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title	category	layout	SPDX-License-Identifier
Coding Style	Contributing	default	LGPL-2.1-or-later

Coding Style

Formatting

8ch indent, no tabs, except for files in man/ which are 2ch indent, and still no tabs, and shell scripts, which are 4ch indent, and no tabs either.
We prefer /* comments */ over // comments in code you commit, please. This way // comments are left for developers to use for local, temporary commenting of code for debug purposes (i.e. uncommittable stuff), making such comments easily discernible from explanatory, documenting code comments (i.e. committable stuff).
Don't break code lines too eagerly. We do not force line breaks at 80ch, all of today's screens should be much larger than that. But then again, don't overdo it, ~109ch should be enough really. The .editorconfig, .vimrc and .dir-locals.el files contained in the repository will set this limit up for you automatically, if you let them (as well as a few other things). Please note that emacs loads .dir-locals.el automatically, but vim needs to be configured to load .vimrc, see that file for instructions.

If you break a function declaration over multiple lines, do it like this:

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:
```
void foo() {
}
```
instead of this:
```
void foo()
{
}
```

Single-line if blocks should not be enclosed in {}. Write this:

if (foobar)
        waldo();

instead of this:

if (foobar) {
        waldo();
}

Do not write foo (), write foo().

else blocks should generally start on the same line as the closing }:

if (foobar) {
        find();
        waldo();
} else
        dont_find_waldo();

Please define flags types like this:

typedef enum FoobarFlags {
        FOOBAR_QUUX  = 1 << 0,
        FOOBAR_WALDO = 1 << 1,
        FOOBAR_XOXO  = 1 << 2,
        …
} FoobarFlags;

i.e. use an enum for it, if possible. Indicate bit values via 1 << expressions, and align them vertically. Define both an enum and a type for it.

If you define (non-flags) enums, follow this template:
```
typedef enum FoobarMode {
        FOOBAR_AAA,
        FOOBAR_BBB,
        FOOBAR_CCC,
        …
        _FOOBAR_MAX,
        _FOOBAR_INVALID = -EINVAL,
} FoobarMode;
```
i.e. define a _MAX enum for the largest defined enum value, plus one. Since this is not a regular enum value, prefix it with _. Also, define a special "invalid" enum value, and set it to -EINVAL. That way the enum type can safely be used to propagate conversion errors.
If you define an enum in a public API, be extra careful, as the size of the enum might change when new values are added, which would break ABI compatibility. Since we typically want to allow adding new enum values to an existing enum type with later API versions, please use the _SD_ENUM_FORCE_S64() macro in the enum definition, which forces the size of the enum to be signed 64-bit wide.
Empty lines to separate code blocks are a good thing, please add them abundantly. However, please stick to one at a time, i.e. multiple empty lines immediately following each other are not OK. Also, we try to keep function calls and their immediate error handling together. Hence:
```
/* → empty line here is good */
r = some_function(…);
/* → empty line here would be bad */
if (r < 0)
        return log_error_errno(r, "Some function failed: %m");
/* → empty line here is good */
```
In shell scripts, do not use whitespace after the redirection operator (>some/file instead of > some/file, <<EOF instead of << EOF).

Code Organization and Semantics

For our codebase we intend to use ISO C11 with GNU extensions (aka "gnu11"). Public APIs (i.e. those we expose via libsystemd.so i.e. systemd/sd-*.h) should only use ISO C89 however (with a very limited set of conservative and common extensions, such as fixed size integer types from <inttypes.h>), so that we don't force consuming programs into C11 mode. (This discrepancy in particular means one thing: internally we use C99 bool booleans, externally C89-compatible int booleans which generally have different size in memory and slightly different semantics, also see below.) Both for internal and external code it's OK to use even newer features and GCC extension than "gnu11", as long as there's reasonable fallback #ifdeffery in place to ensure compatibility is retained with older compilers.
Please name structures in PascalCase (with exceptions, such as public API structs), variables and functions in snake_case.
Avoid static variables, except for caches and very few other cases. Think about thread-safety! While most of our code is never used in threaded environments, at least the library code should make sure it works correctly in them. Instead of doing a lot of locking for that, we tend to prefer using TLS to do per-thread caching (which only works for small, fixed-size cache objects), or we disable caching for any thread that is not the main thread. Use is_main_thread() to detect whether the calling thread is the main thread.
Do not write functions that clobber call-by-reference variables on failure. Use temporary variables for these cases and change the passed in variables only on success. The rule is: never clobber return parameters on failure, always initialize return parameters on success.
Typically, function parameters fit into three categories: input parameters, mutable objects, and call-by-reference return parameters. Input parameters should always carry suitable "const" declarators if they are pointers, to indicate they are input-only and not changed by the function. Return parameters are best prefixed with "ret_", to clarify they are return parameters. (Conversely, please do not prefix parameters that aren't output-only with "ret_", in particular not mutable parameters that are both input as well as output). Example:
```
static int foobar_frobnicate(
                Foobar* object,            /* the associated mutable object */
                const char *input,         /* immutable input parameter */
                char **ret_frobnicated) {  /* return parameter */
        …
        return 0;
}
```
The order in which header files are included doesn't matter too much. systemd-internal headers must not rely on an include order, so it is safe to include them in any order possible. However, to not clutter global includes, and to make sure internal definitions will not affect global headers, please always include the headers of external components first (these are all headers enclosed in <>), followed by our own exported headers (usually everything that's prefixed by sd-), and then followed by internal headers. Furthermore, in all three groups, order all includes alphabetically so duplicate includes can easily be detected.
Please avoid using global variables as much as you can. And if you do use them make sure they are static at least, instead of exported. Especially in library-like code it is important to avoid global variables. Why are global variables bad? They usually hinder generic reusability of code (since they break in threaded programs, and usually would require locking there), and as the code using them has side-effects make programs non-transparent. That said, there are many cases where they explicitly make a lot of sense, and are OK to use. For example, the log level and target in log.c is stored in a global variable, and that's OK and probably expected by most. Also in many cases we cache data in global variables. If you add more caches like this, please be careful however, and think about threading. Only use static variables if you are sure that thread-safety doesn't matter in your case. Alternatively, consider using TLS, which is pretty easy to use with gcc's thread_local concept. It's also OK to store data that is inherently global in global variables, for example, data parsed from command lines, see below.
Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are incompatible with glibc it's on them. However, if there are equivalent POSIX and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there aren't, we are happy to use GNU or Linux APIs, and expect non-GNU implementations of libc to catch up with glibc.

Using C Constructs

Allocate local variables where it makes sense: at the top of the block, or at the point where they can be initialized. Avoid huge variable declaration lists at the top of the function.

As an exception, int r is typically used for a local state variable, but should almost always be declared as the last variable at the top of the function.

{
        uint64_t a;
        int r;

        r = frobnicate(&a);
        if (r < 0)
                …

        uint64_t b = a + 1, c;

        r = foobarify(a, b, &c);
        if (r < 0)
                …

        const char *pretty = prettify(a, b, c);
        …
}

Do not mix multiple variable definitions with function invocations or complicated expressions:

{
        uint64_t x = 7;
        int a;

        a = foobar();
}

instead of:

{
        int a = foobar();
        uint64_t x = 7;
}

Use goto for cleaning up, and only use it for that. I.e. you may only jump to the end of a function, and little else. Never jump backwards!
To minimize strict aliasing violations, we prefer unions over casting.
Instead of using memzero()/memset() to initialize structs allocated on the stack, please try to use c99 structure initializers. It's short, prettier and actually even faster at execution. Hence:
```
struct foobar t = {
        .foo = 7,
        .bar = "bazz",
};
```
instead of:
```
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:
```
p = foobar_unref(p);
```
which will always work regardless if p is initialized or not, and guarantees that p is NULL afterwards, all in just one line.

Common Function Naming

Name destructor functions that destroy an object in full freeing all its memory and associated resources (and thus invalidating the pointer to it) xyz_free(). Example: strv_free().
Name destructor functions that destroy only the referenced content of an object but leave the object itself allocated xyz_done(). If it resets all fields so that the object can be reused later call it xyz_clear().
Functions that decrease the reference counter of an object by one should be called xyz_unref(). Example: json_variant_unref(). Functions that increase the reference counter by one should be called xyz_ref(). Example: json_variant_ref()

Error Handling

Error codes are returned as negative Exxx. e.g. return -EINVAL. There are some exceptions: for constructors, it is OK to return NULL on OOM. For lookup functions, NULL is fine too for "not found".

Be strict with this. When you write a function that can fail due to more than one cause, it really should have an int as the return value for the error code.
libc system calls typically return -1 on error (with the error code in errno), and >= 0 on success. Use the RET_NERRNO() helper if you are looking for a simple way to convert this libc style error returning into systemd style error returning. e.g.
```
…
r = RET_NERRNO(unlink(t));
…
```
or
```
…
r = RET_NERRNO(open("/some/file", O_RDONLY|O_CLOEXEC));
…
```
Do not bother with error checking whether writing to stdout/stderr worked.
Do not log errors from "library" code, only do so from "main program" code. (With one exception: it is OK to log with DEBUG level from any code, with the exception of maybe inner loops).
In public API calls, you must validate all your input arguments for programming error with assert_return() and return a sensible return code. In all other calls, it is recommended to check for programming errors with a more brutal assert(). We are more forgiving to public users than for ourselves! Note that assert() and assert_return() really only should be used for detecting programming errors, not for runtime errors. assert() and assert_return() by usage of _likely_() inform the compiler that it should not expect these checks to fail, and they inform fellow programmers about the expected validity and range of parameters.
When you invoke certain calls like unlink(), or mkdir_p() and you know it is safe to ignore the error it might return (because a later call would detect the failure anyway, or because the error is in an error path and you thus couldn't do anything about it anyway), then make this clear by casting the invocation explicitly to (void). Code checks like Coverity understand that, and will not complain about ignored error codes. Hence, please use this:
```
(void) unlink("/foo/bar/baz");
```
instead of just this:
```
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:
```
if (condition_not_met)
        return (void) log_tests_skipped("Cannot run ...");
```
Don't cast function calls to (void) that return no error conditions. Specifically, the various xyz_unref() calls that return a NULL object shouldn't be cast to (void), since not using the return value does not hide any errors.
When returning a return code from main(), please preferably use EXIT_FAILURE and EXIT_SUCCESS as defined by libc.

Logging

For every function you add, think about whether it is a "logging" function or a "non-logging" function. "Logging" functions do (non-debug) logging on their own, "non-logging" functions never log on their own (except at debug level) and expect their callers to log. All functions in "library" code, i.e. in src/shared/ and suchlike must be "non-logging". Every time a "logging" function calls a "non-logging" function, it should log about the resulting errors. If a "logging" function calls another "logging" function, then it should not generate log messages, so that log messages are not generated twice for the same errors. (Note that debug level logging — at syslog level LOG_DEBUG — is not considered logging in this context, debug logging is generally always fine and welcome.)

If possible, do a combined log & return operation:

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:

n = read(..., s, sizeof s);
if (n != sizeof s)
        return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ...");

Memory Allocation

Always check OOM. There is no excuse. In program code, you can use log_oom() for then printing a short message, but not in "library" code.
Avoid fixed-size string buffers, unless you really know the maximum size and that maximum size is small. It is often nicer to use dynamic memory, alloca_safe() or VLAs. If you do allocate fixed-size strings on the stack, then it is probably only OK if you either use a maximum size such as LINE_MAX, or count in detail the maximum size a string can have. (DECIMAL_STR_MAX and DECIMAL_STR_WIDTH macros are your friends for this!)

Or in other words, if you use char buf[256] then you are likely doing something wrong!
Make use of _cleanup_free_ and friends. It makes your code much nicer to read (and shorter)!
Do not use alloca(), strdupa() or strndupa() directly. Use alloca_safe(), strdupa_safe() or strndupa_safe() instead. (The difference is that the latter include an assertion that the specified size is below a safety threshold, so that the program rather aborts than runs into possible stack overruns.)
Use alloca_safe(), but never forget that it is not OK to invoke alloca_safe() within a loop or within function call parameters. alloca_safe() memory is released at the end of a function, and not at the end of a {} block. Thus, if you invoke it in a loop, you keep increasing the stack pointer without ever releasing memory again. (VLAs have better behavior in this case, so consider using them as an alternative.) Regarding not using alloca_safe() within function parameters, see the BUGS section of the alloca(3) man page.
If you want to concatenate two or more strings, consider using strjoina() or strjoin() rather than asprintf(), as the latter is a lot slower. This matters particularly in inner loops (but note that strjoina() cannot be used there).

Runtime Behaviour

Avoid leaving long-running child processes around, i.e. fork()s that are not followed quickly by an execv() in the child. Resource management is unclear in this case, and memory CoW will result in unexpected penalties in the parent much, much later on.
Don't block execution for arbitrary amounts of time using usleep() or a similar call, unless you really know what you do. Just "giving something some time", or so is a lazy excuse. Always wait for the proper event, instead of doing time-based poll loops.
Whenever installing a signal handler, make sure to set SA_RESTART for it, so that interrupted system calls are automatically restarted, and we minimize hassles with handling EINTR (in particular as EINTR handling is pretty broken on Linux).
When applying C-style unescaping as well as specifier expansion on the same string, always apply the C-style unescaping first, followed by the specifier expansion. When doing the reverse, make sure to escape % in specifier-style first (i.e. % → %%), and then do C-style escaping where necessary.
Be exceptionally careful when formatting and parsing floating point numbers. Their syntax is locale dependent (i.e. 5.000 in en_US is generally understood as 5, while in de_DE as 5000.).
Make sure to enforce limits on every user controllable resource. If the user can allocate resources in your code, your code must enforce some form of limits after which it will refuse operation. It's fine if it is hard-coded (at least initially), but it needs to be there. This is particularly important for objects that unprivileged users may allocate, but also matters for everything else any user may allocate.

Types

Think about the types you use. If a value cannot sensibly be negative, do not use int, but use unsigned. We prefer unsigned form to unsigned int.
Use char only for actual characters. Use uint8_t or int8_t when you actually mean a byte-sized signed or unsigned integers. When referring to a generic byte, we generally prefer the unsigned variant uint8_t. Do not use types based on short. They never make sense. Use int, long, long long, all in unsigned and signed fashion, and the fixed-size types uint8_t, uint16_t, uint32_t, uint64_t, int8_t, int16_t, int32_t and so on, as well as size_t, but nothing else. Do not use kernel types like u32 and so on, leave that to the kernel.
Stay uniform. For example, always use usec_t for time values. Do not mix usec and msec, and usec and whatnot.
Never use the off_t type, and particularly avoid it in public APIs. It's really weirdly defined, as it usually is 64-bit and we don't support it any other way, but it could in theory also be 32-bit. Which one it is depends on a compiler switch chosen by the compiled program, which hence corrupts APIs using it unless they can also follow the program's choice. Moreover, in systemd we should parse values the same way on all architectures and cannot expose off_t values over D-Bus. To avoid any confusion regarding conversion and ABIs, always use simply uint64_t directly.
Unless you allocate an array, double is always a better choice than float. Processors speak double natively anyway, so there is no speed benefit, and on calls like printf() floats get promoted to doubles anyway, so there is no point.
Use the bool type for booleans, not integers. One exception: in public headers (i.e those in src/systemd/sd-*.h) use integers after all, as bool is C99 and in our public APIs we try to stick to C89 (with a few extensions; also see above).

Deadlocks

Do not issue NSS requests (that includes user name and hostname lookups) from PID 1 as this might trigger deadlocks when those lookups involve synchronously talking to services that we would need to start up.
Do not synchronously talk to any other service from PID 1, due to risk of deadlocks.

File Descriptors

When you allocate a file descriptor, it should be made O_CLOEXEC right from the beginning, as none of our files should leak to forked binaries by default. Hence, whenever you open a file, O_CLOEXEC must be specified, right from the beginning. This also applies to sockets. Effectively, this means that all invocations to:
- open() must get O_CLOEXEC passed,
- socket() and socketpair() must get SOCK_CLOEXEC passed,
- recvmsg() must get MSG_CMSG_CLOEXEC set,
- F_DUPFD_CLOEXEC should be used instead of F_DUPFD, and so on,
- invocations of fopen() should take e.
It's a good idea to use O_NONBLOCK when opening 'foreign' regular files, i.e. file system objects that are supposed to be regular files whose paths were specified by the user and hence might actually refer to other types of file system objects. This is a good idea so that we don't end up blocking on 'strange' file nodes, for example, if the user pointed us to a FIFO or device node which may block when opening. Moreover even for actual regular files O_NONBLOCK has a benefit: it bypasses any mandatory lock that might be in effect on the regular file. If in doubt consider turning off O_NONBLOCK again after opening.
These days we generally prefer openat()-style file APIs, i.e. APIs that accept a combination of file descriptor and path string, and where the path (if not absolute) is considered relative to the specified file descriptor. When implementing library calls in similar style, please make sure to imply AT_EMPTY_PATH if an empty or NULL path argument is specified (and convert that latter to an empty string). This differs from the underlying kernel semantics, where AT_EMPTY_PATH must always be specified explicitly, and NULL is not accepted as path.

Command Line

If you parse a command line, and want to store the parsed parameters in global variables, please consider prefixing their names with arg_. We have been following this naming rule in most of our tools, and we should continue to do so, as it makes it easy to identify command line parameter variables, and makes it clear why it is OK that they are global variables.
Command line option parsing:
- Do not print full help() on error, be specific about the error.
- Do not print messages to stdout on error.
- Do not POSIX_ME_HARDER unless necessary, i.e. avoid + in option string.

Exporting Symbols

Variables and functions must be static, unless they have a prototype, and are supposed to be exported.
Public API calls (i.e. functions exported by our shared libraries) must be marked _public_ and need to be prefixed with sd_. No other functions should be prefixed like that.
When exposing public C APIs, be careful what function parameters you make const. For example, a parameter taking a context object should probably not be const, even if you are writing an otherwise read-only accessor function for it. The reason is that making it const fixates the contract that your call won't alter the object ever, as part of the API. However, that's often quite a promise, given that this even prohibits object-internal caching or lazy initialization of object variables. Moreover, it's usually not too useful for client applications. Hence, please be careful and avoid const on object parameters, unless you are very sure const is appropriate.

Referencing Concepts

When referring to a configuration file option in the documentation and such, please always suffix it with =, to indicate that it is a configuration file setting.
When referring to a command line option in the documentation and such, please always prefix with -- or - (as appropriate), to indicate that it is a command line option.
When referring to a file system path that is a directory, please always suffix it with /, to indicate that it is a directory, not a regular file (or other file system object).

Functions to Avoid

Use memzero() or even better zero() instead of memset(..., 0, ...)
Please use streq() and strneq() instead of strcmp(), strncmp() where applicable (i.e. wherever you just care about equality/inequality, not about the sorting order).
Never use strtol(), atoi() and similar calls. Use safe_atoli(), safe_atou32() and suchlike instead. They are much nicer to use in most cases and correctly check for parsing errors.
htonl()/ntohl() and htons()/ntohs() are weird. Please use htobe32() and htobe16() instead, it's much more descriptive, and actually says what really is happening, after all htonl() and htons() don't operate on longs and shorts as their name would suggest, but on uint32_t and uint16_t. Also, "network byte order" is just a weird name for "big endian", hence we might want to call it "big endian" right-away.
Use typesafe_inet_ntop(), typesafe_inet_ntop4(), and typesafe_inet_ntop6() instead of inet_ntop(). But better yet, use the IN_ADDR_TO_STRING(), IN4_ADDR_TO_STRING(), and IN6_ADDR_TO_STRING() macros which allocate an anonymous buffer internally.
Please never use dup(). Use fcntl(fd, F_DUPFD_CLOEXEC, 3) instead. For two reasons: first, you want O_CLOEXEC set on the new fd (see above). Second, dup() will happily duplicate your fd as 0, 1, 2, i.e. stdin, stdout, stderr, should those fds be closed. Given the special semantics of those fds, it's probably a good idea to avoid them. F_DUPFD_CLOEXEC with 3 as parameter avoids them.
Don't use fgets(), it's too hard to properly handle errors such as overly long lines. Use read_line() instead, which is our own function that handles this much more nicely.
Don't invoke exit(), ever. It is not replacement for proper error handling. Please escalate errors up your call chain, and use normal return to exit from the main function of a process. If you fork()ed off a child process, please use _exit() instead of exit(), so that the exit handlers are not run.
Do not use basename() or dirname(). The semantics in corner cases are full of pitfalls, and the fact that there are two quite different versions of basename() (one POSIX and one GNU, of which the latter is much more useful) doesn't make it better either. Use path_extract_filename() and path_extract_directory() instead.
Never use FILENAME_MAX. Use PATH_MAX instead (for checking maximum size of paths) and NAME_MAX (for checking maximum size of filenames). FILENAME_MAX is not POSIX, and is a confusingly named alias for PATH_MAX on Linux. Note that NAME_MAX does not include space for a trailing NUL, but PATH_MAX does. UNIX FTW!

Committing to git

Commit message subject lines should be prefixed with an appropriate component name of some kind. For example, "journal: ", "nspawn: " and so on.
Do not use "Signed-Off-By:" in your commit messages. That's a kernel thing we don't do in the systemd project.

Commenting

The best place for code comments and explanations is in the code itself. Only the second best is in git commit messages. The worst place is in the GitHub PR cover letter. Hence, whenever you type a commit message consider for a moment if what you are typing there wouldn't be a better fit for an in-code comment. And if you type the cover letter of a PR, think hard if this wouldn't be better as a commit message or even code comment. Comments are supposed to be useful for somebody who reviews the code, and hence hiding comments in git commits or PR cover letters makes reviews unnecessarily hard. Moreover, while we rely heavily on GitHub's project management infrastructure we'd like to keep everything that can reasonably be kept in the git repository itself in the git repository, so that we can theoretically move things elsewhere with the least effort possible.
It's OK to reference GitHub PRs, GitHub issues and git commits from code comments. Cross-referencing code, issues, and documentation is a good thing.
Reasonable use of non-ASCII Unicode UTF-8 characters in code comments is welcome. If your code comment contains an emoji or two this will certainly brighten the day of the occasional reviewer of your code. Really! 😊

Threading

We generally avoid using threads, to the level this is possible. In particular in the service manager/PID 1 threads are not OK to use. This is because you cannot mix memory allocation in threads with use of glibc's clone() call, or manual clone()/clone3() system call wrappers. Only glibc's own fork() call will properly synchronize the memory allocation locks around the process clone operation. This means that if a process is cloned via clone()/clone3() and another thread currently has the malloc() lock taken, it will be cloned in locked state to the child, and thus can never be acquired in the child, leading to deadlocks. Hence, when using clone()/clone3() there are only two ways out: never use threads in the parent, or never do memory allocation in the child. For our uses we need clone()/clone3() and hence decided to avoid threads. Of course, sometimes the concurrency threads allow is beneficial, however we suggest forking off worker processes rather than worker threads for this purpose, ideally even with an execve() to remove the CoW trap situation fork() easily triggers.
A corollary of the above is: never use clone() where a fork() would do too. Also consider using posix_spawn() which combines clone() + execve() into one and has nice properties since it avoids becoming a CoW trap by using CLONE_VFORK and CLONE_VM together.
While we avoid forking off threads on our own, writing thread-safe code is a good idea where it might end up running inside of libsystemd.so or similar. Hence, use TLS (i.e. thread_local) where appropriate, and maybe the occasional pthread_once().

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