+---
+title: Coding Style
+---
+
# Coding Style
-- 8ch indent, no tabs, except for files in `man/` which are 2ch indent,
- and still no tabs.
+## 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).
+- 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).
-
-- Variables and functions **must** be static, unless they have a
- prototype, and are supposed to be exported.
-
-- structs in `PascalCase` (with exceptions, such as public API structs),
- variables and functions in `snake_case`.
-
-- The destructors always deregister the object from the next bigger
- object, not the other way around.
-
-- To minimize strict aliasing violations, we prefer unions over casting.
-
-- For robustness reasons, destructors should be able to destruct
- half-initialized objects, too.
-
-- 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.
-
-- 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).
-
-- 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.
-
-- Do not issue NSS requests (that includes user name and host name
- lookups) from PID 1 as this might trigger deadlocks when those
- lookups involve synchronously talking to services that we would need
- to start up.
+ 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.
-- Do not synchronously talk to any other service from PID 1, due to
- risk of deadlocks.
-
-- Avoid fixed-size string buffers, unless you really know the maximum
- size and that maximum size is small. They are a source of errors,
- since they possibly result in truncated strings. It is often nicer
- to use dynamic memory, `alloca()` 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!
-
-- Stay uniform. For example, always use `usec_t` for time
- values. Do not mix `usec` and `msec`, and `usec` and whatnot.
-
-- Make use of `_cleanup_free_` and friends. It makes your code much
- nicer to read (and shorter)!
-
-- 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.).
-
-- Try to use this:
+- Try to write this:
```c
void foo() {
}
```
- But it is OK if you do not.
-
-- Single-line `if` blocks should not be enclosed in `{}`. Use this:
+- Single-line `if` blocks should not be enclosed in `{}`. Write this:
```c
if (foobar)
- Do not write `foo ()`, write `foo()`.
-- 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).
+## Code Organization and Semantics
+
+- 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 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.
+
+- You might wonder what kind of common code belongs in `src/shared/` and what
+ belongs in `src/basic/`. The split is like this: anything that is used to
+ implement the public shared object we provide (sd-bus, sd-login, sd-id128,
+ nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must
+ be located in `src/basic` (those objects are not allowed to link to
+ libsystemd-shared.so). Conversely, anything which is shared between multiple
+ components and does not need to be in `src/basic/`, should be in
+ `src/shared/`.
+
+ To summarize:
+
+ `src/basic/`
+ - may be used by all code in the tree
+ - may not use any code outside of `src/basic/`
+
+ `src/libsystemd/`
+ - may be used by all code in the tree, except for code in `src/basic/`
+ - may not use any code outside of `src/basic/`, `src/libsystemd/`
+
+ `src/shared/`
+ - may be used by all code in the tree, except for code in `src/basic/`,
+ `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
+ `src/journal/` that end up in `libjournal-client.a` convenience library.
+ - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/`
+
+- 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.
-- Preferably allocate stack variables on the top of the block:
+## Using C Constructs
+
+- Preferably allocate local variables on the top of the block:
```c
{
}
```
-- Unless you allocate an array, `double` is always the better choice
- than `float`. Processors speak `double` natively anyway, so this is
- no speed benefit, and on calls like `printf()` `float`s get promoted
- to `double`s anyway, so there is no point.
-
-- Do not mix function invocations with variable definitions in one
- line. Wrong:
+- Do not mix function invocations with variable definitions in one line. Wrong:
```c
{
}
```
-- 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!
-
-- Think about the types you use. If a value cannot sensibly be
- negative, do not use `int`, but use `unsigned`.
+- 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!
-- 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.
-
-- 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.
+- To minimize strict aliasing violations, we prefer unions over casting.
-- 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 he should not expect these checks to fail,
- and they inform fellow programmers about the expected validity and
- range of parameters.
+- 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:
-- 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.
-
-- For every function you add, think about whether it is a "logging"
- function or a "non-logging" function. "Logging" functions do logging
- on their own, "non-logging" function never log on their own 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.
+ ```c
+ struct foobar t = {
+ .foo = 7,
+ .bar = "bazz",
+ };
+ ```
-- If possible, do a combined log & return operation:
+ instead of:
```c
- r = operation(...);
- if (r < 0)
- return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m");
+ struct foobar t;
+ zero(t);
+ t.foo = 7;
+ t.bar = "bazz";
```
- 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:
+- 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.
+
+## 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
- n = read(..., s, sizeof s);
- if (n != sizeof s)
- return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ...");
+ p = foobar_unref(p);
```
-- 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.
-
-- 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.
+ which will always work regardless if `p` is initialized or not,x and
+ guarantees that `p` is `NULL` afterwards, all in just one line.
-- 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.
+## Error Handling
-- 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:
+- 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".
- - `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`.
+ 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.
-- We never use the POSIX version of `basename()` (which glibc defines it in
- `libgen.h`), only the GNU version (which glibc defines in `string.h`).
- The only reason to include `libgen.h` is because `dirname()`
- is needed. Every time you need that please immediately undefine
- `basename()`, and add a comment about it, so that no code ever ends up
- using the POSIX version!
+- Do not bother with error checking whether writing to stdout/stderr worked.
-- 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 extension).
+- 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).
-- 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:
+- 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 he 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");
```
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.
-
-- 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.
-
-- Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)`
- instead. For two reason: 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.
-
-- 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.
+ 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.
-- 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.
+- When returning a return code from `main()`, please preferably use
+ `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc.
-- Use `alloca()`, but never forget that it is not OK to invoke `alloca()`
- within a loop or within function call parameters. `alloca()` 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()` within function parameters, see the
- BUGS section of the `alloca(3)` man page.
+## Logging
-- Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)`
+- For every function you add, think about whether it is a "logging" function or
+ a "non-logging" function. "Logging" functions do logging on their own,
+ "non-logging" function never log on their own 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.
-- 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:
+- If possible, do a combined log & return operation:
```c
- struct foobar t = {
- .foo = 7,
- .bar = "bazz",
- };
+ r = operation(...);
+ if (r < 0)
+ return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m");
```
- instead of:
+ 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
- struct foobar t;
- zero(t);
- t.foo = 7;
- t.bar = "bazz";
+ n = read(..., s, sizeof s);
+ if (n != sizeof s)
+ return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ...");
```
-- When returning a return code from `main()`, please preferably use
- `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc.
+## Memory Allocation
-- 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.
-
-- 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 (;;)`.
-
-- 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.
-
-- Commit message subject lines should be prefixed with an appropriate
- component name of some kind. For example "journal: ", "nspawn: " and
- so on.
+- 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.
-- Do not use "Signed-Off-By:" in your commit messages. That's a kernel
- thing we don't do in the systemd project.
+- Avoid fixed-size string buffers, unless you really know the maximum size and
+ that maximum size is small. They are a source of errors, since they possibly
+ result in truncated strings. It is often nicer to use dynamic memory,
+ `alloca()` 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!)
-- 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.
+ Or in other words, if you use `char buf[256]` then you are likely doing
+ something wrong!
-- 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.
+- Make use of `_cleanup_free_` and friends. It makes your code much nicer to
+ read (and shorter)!
-- 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.
+- Use `alloca()`, but never forget that it is not OK to invoke `alloca()`
+ within a loop or within function call parameters. `alloca()` 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()`
+ 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).
-- 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.
+## Runtime Behaviour
-- 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.
+- 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.
-- 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.
+- 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 fist, 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 allocated.
-
-- `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.
+ 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 allocated.
+
+## 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.
-- You might wonder what kind of common code belongs in `src/shared/` and what
- belongs in `src/basic/`. The split is like this: anything that is used to
- implement the public shared object we provide (sd-bus, sd-login, sd-id128,
- nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must
- be located in `src/basic` (those objects are not allowed to link to
- libsystemd-shared.so). Conversely, anything which is shared between multiple
- components and does not need to be in `src/basic/`, should be in
- `src/shared/`.
+- 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.
- To summarize:
+- 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 extension).
- `src/basic/`
- - may be used by all code in the tree
- - may not use any code outside of `src/basic/`
+## Deadlocks
- `src/libsystemd/`
- - may be used by all code in the tree, except for code in `src/basic/`
- - may not use any code outside of `src/basic/`, `src/libsystemd/`
+- Do not issue NSS requests (that includes user name and host name lookups)
+ from PID 1 as this might trigger deadlocks when those lookups involve
+ synchronously talking to services that we would need to start up.
- `src/shared/`
- - may be used by all code in the tree, except for code in `src/basic/`,
- `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
- `src/journal/` that end up in `libjournal-client.a` convenience library.
- - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/`
+- Do not synchronously talk to any other service from PID 1, due to risk of
+ deadlocks.
-- 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.
+## File Descriptors
-- 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 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:
-- When applying C-style unescaping as well as specifier expansion on the same
- string, always apply the C-style unescaping fist, 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.
+ - `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 where
- 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
+- 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
+ where 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.
+ 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
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.
+
+- Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)` instead. For
+ two reason: 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 nicer.
+
+- 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.
+
+- We never use the POSIX version of `basename()` (which glibc defines it in
+ `libgen.h`), only the GNU version (which glibc defines in `string.h`). The
+ only reason to include `libgen.h` is because `dirname()` is needed. Every
+ time you need that please immediately undefine `basename()`, and add a
+ comment about it, so that no code ever ends up using the POSIX version!
+
+# 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.
projects / thirdparty / systemd.git / blobdiff