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CODING_STYLE.md

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General Notes

  • The coding style guide refers to new code. But legacy code can be cleaned up and we are happy to take those patches.

  • Just because there is still code in LXC that doesn't adhere to the coding standards outlined here does not license not adhering to the coding style. In other words: please stick to the coding style.

  • Maintainers are free to ignore rules specified here when merging pull requests. This guideline might seem a little weird but it exits to ease new developers into the code base and to prevent unnecessary bikeshedding. If a maintainer feels hat enforcing a specific rule in a given commit would do more harm than good they should always feel free to ignore the rule.

    Furthermore, when merging pull requests that do not adhere to our coding style maintainers should feel free to grab the commit, adapt it to our coding style and add their Signed-off-by line to it. This is especially helpful to make it easier for first-time contributors and to prevent having pull requests being stuck in the merge queue because of minor details.

  • We currently do not provide automatic coding style checks but if a suitable tool is found we are happy to integrate it into our test suite. It is possible and recommended to use the clang-format binary to check your code. The following options are an approximation of the coding style used here. Simply create a file called .clang-format in your home directory with the following options:

    cat << EOF > "${HOME}"/.clang-format
AlignEscapedNewlines: Left
BreakBeforeBraces: Attach
AlwaysBreakBeforeMultilineStrings: false
BreakBeforeBinaryOperators: None
MaxEmptyLinesToKeep: 1
PenaltyBreakBeforeFirstCallParameter: 1000000
BinPackArguments: true
BinPackParameters: true
AllowAllParametersOfDeclarationOnNextLine: false
AlignAfterOpenBracket: true
SpacesInSquareBrackets: false
SpacesInCStyleCastParentheses: false
SpaceInEmptyParentheses: false
SpaceBeforeParens: ControlStatements
SpaceAfterCStyleCast: false
SortIncludes: true
PenaltyReturnTypeOnItsOwnLine: 10000
PenaltyExcessCharacter: 10
Language: Cpp
ForEachMacros: ['lxc_list_for_each', 'lxc_list_for_each_safe']
AllowShortLoopsOnASingleLine: false
AllowShortIfStatementsOnASingleLine: false
AllowShortFunctionsOnASingleLine: None
AllowShortCaseLabelsOnASingleLine: false
AllowShortBlocksOnASingleLine: false
BasedOnStyle: LLVM
TabWidth: 8
IndentWidth: 8
UseTab: Always
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
EOF

    However, it will not handle all cases correctly. For example, most struct initializations will not be correct. In such cases please refer to the coding style here.

Only Use Tabs

  • LXC uses tabs.

Only use /* */ Style Comments

  • Any comments that are added must use /* */.
  • All comments should start on the same line as the opening /*.
  • Single-line comments should simply be placed between /* */. For example: 
    /* Define pivot_root() if missing from the C library */
  • Multi-line comments should end with the closing */ on a separate line. For example: 
    /* At this point the old-root is mounted on top of our new-root
 * To unmounted it we must not be chdir()ed into it, so escape back
 * to old-root.
 */

Try To Wrap At 80chars

  • This is not strictly enforced. It is perfectly valid to sometimes overflow this limit if it helps clarity. Nonetheless, try to stick to it and use common sense to decide when not to.

Error Messages

  • Error messages must start with a capital letter and must not end with a punctuation sign.
  • They should be descriptive, without being needlessly long. It is best to just use already existing error messages as examples.
  • Examples of acceptable error messages are: 
    SYSERROR("Failed to create directory \"%s\"", path);
WARN("\"/dev\" directory does not exist. Proceeding without autodev being set up");

Return Error Codes

  • When writing a function that can fail in a non-binary way try to return meaningful negative error codes (e.g. return -EINVAL;).

All Unexported Functions Must Be Declared static

  • Functions which are only used in the current file and are not exported within the codebase need to be declared with the static attribute.

All Exported Functions Must Be Declared extern In A Header File

  • Functions which are used in different files in the library should be declared in a suitable *.c file and exposed in a suitable *.h file. When defining the function in the *.c file the function signature should not be preceded by the extern keyword. When declaring the function signature in the *.h file it must be preceded by the extern keyword. For example: 
    /* Valid function definition in a *.c file */
ssize_t lxc_write_nointr(int fd, const void* buf, size_t count)
{
        ssize_t ret;
again:
        ret = write(fd, buf, count);
        if (ret < 0 && errno == EINTR)
                goto again;
        return ret;
}

/* Valid function declaration in a *.h file */
extern ssize_t lxc_write_nointr(int fd, const void* buf, size_t count);

All Names Must Be In lower_case

  • All functions and variable names must use lower case.

Declaring Variables

  • variables should be declared at the top of the function or at the beginning of a new scope but never in the middle of a scope
  1. uninitialized variables
  • put base types before complex types
  • put standard types defined by libc before types defined by LXC
  • put multiple declarations of the same type on the same line
  1. initialized variables
  • put base types before complex types
  • put standard types defined by libc before types defined by LXC
  • put multiple declarations of the same type on the same line
  • Examples of good declarations can be seen in the following function: 
    int lxc_clear_procs(struct lxc_conf *c, const char *key)
{
        struct lxc_list *it, *next;
        bool all = false;
        const char *k = NULL;

        if (strcmp(key, "lxc.proc") == 0)
                all = true;
        else if (strncmp(key, "lxc.proc.", sizeof("lxc.proc.") - 1) == 0)
                k = key + sizeof("lxc.proc.") - 1;
        else
                return -1;

        lxc_list_for_each_safe(it, &c->procs, next) {
                struct lxc_proc *proc = it->elem;

                if (!all && strcmp(proc->filename, k) != 0)
                        continue;
                lxc_list_del(it);
                free(proc->filename);
                free(proc->value);
                free(proc);
                free(it);
        }

        return 0;
}

Single-line if blocks should not be enclosed in {}

  • This also affects if-else ladders if and only if all constituting conditions are single-line conditions. If there is at least one non-single-line condition {} must be used.
  • For example: 
    /* no brackets needed */
if (size > INT_MAX)
        return -EFBIG;

/* The else branch has more than one-line and so needs {}. This entails that
 * the if branch also needs to have {}.
 */
if ( errno == EROFS ) {
        WARN("Warning: Read Only file system while creating %s", path);
} else {
        SYSERROR("Error creating %s", path);
        return -1;
}

/* also fine */
for (i = 0; list[i]; i++)
      if (strcmp(list[i], entry) == 0)
              return true;

/* also fine */
if (ret < 0)
        WARN("Failed to set FD_CLOEXEC flag on slave fd %d of "
             "pty device \"%s\": %s", pty_info->slave,
             pty_info->name, strerror(errno));

/* also fine */
if (ret == 0)
        for (i = 0; i < sizeof(limit_opt)/sizeof(limit_opt[0]); ++i) {
                if (strcmp(res, limit_opt[i].name) == 0)
                        return limit_opt[i].value;
        }

Functions Not Returning Booleans Must Assign Return Value Before Performing Checks

  • When checking whether a function not returning booleans was successful or not the returned value must be assigned before it is checked (str{n}cmp() functions being one notable exception). For example: 
    /* assign value to "ret" first */
ret = mount(sourcepath, cgpath, "cgroup", remount_flags, NULL);
/* check whether function was successful */
if (ret < 0) {
        SYSERROR("Failed to remount \"%s\" ro", cgpath);
        free(sourcepath);
        return -1;
}
    Functions returning booleans can be checked directly. For example: 
    extern bool lxc_string_in_array(const char *needle, const char **haystack);

/* check right away */
if (lxc_string_in_array("ns", (const char **)h->subsystems))
        continue;

Non-Boolean Functions That Behave Like Boolean Functions Must Explicitly Check Against A Value

  • This rule mainly exists for str{n}cmp() type functions. In most cases they are used like a boolean function to check whether a string matches or not. But they return an integer. It is perfectly fine to check str{n}cmp() functions directly but you must compare explicitly against a value. That is to say, while they are conceptually boolean functions they shouldn't be treated as such since they don't really behave like boolean functions. So if (!str{n}cmp()) and if (str{n}cmp()) checks must not be used. Good examples are found in the following functions: 
    static int set_config_hooks(const char *key, const char *value,
                            struct lxc_conf *lxc_conf, void *data)

        char *copy;

        if (lxc_config_value_empty(value))
                return lxc_clear_hooks(lxc_conf, key);

        if (strcmp(key + 4, "hook") == 0) {
                ERROR("lxc.hook must not have a value");
                return -1;
        }

        copy = strdup(value);
        if (!copy)
                return -1;

        if (strcmp(key + 9, "pre-start") == 0)
                return add_hook(lxc_conf, LXCHOOK_PRESTART, copy);
        else if (strcmp(key + 9, "start-host") == 0)
                return add_hook(lxc_conf, LXCHOOK_START_HOST, copy);
        else if (strcmp(key + 9, "pre-mount") == 0)
                return add_hook(lxc_conf, LXCHOOK_PREMOUNT, copy);
        else if (strcmp(key + 9, "autodev") == 0)
                return add_hook(lxc_conf, LXCHOOK_AUTODEV, copy);
        else if (strcmp(key + 9, "mount") == 0)
                return add_hook(lxc_conf, LXCHOOK_MOUNT, copy);
        else if (strcmp(key + 9, "start") == 0)
                return add_hook(lxc_conf, LXCHOOK_START, copy);
        else if (strcmp(key + 9, "stop") == 0)
                return add_hook(lxc_conf, LXCHOOK_STOP, copy);
        else if (strcmp(key + 9, "post-stop") == 0)
                return add_hook(lxc_conf, LXCHOOK_POSTSTOP, copy);
        else if (strcmp(key + 9, "clone") == 0)
                return add_hook(lxc_conf, LXCHOOK_CLONE, copy);
        else if (strcmp(key + 9, "destroy") == 0)
                return add_hook(lxc_conf, LXCHOOK_DESTROY, copy);

        free(copy);
        return -1;
}

Do Not Use C99 Variable Length Arrays (VLA)

  • They are made optional and there is no guarantee that future C standards will support them.

Use Standard libc Macros When Exiting

  • libc provides EXIT_FAILURE and EXIT_SUCCESS. Use them whenever possible in the child of fork()ed process or when exiting from a main() function.

Use gotos

gotos are an essential language construct of C and are perfect to perform cleanup operations or simplify the logic of functions. However, here are the rules to use them:

  • use descriptive goto labels. For example, if you know that this label is only used as an error path you should use something like on_error instead of out as label name.
  • only jump downwards unless you are handling EAGAIN errors and want to avoid do-while constructs.
  • An example of a good usage of goto is: 
    static int set_config_idmaps(const char *key, const char *value,
                           struct lxc_conf *lxc_conf, void *data)
{
        unsigned long hostid, nsid, range;
        char type;
        int ret;
        struct lxc_list *idmaplist = NULL;
        struct id_map *idmap = NULL;

        if (lxc_config_value_empty(value))
                return lxc_clear_idmaps(lxc_conf);

        idmaplist = malloc(sizeof(*idmaplist));
        if (!idmaplist)
                goto on_error;

        idmap = malloc(sizeof(*idmap));
        if (!idmap)
                goto on_error;
        memset(idmap, 0, sizeof(*idmap));

        ret = parse_idmaps(value, &type, &nsid, &hostid, &range);
        if (ret < 0) {
                ERROR("Failed to parse id mappings");
                goto on_error;
        }

        INFO("Read uid map: type %c nsid %lu hostid %lu range %lu", type, nsid, hostid, range);
        if (type == 'u')
                idmap->idtype = ID_TYPE_UID;
        else if (type == 'g')
                idmap->idtype = ID_TYPE_GID;
        else
                goto on_error;

        idmap->hostid = hostid;
        idmap->nsid = nsid;
        idmap->range = range;
        idmaplist->elem = idmap;
        lxc_list_add_tail(&lxc_conf->id_map, idmaplist);

        if (!lxc_conf->root_nsuid_map && idmap->idtype == ID_TYPE_UID)
                if (idmap->nsid == 0)
                        lxc_conf->root_nsuid_map = idmap;


        if (!lxc_conf->root_nsgid_map && idmap->idtype == ID_TYPE_GID)
                if (idmap->nsid == 0)
                        lxc_conf->root_nsgid_map = idmap;

        idmap = NULL;

        return 0;

on_error:
        free(idmaplist);
        free(idmap);

        return -1;
}

Use Booleans instead of integers

  • When something can be conceptualized in a binary way use a boolean not an integer.

Cleanup Functions Must Handle The Object's Null Type And Being Passed Already Cleaned Up Objects

  • If you implement a custom cleanup function to e.g. free a complex type you declared you must ensure that the object's null type is handled and treated as a NOOP. For example: 
    void lxc_free_array(void **array, lxc_free_fn element_free_fn)
{
        void **p;
        for (p = array; p && *p; p++)
                element_free_fn(*p);
        free((void*)array);
}
  • Cleanup functions should also expect to be passed already cleaned up objects. One way to handle this cleanly is to initialize the cleaned up variable to a special value that signals the function that the element has already been freed on the next call. For example, the following function cleans up file descriptors and sets the already closed file descriptors to -EBADF. On the next call it can simply check whether the file descriptor is positive and move on if it isn't: 
    static void lxc_put_attach_clone_payload(struct attach_clone_payload *p)
{
        if (p->ipc_socket >= 0) {
                shutdown(p->ipc_socket, SHUT_RDWR);
                close(p->ipc_socket);
                p->ipc_socket = -EBADF;
        }

        if (p->pty_fd >= 0) {
                close(p->pty_fd);
                p->pty_fd = -EBADF;
        }

        if (p->init_ctx) {
                lxc_proc_put_context_info(p->init_ctx);
                p->init_ctx = NULL;
        }
}

Cast to (void) When Intentionally Ignoring Return Values

  • There are cases where you do not care about the return value of a function. Please cast the return value to (void) when doing so.
  • Standard library functions or functions which are known to be ignored by default do not need to be cast to (void). Classical candidates are close() and fclose().
  • A good example is: 
    for (i = 0; hierarchies[i]; i++) {
        char *fullpath;
        char *path = hierarchies[i]->fullcgpath;

        ret = chowmod(path, destuid, nsgid, 0755);
        if (ret < 0)
                return -1;

        /* failures to chown() these are inconvenient but not
         * detrimental we leave these owned by the container launcher,
         * so that container root can write to the files to attach.  we
         * chmod() them 664 so that container systemd can write to the
         * files (which systemd in wily insists on doing).
         */

        if (hierarchies[i]->version == cgroup_super_magic) {
                fullpath = must_make_path(path, "tasks", null);
                (void)chowmod(fullpath, destuid, nsgid, 0664);
                free(fullpath);
        }

        fullpath = must_make_path(path, "cgroup.procs", null);
        (void)chowmod(fullpath, destuid, 0, 0664);
        free(fullpath);

        if (hierarchies[i]->version != cgroup2_super_magic)
                continue;

        fullpath = must_make_path(path, "cgroup.subtree_control", null);
        (void)chowmod(fullpath, destuid, nsgid, 0664);
        free(fullpath);

        fullpath = must_make_path(path, "cgroup.threads", null);
        (void)chowmod(fullpath, destuid, nsgid, 0664);
        free(fullpath);
}

Use for (;;) instead of while (1) or while (true)

  • Let's be honest, it is really the only sensible way to do this.

Use The Set Of Supported DCO Statements

  • Signed-off-by: Random J Developer [email protected]
    • You did write this code or have the right to contribute it to LXC.
  • Acked-by: Random J Developer [email protected]
    • You did read the code and think it is correct. This is usually only used by maintainers or developers that have made significant contributions and can vouch for the correctness of someone else's code.
  • Reviewed-by: Random J Developer [email protected]
    • You did review the code and vouch for its correctness, i.e. you'd be prepared to fix bugs it might cause. This is usually only used by maintainers or developers that have made significant contributions and can vouch for the correctness of someone else's code.
  • Co-developed-by: Random J Developer [email protected]
    • The code can not be reasonably attributed to a single developer, i.e. you worked on this together.
  • Tested-by: Random J Developer [email protected]
    • You verified that the code fixes a given bug or is behaving as advertised.
  • Reported-by: Random J Developer [email protected]
    • You found and reported the bug.
  • Suggested-by: Random J Developer [email protected]
    • You wrote the code but someone contributed the idea. This line is usually overlooked but it is a sign of good etiquette and coding ethics: if someone helped you solve a problem or had a clever idea do not silently claim it by slapping your Signed-off-by underneath. Be honest and add a Suggested-by.

Commit Message Outline

  • You must stick to the 80chars limit especially in the title of the commit message.
  • Please use English commit messages only.
  • use meaningful commit messages.
  • Use correct spelling and grammar. If you are not a native speaker and/or feel yourself struggling with this it is perfectly fine to point this out and there's no need to apologize. Usually developers will be happy to pull your branch and adopt the commit message.
  • Please always use the affected file (without the file type suffix) or module as a prefix in the commit message.
  • Examples of good commit messages are: 
    commit b87243830e3b5e95fa31a17cf1bfebe55353bf13
Author: Felix Abecassis <[email protected]>
Date:   Fri Feb 2 06:19:13 2018 -0800

    hooks: change the semantic of NVIDIA_VISIBLE_DEVICES=""

    With LXC, you can override the value of an environment variable to
    null, but you can't unset an existing variable.

    The NVIDIA hook was previously activated when NVIDIA_VISIBLE_DEVICES
    was set to null. As a result, it was not possible to disable the hook
    by overriding the environment variable in the configuration.

    The hook can now be disabled by setting NVIDIA_VISIBLE_DEVICES to
    null or to the new special value "void".

    Signed-off-by: Felix Abecassis <[email protected]>


commit d6337a5f9dc7311af168aa3d586fdf239f5a10d3
Author: Christian Brauner <[email protected]>
Date:   Wed Jan 31 16:25:11 2018 +0100

    cgroups: get controllers on the unified hierarchy

    Signed-off-by: Christian Brauner <[email protected]>

Use _exit() To Terminate fork()ed Child Processes

  • When fork()ing off a child process use _exit() to terminate it instead of exit(). The exit() function is not thread-safe and thus not suited for the shared library which must ensure that it is thread-safe.

Keep Arrays of structs Aligned Horizontally When Initializing

  • Arrays of structs are: 
    struct foo_struct {
      int n;
      int m;
      int p;
};

struct foo_struct new_instance[] = {
        { 1, 2, 3 },
        { 4, 5, 6 },
        { 7, 8, 9 },
};
  • Leave a single space after the opening { and before closing } of the largest member of the last column.
  • Always leave a single space between the largest member of the current column and the member in the next column.
  • A good example is 
    struct signame {
        int num;
        const char *name;
};

static const struct signame signames[] = {
        { SIGHUP,    "HUP"    },
        { SIGINT,    "INT"    },
        { SIGQUIT,   "QUIT"   },
        { SIGILL,    "ILL"    },
        { SIGABRT,   "ABRT"   },
        { SIGFPE,    "FPE"    },
        { SIGKILL,   "KILL"   },
        { SIGSEGV,   "SEGV"   },
        { SIGPIPE,   "PIPE"   },
        { SIGALRM,   "ALRM"   },
        { SIGTERM,   "TERM"   },
        { SIGUSR1,   "USR1"   },
        { SIGUSR2,   "USR2"   },
        { SIGCHLD,   "CHLD"   },
        { SIGCONT,   "CONT"   },
        { SIGSTOP,   "STOP"   },
        { SIGTSTP,   "TSTP"   },
        { SIGTTIN,   "TTIN"   },
        { SIGTTOU,   "TTOU"   },
#ifdef SIGTRAP
        { SIGTRAP,   "TRAP"   },
#endif
#ifdef SIGIOT
        { SIGIOT,    "IOT"    },
#endif
#ifdef SIGEMT
        { SIGEMT,    "EMT"    },
#endif
#ifdef SIGBUS
        { SIGBUS,    "BUS"    },
#endif
#ifdef SIGSTKFLT
        { SIGSTKFLT, "STKFLT" },
#endif
#ifdef SIGCLD
        { SIGCLD,    "CLD"    },
#endif
#ifdef SIGURG
        { SIGURG,    "URG"    },
#endif
#ifdef SIGXCPU
        { SIGXCPU,   "XCPU"   },
#endif
#ifdef SIGXFSZ
        { SIGXFSZ,   "XFSZ"   },
#endif
#ifdef SIGVTALRM
        { SIGVTALRM, "VTALRM" },
#endif
#ifdef SIGPROF
        { SIGPROF,   "PROF"   },
#endif
#ifdef SIGWINCH
        { SIGWINCH,  "WINCH"  },
#endif
#ifdef SIGIO
        { SIGIO,     "IO"     },
#endif
#ifdef SIGPOLL
        { SIGPOLL,   "POLL"   },
#endif
#ifdef SIGINFO
        { SIGINFO,   "INFO"   },
#endif
#ifdef SIGLOST
        { SIGLOST,   "LOST"   },
#endif
#ifdef SIGPWR
        { SIGPWR,    "PWR"    },
#endif
#ifdef SIGUNUSED
        { SIGUNUSED, "UNUSED" },
#endif
#ifdef SIGSYS
        { SIGSYS,    "SYS"    },
#endif
};