 ## Functions to Avoid
 503
 504 - Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)`
 505
 506 - Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where
 507   applicable (i.e. wherever you just care about equality/inequality, not about
 508   the sorting order).
 509
 510 - Never use `strtol()`, `atoi()` and similar calls. Use `safe_atoli()`,
 511   `safe_atou32()` and suchlike instead. They are much nicer to use in most
 512   cases and correctly check for parsing errors.
 513
 514 - `htonl()`/`ntohl()` and `htons()`/`ntohs()` are weird. Please use `htobe32()`
 515   and `htobe16()` instead, it's much more descriptive, and actually says what
 516   really is happening, after all `htonl()` and `htons()` don't operate on
 517   `long`s and `short`s as their name would suggest, but on `uint32_t` and
 518   `uint16_t`. Also, "network byte order" is just a weird name for "big endian",
 519   hence we might want to call it "big endian" right-away.
 520
 521 - Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)` instead. For
 522   two reason: first, you want `O_CLOEXEC` set on the new `fd` (see
 523   above). Second, `dup()` will happily duplicate your `fd` as 0, 1, 2,
 524   i.e. stdin, stdout, stderr, should those `fd`s be closed. Given the special
 525   semantics of those `fd`s, it's probably a good idea to avoid
 526   them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them.
 527
 528 - Don't use `fgets()`, it's too hard to properly handle errors such as overly
 529   long lines. Use `read_line()` instead, which is our own function that handles
 530   this much nicer.
 531
 532 - Don't invoke `exit()`, ever. It is not replacement for proper error
 533   handling. Please escalate errors up your call chain, and use normal `return`
 534   to exit from the main function of a process. If you `fork()`ed off a child
 535   process, please use `_exit()` instead of `exit()`, so that the exit handlers
 536   are not run.
 537
 538 - We never use the POSIX version of `basename()` (which glibc defines it in
 539   `libgen.h`), only the GNU version (which glibc defines in `string.h`).  The
 540   only reason to include `libgen.h` is because `dirname()` is needed. Every
 541   time you need that please immediately undefine `basename()`, and add a
 542   comment about it, so that no code ever ends up using the POSIX version!
 543
 544 # Committing to git
 545
 546 - Commit message subject lines should be prefixed with an appropriate component
 547   name of some kind. For example "journal: ", "nspawn: " and so on.
 548
 549 - Do not use "Signed-Off-By:" in your commit messages. That's a kernel thing we
 550   don't do in the systemd project.