CODING_STYLE.md

   1 #### General Notes
   2
   3 - The coding style guide refers to new code. But legacy code can be cleaned up
   4   and we are happy to take those patches.
   5 - Just because there is still code in LXC that doesn't adhere to the coding
   6   standards outlined here does not license not adhering to the coding style. In
   7   other words: please stick to the coding style.
   8 - Maintainers are free to ignore rules specified here when merging pull
   9   requests. This guideline might seem a little weird but it exits to ease new
  10   developers into the code base and to prevent unnecessary bikeshedding. If
  11   a maintainer feels hat enforcing a specific rule in a given commit would do
  12   more harm than good they should always feel free to ignore the rule.
  13
  14   Furthermore, when merging pull requests that do not adhere to our coding
  15   style maintainers should feel free to grab the commit, adapt it to our coding
  16   style and add their Signed-off-by line to it. This is especially helpful to
  17   make it easier for first-time contributors and to prevent having pull
  18   requests being stuck in the merge queue because of minor details.
  19
  20 #### Only Use Tabs
  21
  22 - LXC uses tabs.
  23
  24 #### Only use `/* */` Style Comments
  25
  26 - Any comments that are added must use `/* */`.
  27 - All comments should start on the same line as the opening `/*`.
  28 - Single-line comments should simply be placed between `/* */`. For example:
  29   ```
  30   /* Define pivot_root() if missing from the C library */
  31   ```
  32 - Multi-line comments should end with the closing `*/` on a separate line. For
  33   example:
  34   ```
  35   /* At this point the old-root is mounted on top of our new-root
  36    * To unmounted it we must not be chdir()ed into it, so escape back
  37    * to old-root.
  38    */
  39   ```
  40
  41 #### Try To Wrap At 80chars
  42
  43 - This is not strictly enforced. It is perfectly valid to sometimes
  44   overflow this limit if it helps clarity. Nonetheless, try to stick to it
  45   and use common sense to decide when not to.
  46
  47 #### Error Messages
  48
  49 - Error messages must start with a capital letter and must **not** end with a
  50   punctuation sign.
  51 - They should be descriptive, without being needlessly long. It is best to just
  52   use already existing error messages as examples.
  53 - Examples of acceptable error messages are:
  54   ```
  55   SYSERROR("Failed to create directory \"%s\"", path);
  56   WARN("\"/dev\" directory does not exist. Proceeding without autodev being set up");
  57   ```
  58
  59 #### Return Error Codes
  60
  61 - When writing a function that can fail in a non-binary way try to return
  62   meaningful negative error codes (e.g. `return -EINVAL;`).
  63
  64 #### All Unexported Functions Must Be Declared `static`
  65
  66 - Functions which are only used in the current file and are not exported
  67   within the codebase need to be declared with the `static` attribute.
  68
  69 #### All Exported Functions Must Be Declared `extern` In A Header File
  70
  71 - Functions which are used in different files in the library should be declared
  72   in a suitable `*.c` file and exposed in a suitable `*.h` file. When defining
  73   the function in the `*.c` file the function signature should not be preceded
  74   by the `extern` keyword. When declaring the function signature in the `*.h`
  75   file it must be preceded by the `extern` keyword. For example:
  76   ```
  77   /* Valid function definition in a *.c file */
  78   ssize_t lxc_write_nointr(int fd, const void* buf, size_t count)
  79   {
  80           ssize_t ret;
  81   again:
  82           ret = write(fd, buf, count);
  83           if (ret < 0 && errno == EINTR)
  84                   goto again;
  85           return ret;
  86   }
  87
  88   /* Valid function declaration in a *.h file */
  89   extern ssize_t lxc_write_nointr(int fd, const void* buf, size_t count);
  90   ```
  91
  92 #### All Names Must Be In lower_case
  93
  94 - All functions and variable names must use lower case.
  95
  96 #### Declaring Variables
  97
  98 - variables should be declared at the top of the function or at the beginning
  99   of a new scope but **never** in the middle of a scope
 100 1. uninitialized variables
 101   - put base types before complex types
 102   - put standard types defined by libc before types defined by LXC
 103   - put multiple declarations of the same type on the same line
 104 2. initialized variables
 105   - put base types before complex types
 106   - put standard types defined by libc before types defined by LXC
 107   - put multiple declarations of the same type on the same line
 108 - Examples of good declarations can be seen in the following function:
 109   ```
 110   int lxc_clear_procs(struct lxc_conf *c, const char *key)
 111   {
 112           struct lxc_list *it, *next;
 113           bool all = false;
 114           const char *k = NULL;
 115
 116           if (strcmp(key, "lxc.proc") == 0)
 117                   all = true;
 118           else if (strncmp(key, "lxc.proc.", sizeof("lxc.proc.") - 1) == 0)
 119                   k = key + sizeof("lxc.proc.") - 1;
 120           else
 121                   return -1;
 122
 123           lxc_list_for_each_safe(it, &c->procs, next) {
 124                   struct lxc_proc *proc = it->elem;
 125
 126                   if (!all && strcmp(proc->filename, k) != 0)
 127                           continue;
 128                   lxc_list_del(it);
 129                   free(proc->filename);
 130                   free(proc->value);
 131                   free(proc);
 132                   free(it);
 133           }
 134
 135           return 0;
 136   }
 137     ```
 138
 139 #### Single-line `if` blocks should not be enclosed in `{}`
 140
 141 - This also affects `if-else` ladders if and only if all constituting
 142   conditions are
 143   single-line conditions. If there is at least one non-single-line
 144   condition `{}` must be used.
 145 - For example:
 146   ```
 147   /* no brackets needed */
 148   if (size > INT_MAX)
 149           return -EFBIG;
 150
 151   /* The else branch has more than one-line and so needs {}. This entails that
 152    * the if branch also needs to have {}.
 153    */
 154   if ( errno == EROFS ) {
 155           WARN("Warning: Read Only file system while creating %s", path);
 156   } else {
 157           SYSERROR("Error creating %s", path);
 158           return -1;
 159   }
 160
 161   /* also fine */
 162   for (i = 0; list[i]; i++)
 163         if (strcmp(list[i], entry) == 0)
 164                 return true;
 165
 166   /* also fine */
 167   if (ret < 0)
 168           WARN("Failed to set FD_CLOEXEC flag on slave fd %d of "
 169                "pty device \"%s\": %s", pty_info->slave,
 170                pty_info->name, strerror(errno));
 171
 172   /* also fine */
 173   if (ret == 0)
 174           for (i = 0; i < sizeof(limit_opt)/sizeof(limit_opt[0]); ++i) {
 175                   if (strcmp(res, limit_opt[i].name) == 0)
 176                           return limit_opt[i].value;
 177           }
 178   ```
 179
 180 #### Functions Not Returning Booleans Must Assigned Return Value Before Performing Checks
 181
 182 - When checking whether a function not returning booleans was successful or not
 183   the returned value must be assigned before it is checked (`str{n}cmp()`
 184   functions being one notable exception). For example:
 185   ```
 186   /* assign value to "ret" first */
 187   ret = mount(sourcepath, cgpath, "cgroup", remount_flags, NULL);
 188   /* check whether function was successful */
 189   if (ret < 0) {
 190           SYSERROR("Failed to remount \"%s\" ro", cgpath);
 191           free(sourcepath);
 192           return -1;
 193   }
 194   ```
 195   Functions returning booleans can be checked directly. For example:
 196   ```
 197   extern bool lxc_string_in_array(const char *needle, const char **haystack);
 198
 199   /* check right away */
 200   if (lxc_string_in_array("ns", (const char **)h->subsystems))
 201           continue;
 202   ```
 203
 204 #### Do Not Use C99 Variable Length Arrays (VLA)
 205
 206 - They are made optional and there is no guarantee that future C standards
 207   will support them.
 208
 209 #### Use Standard libc Macros When Exiting
 210
 211 - libc provides `EXIT_FAILURE` and `EXIT_SUCCESS`. Use them whenever possible
 212   in the child of `fork()`ed process or when exiting from a `main()` function.
 213
 214 #### Use `goto`s
 215
 216 `goto`s are an essential language construct of C and are perfect to perform
 217 cleanup operations or simplify the logic of functions. However, here are the
 218 rules to use them:
 219 - use descriptive `goto` labels.
 220   For example, if you know that this label is only used as an error path you
 221   should use something like `on_error` instead of `out` as label name.
 222 - **only** jump downwards unless you are handling `EAGAIN` errors and want to
 223   avoid `do-while` constructs.
 224 - An example of a good usage of `goto` is:
 225   ```
 226   static int set_config_idmaps(const char *key, const char *value,
 227                              struct lxc_conf *lxc_conf, void *data)
 228   {
 229           unsigned long hostid, nsid, range;
 230           char type;
 231           int ret;
 232           struct lxc_list *idmaplist = NULL;
 233           struct id_map *idmap = NULL;
 234
 235           if (lxc_config_value_empty(value))
 236                   return lxc_clear_idmaps(lxc_conf);
 237
 238           idmaplist = malloc(sizeof(*idmaplist));
 239           if (!idmaplist)
 240                   goto on_error;
 241
 242           idmap = malloc(sizeof(*idmap));
 243           if (!idmap)
 244                   goto on_error;
 245           memset(idmap, 0, sizeof(*idmap));
 246
 247           ret = parse_idmaps(value, &type, &nsid, &hostid, &range);
 248           if (ret < 0) {
 249                   ERROR("Failed to parse id mappings");
 250                   goto on_error;
 251           }
 252
 253           INFO("Read uid map: type %c nsid %lu hostid %lu range %lu", type, nsid, hostid, range);
 254           if (type == 'u')
 255                   idmap->idtype = ID_TYPE_UID;
 256           else if (type == 'g')
 257                   idmap->idtype = ID_TYPE_GID;
 258           else
 259                   goto on_error;
 260
 261           idmap->hostid = hostid;
 262           idmap->nsid = nsid;
 263           idmap->range = range;
 264           idmaplist->elem = idmap;
 265           lxc_list_add_tail(&lxc_conf->id_map, idmaplist);
 266
 267           if (!lxc_conf->root_nsuid_map && idmap->idtype == ID_TYPE_UID)
 268                   if (idmap->nsid == 0)
 269                           lxc_conf->root_nsuid_map = idmap;
 270
 271
 272           if (!lxc_conf->root_nsgid_map && idmap->idtype == ID_TYPE_GID)
 273                   if (idmap->nsid == 0)
 274                           lxc_conf->root_nsgid_map = idmap;
 275
 276           idmap = NULL;
 277
 278           return 0;
 279
 280   on_error:
 281           free(idmaplist);
 282           free(idmap);
 283
 284           return -1;
 285   }
 286   ```
 287
 288 #### Use Booleans instead of integers
 289
 290 - When something can be conceptualized in a binary way use a boolean not
 291   an integer.
 292
 293 #### Cleanup Functions Must Handle The Object's Null Type And Being Passed Already Cleaned Up Objects
 294
 295 - If you implement a custom cleanup function to e.g. free a complex type
 296   you declared you must ensure that the object's null type is handled and
 297   treated as a NOOP. For example:
 298   ```
 299   void lxc_free_array(void **array, lxc_free_fn element_free_fn)
 300   {
 301           void **p;
 302           for (p = array; p && *p; p++)
 303                   element_free_fn(*p);
 304           free((void*)array);
 305   }
 306   ```
 307 - Cleanup functions should also expect to be passed already cleaned up objects.
 308   One way to handle this cleanly is to initialize the cleaned up variable to
 309   a special value that signals the function that the element has already been
 310   freed on the next call. For example, the following function cleans up file
 311   descriptors and sets the already closed file descriptors to `-EBADF`. On the
 312   next call it can simply check whether the file descriptor is positive and
 313   move on if it isn't:
 314   ```
 315   static void lxc_put_attach_clone_payload(struct attach_clone_payload *p)
 316   {
 317           if (p->ipc_socket >= 0) {
 318                   shutdown(p->ipc_socket, SHUT_RDWR);
 319                   close(p->ipc_socket);
 320                   p->ipc_socket = -EBADF;
 321           }
 322
 323           if (p->pty_fd >= 0) {
 324                   close(p->pty_fd);
 325                   p->pty_fd = -EBADF;
 326           }
 327
 328           if (p->init_ctx) {
 329                   lxc_proc_put_context_info(p->init_ctx);
 330                   p->init_ctx = NULL;
 331           }
 332   }
 333   ```
 334
 335 ### Cast to `(void)` When Intentionally Ignoring Return Values
 336
 337 - There are cases where you do not care about the return value of a function.
 338   Please cast the return value to `(void)` when doing so.
 339 - Standard library functions or functions which are known to be ignored by
 340   default do not need to be cast to `(void)`. Classical candidates are
 341   `close()` and `fclose()`.
 342 - A good example is:
 343   ```
 344   for (i = 0; hierarchies[i]; i++) {
 345           char *fullpath;
 346           char *path = hierarchies[i]->fullcgpath;
 347
 348           ret = chowmod(path, destuid, nsgid, 0755);
 349           if (ret < 0)
 350                   return -1;
 351
 352           /* failures to chown() these are inconvenient but not
 353            * detrimental we leave these owned by the container launcher,
 354            * so that container root can write to the files to attach.  we
 355            * chmod() them 664 so that container systemd can write to the
 356            * files (which systemd in wily insists on doing).
 357            */
 358
 359           if (hierarchies[i]->version == cgroup_super_magic) {
 360                   fullpath = must_make_path(path, "tasks", null);
 361                   (void)chowmod(fullpath, destuid, nsgid, 0664);
 362                   free(fullpath);
 363           }
 364
 365           fullpath = must_make_path(path, "cgroup.procs", null);
 366           (void)chowmod(fullpath, destuid, 0, 0664);
 367           free(fullpath);
 368
 369           if (hierarchies[i]->version != cgroup2_super_magic)
 370                   continue;
 371
 372           fullpath = must_make_path(path, "cgroup.subtree_control", null);
 373           (void)chowmod(fullpath, destuid, nsgid, 0664);
 374           free(fullpath);
 375
 376           fullpath = must_make_path(path, "cgroup.threads", null);
 377           (void)chowmod(fullpath, destuid, nsgid, 0664);
 378           free(fullpath);
 379   }
 380   ```
 381
 382 #### Use `_exit()` in `fork()`ed Processes
 383
 384 - This has multiple reasons but the gist is:
 385   - `exit()` is not thread-safe
 386   - `exit()` in libc runs exit handlers which might interfer with the parents
 387     state
 388
 389 #### Use `for (;;)` instead of `while (1)` or `while (true)`
 390
 391 - Let's be honest, it is really the only sensible way to do this.
 392
 393 #### Use The Set Of Supported DCO Statements
 394
 395 - Signed-off-by: Random J Developer <random@developer.org>
 396   - You did write this code or have the right to contribute it to LXC.
 397 - Acked-by: Random J Developer <random@developer.org>
 398   - You did read the code and think it is correct. This is usually only used by
 399     maintainers or developers that have made significant contributions and can
 400     vouch for the correctness of someone else's code.
 401 - Reviewed-by: Random J Developer <random@developer.org>
 402   - You did review the code and vouch for its correctness, i.e. you'd be
 403     prepared to fix bugs it might cause. This is usually only used by
 404     maintainers or developers that have made significant contributions and can
 405     vouch for the correctness of someone else's code.
 406 - Co-developed-by: Random J Developer <random@developer.org>
 407   - The code can not be reasonably attributed to a single developer, i.e.
 408     you worked on this together.
 409 - Tested-by: Random J Developer <random@developer.org>
 410   - You verified that the code fixes a given bug or is behaving as advertised.
 411 - Reported-by: Random J Developer <random@developer.org>
 412   - You found and reported the bug.
 413 - Suggested-by: Random J Developer <random@developer.org>
 414   - You wrote the code but someone contributed the idea. This line is usually
 415     overlooked but it is a sign of good etiquette and coding ethics: if someone
 416     helped you solve a problem or had a clever idea do not silently claim it by
 417     slapping your Signed-off-by underneath. Be honest and add a Suggested-by.
 418
 419 #### Commit Message Outline
 420
 421 - You **must** stick to the 80chars limit especially in the title of the commit
 422   message.
 423 - Please use English commit messages only.
 424 - use meaningful commit messages.
 425 - Use correct spelling and grammar.
 426   If you are not a native speaker and/or feel yourself struggling with this it
 427   is perfectly fine to point this out and there's no need to apologize. Usually
 428   developers will be happy to pull your branch and adopt the commit message.
 429 - Please always use the affected file (without the file type suffix) or module
 430   as a prefix in the commit message.
 431 - Examples of good commit messages are:
 432   ```
 433   commit b87243830e3b5e95fa31a17cf1bfebe55353bf13
 434   Author: Felix Abecassis <fabecassis@nvidia.com>
 435   Date:   Fri Feb 2 06:19:13 2018 -0800
 436
 437       hooks: change the semantic of NVIDIA_VISIBLE_DEVICES=""
 438
 439       With LXC, you can override the value of an environment variable to
 440       null, but you can't unset an existing variable.
 441
 442       The NVIDIA hook was previously activated when NVIDIA_VISIBLE_DEVICES
 443       was set to null. As a result, it was not possible to disable the hook
 444       by overriding the environment variable in the configuration.
 445
 446       The hook can now be disabled by setting NVIDIA_VISIBLE_DEVICES to
 447       null or to the new special value "void".
 448
 449       Signed-off-by: Felix Abecassis <fabecassis@nvidia.com>
 450
 451
 452   commit d6337a5f9dc7311af168aa3d586fdf239f5a10d3
 453   Author: Christian Brauner <christian.brauner@ubuntu.com>
 454   Date:   Wed Jan 31 16:25:11 2018 +0100
 455
 456       cgroups: get controllers on the unified hierarchy
 457
 458       Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
 459
 460   ```