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839 lines
35 KiB
Markdown
839 lines
35 KiB
Markdown
---
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title: Coding Style
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category: Contributing
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layout: default
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SPDX-License-Identifier: LGPL-2.1-or-later
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---
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# Coding Style
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## Formatting
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- 8ch indent, no tabs, except for files in `man/` which are 2ch indent, and
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still no tabs, and shell scripts, which are 4ch indent, and no tabs either.
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- We prefer `/* comments */` over `// comments` in code you commit,
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please. This way `// comments` are left for developers to use for local,
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temporary commenting of code for debug purposes (i.e. uncommittable stuff),
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making such comments easily discernible from explanatory, documenting code
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comments (i.e. committable stuff).
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- Don't break code lines too eagerly. We do **not** force line breaks at 80ch,
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all of today's screens should be much larger than that. But then again, don't
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overdo it, ~109ch should be enough really. The `.editorconfig`, `.vimrc` and
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`.dir-locals.el` files contained in the repository will set this limit up for
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you automatically, if you let them (as well as a few other things). Please
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note that emacs loads `.dir-locals.el` automatically, but vim needs to be
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configured to load `.vimrc`, see that file for instructions.
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- If you break a function declaration over multiple lines, do it like this:
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```c
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void some_function(
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int foo,
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bool bar,
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char baz) {
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int a, b, c;
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```
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(i.e. use double indentation — 16 spaces — for the parameter list.)
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- Try to write this:
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```c
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void foo() {
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}
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```
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instead of this:
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```c
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void foo()
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{
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}
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```
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- Function return types should be seen/written as whole, i.e. write this:
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```c
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const char* foo(const char *input);
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```
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instead of this:
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```c
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const char *foo(const char *input);
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```
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- Single-line `if` blocks should not be enclosed in `{}`. Write this:
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```c
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if (foobar)
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waldo();
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```
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instead of this:
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```c
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if (foobar) {
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waldo();
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}
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```
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- Do not write `foo ()`, write `foo()`.
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- `else` blocks should generally start on the same line as the closing `}`:
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```c
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if (foobar) {
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find();
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waldo();
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} else
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dont_find_waldo();
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```
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- Please define flags types like this:
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```c
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typedef enum FoobarFlags {
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FOOBAR_QUUX = 1 << 0,
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FOOBAR_WALDO = 1 << 1,
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FOOBAR_XOXO = 1 << 2,
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…
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} FoobarFlags;
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```
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i.e. use an enum for it, if possible. Indicate bit values via `1 <<`
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expressions, and align them vertically. Define both an enum and a type for
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it.
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- If you define (non-flags) enums, follow this template:
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```c
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typedef enum FoobarMode {
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FOOBAR_AAA,
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FOOBAR_BBB,
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FOOBAR_CCC,
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…
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_FOOBAR_MAX,
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_FOOBAR_INVALID = -EINVAL,
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} FoobarMode;
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```
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i.e. define a `_MAX` enum for the largest defined enum value, plus one. Since
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this is not a regular enum value, prefix it with `_`. Also, define a special
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"invalid" enum value, and set it to `-EINVAL`. That way the enum type can
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safely be used to propagate conversion errors.
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- If you define an enum in a public API, be extra careful, as the size of the
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enum might change when new values are added, which would break ABI
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compatibility. Since we typically want to allow adding new enum values to an
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existing enum type with later API versions, please use the
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`_SD_ENUM_FORCE_S64()` macro in the enum definition, which forces the size of
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the enum to be signed 64-bit wide.
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- Empty lines to separate code blocks are a good thing, please add them
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abundantly. However, please stick to one at a time, i.e. multiple empty lines
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immediately following each other are not OK. Also, we try to keep function
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calls and their immediate error handling together. Hence:
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```c
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/* → empty line here is good */
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r = some_function(…);
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/* → empty line here would be bad */
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if (r < 0)
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return log_error_errno(r, "Some function failed: %m");
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/* → empty line here is good */
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```
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- In shell scripts, do not use whitespace after the redirection operator
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(`>some/file` instead of `> some/file`, `<<EOF` instead of `<< EOF`).
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## Code Organization and Semantics
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- For our codebase we intend to use ISO C11 *with* GNU extensions (aka
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"gnu11"). Public APIs (i.e. those we expose via `libsystemd.so`
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i.e. `systemd/sd-*.h`) should only use ISO C89 however (with a very limited
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set of conservative and common extensions, such as fixed size integer types
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from `<inttypes.h>`), so that we don't force consuming programs into C11
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mode. (This discrepancy in particular means one thing: internally we use C99
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`bool` booleans, externally C89-compatible `int` booleans which generally
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have different size in memory and slightly different semantics, also see
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below.) Both for internal and external code it's OK to use even newer
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features and GCC extension than "gnu11", as long as there's reasonable
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fallback #ifdeffery in place to ensure compatibility is retained with older
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compilers.
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- Please name structures in `PascalCase` (with exceptions, such as public API
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structs), variables and functions in `snake_case`.
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- Avoid static variables, except for caches and very few other cases. Think
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about thread-safety! While most of our code is never used in threaded
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environments, at least the library code should make sure it works correctly
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in them. Instead of doing a lot of locking for that, we tend to prefer using
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TLS to do per-thread caching (which only works for small, fixed-size cache
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objects), or we disable caching for any thread that is not the main
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thread. Use `is_main_thread()` to detect whether the calling thread is the
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main thread.
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- Typically, function parameters fit into four categories: input parameters,
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mutable objects, call-by-reference return parameters that are initialized on
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success, and call-by-reference return parameters that are initialized on
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failure. Input parameters should always carry suitable `const` declarators if
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they are pointers, to indicate they are input-only and not changed by the
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function. The name of return parameters that are initialized on success
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should be prefixed with `ret_`, to clarify they are return parameters. The
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name of return parameters that are initialized on failure should be prefixed
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with `reterr_`. (Examples of such parameters: those which carry additional
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error information, such as the row/column of parse errors or so). –
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Conversely, please do not prefix parameters that aren't output-only with
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`ret_` or `reterr_`, in particular not mutable parameters that are both input
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as well as output.
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Example:
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```c
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static int foobar_frobnicate(
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Foobar *object, /* the associated mutable object */
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const char *input, /* immutable input parameter */
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char **ret_frobnicated, /* return parameter on success */
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unsigned *reterr_line, /* return parameter on failure */
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unsigned *reterr_column) { /* ditto */
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…
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return 0;
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}
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```
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- Do not write functions that clobber call-by-reference success return
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parameters on failure (i.e. `ret_xyz`, see above), or that clobber
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call-by-reference failure return parameters on success
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(i.e. `reterr_xyz`). Use temporary variables for these cases and change the
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passed in variables only in the right condition. The rule is: never clobber
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success return parameters on failure, always initialize success return
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parameters on success (and the reverse for failure return parameters, of
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course).
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- Please put `reterr_` return parameters in the function parameter list last,
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and `ret_` return parameters immediately before that.
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Good:
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```c
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static int do_something(
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const char *input,
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const char *ret_on_success,
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const char *reterr_on_failure);
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```
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Not good:
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```c
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static int do_something(
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const char *reterr_on_failure,
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const char *ret_on_success,
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const char *input);
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```
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- The order in which header files are included doesn't matter too
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much. systemd-internal headers must not rely on an include order, so it is
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safe to include them in any order possible. However, to not clutter global
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includes, and to make sure internal definitions will not affect global
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headers, please always include the headers of external components first
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(these are all headers enclosed in <>), followed by our own exported headers
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(usually everything that's prefixed by `sd-`), and then followed by internal
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headers. Furthermore, in all three groups, order all includes alphabetically
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so duplicate includes can easily be detected.
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- Please avoid using global variables as much as you can. And if you do use
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them make sure they are static at least, instead of exported. Especially in
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library-like code it is important to avoid global variables. Why are global
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variables bad? They usually hinder generic reusability of code (since they
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break in threaded programs, and usually would require locking there), and as
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the code using them has side-effects make programs non-transparent. That
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said, there are many cases where they explicitly make a lot of sense, and are
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OK to use. For example, the log level and target in `log.c` is stored in a
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global variable, and that's OK and probably expected by most. Also in many
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cases we cache data in global variables. If you add more caches like this,
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please be careful however, and think about threading. Only use static
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variables if you are sure that thread-safety doesn't matter in your
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case. Alternatively, consider using TLS, which is pretty easy to use with
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gcc's `thread_local` concept. It's also OK to store data that is inherently
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global in global variables, for example, data parsed from command lines, see
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below.
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- Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are
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incompatible with glibc it's on them. However, if there are equivalent POSIX
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and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there
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aren't, we are happy to use GNU or Linux APIs, and expect non-GNU
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implementations of libc to catch up with glibc.
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## Using C Constructs
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- Allocate local variables where it makes sense: at the top of the block, or at
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the point where they can be initialized. Avoid huge variable declaration
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lists at the top of the function.
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As an exception, `int r` is typically used for a local state variable, but
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should almost always be declared as the last variable at the top of the
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function.
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```c
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{
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uint64_t a;
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int r;
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r = frobnicate(&a);
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if (r < 0)
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…
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uint64_t b = a + 1, c;
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r = foobarify(a, b, &c);
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if (r < 0)
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…
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const char *pretty = prettify(a, b, c);
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…
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}
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```
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- Do not mix multiple variable definitions with function invocations or
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complicated expressions:
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```c
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{
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uint64_t x = 7;
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int a;
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a = foobar();
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}
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```
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instead of:
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```c
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{
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int a = foobar();
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uint64_t x = 7;
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}
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```
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- Use `goto` for cleaning up, and only use it for that. I.e. you may only jump
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to the end of a function, and little else. Never jump backwards!
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- To minimize strict aliasing violations, we prefer unions over casting.
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- Instead of using `memzero()`/`memset()` to initialize structs allocated on
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the stack, please try to use c99 structure initializers. It's short, prettier
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and actually even faster at execution. Hence:
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```c
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struct foobar t = {
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.foo = 7,
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.bar = "bazz",
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};
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```
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instead of:
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```c
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struct foobar t;
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zero(t);
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t.foo = 7;
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t.bar = "bazz";
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```
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- To implement an endless loop, use `for (;;)` rather than `while (1)`. The
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latter is a bit ugly anyway, since you probably really meant `while
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(true)`. To avoid the discussion what the right always-true expression for an
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infinite while loop is, our recommendation is to simply write it without any
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such expression by using `for (;;)`.
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- To determine the length of a constant string `"foo"`, don't bother with
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`sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize
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the call away for fixed strings). The only exception is when declaring an
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array. In that case use `STRLEN()`, which evaluates to a static constant and
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doesn't force the compiler to create a VLA.
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|
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- Please use C's downgrade-to-bool feature only for expressions that are
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actually booleans (or "boolean-like"), and not for variables that are really
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numeric. Specifically, if you have an `int b` and it's only used in a boolean
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sense, by all means check its state with `if (b) …` — but if `b` can actually
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have more than two semantic values, and you want to compare for non-zero,
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then please write that explicitly with `if (b != 0) …`. This helps readability
|
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as the value range and semantical behaviour is directly clear from the
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condition check. As a special addition: when dealing with pointers which you
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want to check for non-NULL-ness, you may also use downgrade-to-bool feature.
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- Please do not use yoda comparisons, i.e. please prefer the more readable `if
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(a == 7)` over the less readable `if (7 == a)`.
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## Destructors
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- The destructors always deregister the object from the next bigger object, not
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the other way around.
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- For robustness reasons, destructors should be able to destruct
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half-initialized objects, too.
|
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- When you define a destructor or `unref()` call for an object, please accept a
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`NULL` object and simply treat this as NOP. This is similar to how libc
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`free()` works, which accepts `NULL` pointers and becomes a NOP for them. By
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following this scheme a lot of `if` checks can be removed before invoking
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your destructor, which makes the code substantially more readable and robust.
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|
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- Related to this: when you define a destructor or `unref()` call for an
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object, please make it return the same type it takes and always return `NULL`
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from it. This allows writing code like this:
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|
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```c
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p = foobar_unref(p);
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```
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which will always work regardless if `p` is initialized or not, and
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guarantees that `p` is `NULL` afterwards, all in just one line.
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## Common Function Naming
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- Name destructor functions that destroy an object in full freeing all its
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memory and associated resources (and thus invalidating the pointer to it)
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`xyz_free()`. Example: `strv_free()`.
|
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- Name destructor functions that destroy only the referenced content of an
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object but leave the object itself allocated `xyz_done()`. If it resets all
|
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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
|
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called `xyz_unref()`. Example: `json_variant_unref()`. Functions that
|
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increase the reference counter by one should be called `xyz_ref()`. Example:
|
||
`json_variant_ref()`
|
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|
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## Error Handling
|
||
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- Error codes are returned as negative `Exxx`. e.g. `return -EINVAL`. There are
|
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some exceptions: for constructors, it is OK to return `NULL` on OOM. For
|
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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
|
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…
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r = RET_NERRNO(unlink(t));
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…
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```
|
||
|
||
or
|
||
|
||
```c
|
||
…
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||
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`. 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()` `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.
|
||
|
||
- 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
|
||
`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 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()`.
|
||
|
||
## Tests
|
||
|
||
- Use the assertion macros from `tests.h` (`ASSERT_GE()`, `ASSERT_OK()`, ...) to
|
||
make sure a descriptive error is logged when an assertion fails. If no assertion
|
||
macro exists for your specific use case, please add a new assertion macro in a
|
||
separate commit.
|
||
|
||
- When modifying existing tests, please convert the test to use the new assertion
|
||
macros from `tests.h` if it is not already using those.
|