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+---
+title: Coding Style
+category: Contributing
+layout: default
+SPDX-License-Identifier: 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:
+
+ ```c
+ void some_function(
+ int foo,
+ bool bar,
+ char baz) {
+
+ int a, b, c;
+ ```
+
+ (i.e. use double indentation — 16 spaces — for the parameter list.)
+
+- Try to write this:
+
+ ```c
+ void foo() {
+ }
+ ```
+
+ instead of this:
+
+ ```c
+ void foo()
+ {
+ }
+ ```
+
+- Single-line `if` blocks should not be enclosed in `{}`. Write this:
+
+ ```c
+ if (foobar)
+ waldo();
+ ```
+
+ instead of this:
+
+ ```c
+ if (foobar) {
+ waldo();
+ }
+ ```
+
+- Do not write `foo ()`, write `foo()`.
+
+- `else` blocks should generally start on the same line as the closing `}`:
+ ```c
+ if (foobar) {
+ find();
+ waldo();
+ } else
+ dont_find_waldo();
+ ```
+
+- Please define flags types like this:
+
+ ```c
+ 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:
+
+ ```c
+ 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 64bit wide.
+
+## 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:
+
+ ```c
+ static int foobar_frobnicate(
+ Foobar* object, /* the associated mutable object */
+ const char *input, /* immutable input parameter */
+ char **ret_frobnicated) { /* return parameter */
+ …
+ return 0;
+ }
+ ```
+
+- The order in which header files are included doesn't matter too
+ much. systemd-internal headers must not rely on an include order, so it is
+ safe to include them in any order possible. However, to not clutter global
+ includes, and to make sure internal definitions will not affect global
+ headers, please always include the headers of external components first
+ (these are all headers enclosed in <>), followed by our own exported headers
+ (usually everything that's prefixed by `sd-`), and then followed by internal
+ headers. Furthermore, in all three groups, order all includes alphabetically
+ so duplicate includes can easily be detected.
+
+- Please avoid using global variables as much as you can. And if you do use
+ them make sure they are static at least, instead of exported. Especially in
+ library-like code it is important to avoid global variables. Why are global
+ variables bad? They usually hinder generic reusability of code (since they
+ break in threaded programs, and usually would require locking there), and as
+ the code using them has side-effects make programs non-transparent. That
+ said, there are many cases where they explicitly make a lot of sense, and are
+ OK to use. For example, the log level and target in `log.c` is stored in a
+ global variable, and that's OK and probably expected by most. Also in many
+ cases we cache data in global variables. If you add more caches like this,
+ please be careful however, and think about threading. Only use static
+ variables if you are sure that thread-safety doesn't matter in your
+ case. Alternatively, consider using TLS, which is pretty easy to use with
+ gcc's `thread_local` concept. It's also OK to store data that is inherently
+ global in global variables, for example data parsed from command lines, see
+ below.
+
+- 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, `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.
+
+ ```c
+ {
+ 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:
+
+ ```c
+ {
+ uint64_t x = 7;
+ int a;
+
+ a = foobar();
+ }
+ ```
+
+ instead of:
+
+ ```c
+ {
+ 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:
+
+ ```c
+ struct foobar t = {
+ .foo = 7,
+ .bar = "bazz",
+ };
+ ```
+
+ instead of:
+
+ ```c
+ struct foobar t;
+ zero(t);
+ t.foo = 7;
+ t.bar = "bazz";
+ ```
+
+- To implement an endless loop, use `for (;;)` rather than `while (1)`. The
+ latter is a bit ugly anyway, since you probably really meant `while
+ (true)`. To avoid the discussion what the right always-true expression for an
+ infinite while loop is, our recommendation is to simply write it without any
+ such expression by using `for (;;)`.
+
+- To determine the length of a constant string `"foo"`, don't bother with
+ `sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize
+ the call away for fixed strings). The only exception is when declaring an
+ array. In that case use `STRLEN()`, which evaluates to a static constant and
+ doesn't force the compiler to create a VLA.
+
+- Please use C's downgrade-to-bool feature only for expressions that are
+ actually booleans (or "boolean-like"), and not for variables that are really
+ numeric. Specifically, if you have an `int b` and it's only used in a boolean
+ sense, by all means check its state with `if (b) …` — but if `b` can actually
+ have more than two semantic values, and you want to compare for non-zero,
+ then please write that explicitly with `if (b != 0) …`. This helps readability
+ as the value range and semantical behaviour is directly clear from the
+ condition check. As a special addition: when dealing with pointers which you
+ want to check for non-NULL-ness, you may also use downgrade-to-bool feature.
+
+- Please do not use yoda comparisons, i.e. please prefer the more readable `if
+ (a == 7)` over the less readable `if (7 == a)`.
+
+## Destructors
+
+- The destructors always deregister the object from the next bigger object, not
+ the other way around.
+
+- For robustness reasons, destructors should be able to destruct
+ half-initialized objects, too.
+
+- When you define a destructor or `unref()` call for an object, please accept a
+ `NULL` object and simply treat this as NOP. This is similar to how libc
+ `free()` works, which accepts `NULL` pointers and becomes a NOP for them. By
+ following this scheme a lot of `if` checks can be removed before invoking
+ your destructor, which makes the code substantially more readable and robust.
+
+- Related to this: when you define a destructor or `unref()` call for an
+ object, please make it return the same type it takes and always return `NULL`
+ from it. This allows writing code like this:
+
+ ```c
+ p = foobar_unref(p);
+ ```
+
+ which will always work regardless if `p` is initialized or not, and
+ guarantees that `p` is `NULL` afterwards, all in just one line.
+
+## 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.
+
+ ```c
+ …
+ r = RET_NERRNO(unlink(t));
+ …
+ ```
+
+ or
+
+ ```c
+ …
+ 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:
+
+ ```c
+ (void) unlink("/foo/bar/baz");
+ ```
+
+ instead of just this:
+
+ ```c
+ unlink("/foo/bar/baz");
+ ```
+
+ When returning from a `void` function, you may also want to shorten the error
+ path boilerplate by returning a function invocation cast to `(void)` like so:
+
+ ```c
+ if (condition_not_met)
+ return (void) log_tests_skipped("Cannot run ...");
+ ```
+
+ Don't cast function calls to `(void)` that return no error
+ conditions. Specifically, the various `xyz_unref()` calls that return a
+ `NULL` object shouldn't be cast to `(void)`, since not using the return value
+ does not hide any errors.
+
+- When returning a return code from `main()`, please preferably use
+ `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc.
+
+## Logging
+
+- For every function you add, think about whether it is a "logging" function or
+ a "non-logging" function. "Logging" functions do (non-debug) logging on their
+ own, "non-logging" 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:
+
+ ```c
+ r = operation(...);
+ if (r < 0)
+ return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m");
+ ```
+
+ If the error value is "synthetic", i.e. it was not received from
+ the called function, use `SYNTHETIC_ERRNO` wrapper to tell the logging
+ system to not log the errno value, but still return it:
+
+ ```c
+ n = read(..., s, sizeof s);
+ if (n != sizeof s)
+ return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ...");
+ ```
+
+## Memory Allocation
+
+- Always check OOM. There is no excuse. In program code, you can use
+ `log_oom()` for then printing a short message, but not in "library" code.
+
+- Avoid fixed-size string buffers, unless you really know the maximum size and
+ that maximum size is small. 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`.
+
+- Use `char` only for actual characters. Use `uint8_t` or `int8_t` when you
+ actually mean a byte-sized signed or unsigned integers. When referring to a
+ generic byte, we generally prefer the unsigned variant `uint8_t`. Do not use
+ types based on `short`. They *never* make sense. Use `int`, `long`, `long
+ long`, all in unsigned and signed fashion, and the fixed-size types
+ `uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`, `int8_t`, `int16_t`, `int32_t`
+ and so on, as well as `size_t`, but nothing else. Do not use kernel types
+ like `u32` and so on, leave that to the kernel.
+
+- Stay uniform. For example, always use `usec_t` for time values. Do not mix
+ `usec` and `msec`, and `usec` and whatnot.
+
+- Never use the `off_t` type, and particularly avoid it in public APIs. It's
+ really weirdly defined, as it usually is 64-bit and we don't support it any
+ other way, but it could in theory also be 32-bit. Which one it is depends on
+ a compiler switch chosen by the compiled program, which hence corrupts APIs
+ using it unless they can also follow the program's choice. Moreover, in
+ systemd we should parse values the same way on all architectures and cannot
+ expose `off_t` values over D-Bus. To avoid any confusion regarding conversion
+ and ABIs, always use simply `uint64_t` directly.
+
+- Unless you allocate an array, `double` is always a better choice than
+ `float`. Processors speak `double` natively anyway, so there is no speed
+ benefit, and on calls like `printf()` `float`s get promoted to `double`s
+ anyway, so there is no point.
+
+- Use the bool type for booleans, not integers. One exception: in public
+ headers (i.e those in `src/systemd/sd-*.h`) use integers after all, as `bool`
+ is C99 and in our public APIs we try to stick to C89 (with a few 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.
+
+## Command Line
+
+- If you parse a command line, and want to store the parsed parameters in
+ global variables, please consider prefixing their names with `arg_`. We have
+ been following this naming rule in most of our tools, and we should continue
+ to do so, as it makes it easy to identify command line parameter variables,
+ and makes it clear why it is OK that they are global variables.
+
+- Command line option parsing:
+ - Do not print full `help()` on error, be specific about the error.
+ - Do not print messages to stdout on error.
+ - Do not POSIX_ME_HARDER unless necessary, i.e. avoid `+` in option string.
+
+## Exporting Symbols
+
+- Variables and functions **must** be static, unless they have a prototype, and
+ are supposed to be exported.
+
+- Public API calls (i.e. functions exported by our shared libraries)
+ must be marked `_public_` and need to be prefixed with `sd_`. No
+ other functions should be prefixed like that.
+
+- When exposing public C APIs, be careful what function parameters you make
+ `const`. For example, a parameter taking a context object should probably not
+ be `const`, even if you are writing an otherwise read-only accessor function
+ for it. The reason is that making it `const` fixates the contract that your
+ call won't alter the object ever, as part of the API. However, that's often
+ quite a promise, given that this even prohibits object-internal caching or
+ lazy initialization of object variables. Moreover, it's usually not too
+ useful for client applications. Hence, please be careful and avoid `const` on
+ object parameters, unless you are very sure `const` is appropriate.
+
+## Referencing Concepts
+
+- When referring to a configuration file option in the documentation and such,
+ please always suffix it with `=`, to indicate that it is a configuration file
+ setting.
+
+- When referring to a command line option in the documentation and such, please
+ always prefix with `--` or `-` (as appropriate), to indicate that it is a
+ command line option.
+
+- When referring to a file system path that is a directory, please always
+ suffix it with `/`, to indicate that it is a directory, not a regular file
+ (or other file system object).
+
+## Functions to Avoid
+
+- Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)`
+
+- Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where
+ applicable (i.e. wherever you just care about equality/inequality, not about
+ the sorting order).
+
+- Never use `strtol()`, `atoi()` and similar calls. Use `safe_atoli()`,
+ `safe_atou32()` and suchlike instead. They are much nicer to use in most
+ cases and correctly check for parsing errors.
+
+- `htonl()`/`ntohl()` and `htons()`/`ntohs()` are weird. Please use `htobe32()`
+ and `htobe16()` instead, it's much more descriptive, and actually says what
+ really is happening, after all `htonl()` and `htons()` don't operate on
+ `long`s and `short`s as their name would suggest, but on `uint32_t` and
+ `uint16_t`. Also, "network byte order" is just a weird name for "big endian",
+ hence we might want to call it "big endian" right-away.
+
+- 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 `fd`s be closed. Given the special
+ semantics of those `fd`s, it's probably a good idea to avoid
+ them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them.
+
+- Don't use `fgets()`, it's too hard to properly handle errors such as overly
+ long lines. Use `read_line()` instead, which is our own function that handles
+ this much more nicely.
+
+- Don't invoke `exit()`, ever. It is not replacement for proper error
+ handling. Please escalate errors up your call chain, and use normal `return`
+ to exit from the main function of a process. If you `fork()`ed off a child
+ process, please use `_exit()` instead of `exit()`, so that the exit handlers
+ are not run.
+
+- 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 bette 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 elswhere 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! 😊