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525 lines
22 KiB
Markdown
525 lines
22 KiB
Markdown
---
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title: Coding Style
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---
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# Coding Style
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- 8ch indent, no tabs, except for files in `man/` which are 2ch indent,
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and still no tabs.
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- We prefer `/* comments */` over `// comments` in code you commit, please. This
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way `// comments` are left for developers to use for local, temporary
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commenting of code for debug purposes (i.e. uncommittable stuff), making such
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comments easily discernible from explanatory, documenting code comments
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(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).
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- Variables and functions **must** be static, unless they have a
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prototype, and are supposed to be exported.
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- structs in `PascalCase` (with exceptions, such as public API structs),
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variables and functions in `snake_case`.
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- The destructors always deregister the object from the next bigger
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object, not the other way around.
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- To minimize strict aliasing violations, we prefer unions over casting.
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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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- Error codes are returned as negative `Exxx`. e.g. `return -EINVAL`. There
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are some exceptions: for constructors, it is OK to return `NULL` on
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OOM. For lookup functions, `NULL` is fine too for "not found".
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Be strict with this. When you write a function that can fail due to
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more than one cause, it *really* should have an `int` as the return value
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for the error code.
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- Do not bother with error checking whether writing to stdout/stderr
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worked.
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- Do not log errors from "library" code, only do so from "main
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program" code. (With one exception: it is OK to log with DEBUG level
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from any code, with the exception of maybe inner loops).
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- Always check OOM. There is no excuse. In program code, you can use
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`log_oom()` for then printing a short message, but not in "library" code.
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- Do not issue NSS requests (that includes user name and host name
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lookups) from PID 1 as this might trigger deadlocks when those
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lookups involve synchronously talking to services that we would need
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to start up.
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- Do not synchronously talk to any other service from PID 1, due to
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risk of deadlocks.
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- Avoid fixed-size string buffers, unless you really know the maximum
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size and that maximum size is small. They are a source of errors,
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since they possibly result in truncated strings. It is often nicer
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to use dynamic memory, `alloca()` or VLAs. If you do allocate fixed-size
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strings on the stack, then it is probably only OK if you either
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use a maximum size such as `LINE_MAX`, or count in detail the maximum
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size a string can have. (`DECIMAL_STR_MAX` and `DECIMAL_STR_WIDTH`
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macros are your friends for this!)
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Or in other words, if you use `char buf[256]` then you are likely
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doing something wrong!
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- Stay uniform. For example, always use `usec_t` for time
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values. Do not mix `usec` and `msec`, and `usec` and whatnot.
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- Make use of `_cleanup_free_` and friends. It makes your code much
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nicer to read (and shorter)!
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- Be exceptionally careful when formatting and parsing floating point
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numbers. Their syntax is locale dependent (i.e. `5.000` in en_US is
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generally understood as 5, while in de_DE as 5000.).
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- Try to use 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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But it is OK if you do not.
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- Single-line `if` blocks should not be enclosed in `{}`. Use 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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- Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where
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applicable (i.e. wherever you just care about equality/inequality, not about
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the sorting order).
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- Preferably allocate stack variables on the top of the block:
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```c
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{
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int a, b;
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a = 5;
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b = a;
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}
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```
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- Unless you allocate an array, `double` is always a better choice
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than `float`. Processors speak `double` natively anyway, so there is
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no speed benefit, and on calls like `printf()` `float`s get promoted
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to `double`s anyway, so there is no point.
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- Do not mix function invocations with variable definitions in one
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line. Wrong:
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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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Right:
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```c
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{
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int a;
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uint64_t x = 7;
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a = foobar();
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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
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only jump to the end of a function, and little else. Never jump
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backwards!
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- Think about the types you use. If a value cannot sensibly be
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negative, do not use `int`, but use `unsigned`.
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- Use `char` only for actual characters. Use `uint8_t` or `int8_t`
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when you actually mean a byte-sized signed or unsigned
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integers. When referring to a generic byte, we generally prefer the
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unsigned variant `uint8_t`. Do not use types based on `short`. They
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*never* make sense. Use `int`, `long`, `long long`, all in
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unsigned and signed fashion, and the fixed-size types
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`uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`, `int8_t`, `int16_t`, `int32_t` and so on,
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as well as `size_t`, but nothing else. Do not use kernel types like
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`u32` and so on, leave that to the kernel.
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- Public API calls (i.e. functions exported by our shared libraries)
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must be marked `_public_` and need to be prefixed with `sd_`. No
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other functions should be prefixed like that.
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- In public API calls, you **must** validate all your input arguments for
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programming error with `assert_return()` and return a sensible return
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code. In all other calls, it is recommended to check for programming
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errors with a more brutal `assert()`. We are more forgiving to public
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users than for ourselves! Note that `assert()` and `assert_return()`
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really only should be used for detecting programming errors, not for
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runtime errors. `assert()` and `assert_return()` by usage of `_likely_()`
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inform the compiler that he should not expect these checks to fail,
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and they inform fellow programmers about the expected validity and
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range of parameters.
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- Never use `strtol()`, `atoi()` and similar calls. Use `safe_atoli()`,
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`safe_atou32()` and suchlike instead. They are much nicer to use in
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most cases and correctly check for parsing errors.
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- For every function you add, think about whether it is a "logging"
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function or a "non-logging" function. "Logging" functions do logging
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on their own, "non-logging" function never log on their own and
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expect their callers to log. All functions in "library" code,
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i.e. in `src/shared/` and suchlike must be "non-logging". Every time a
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"logging" function calls a "non-logging" function, it should log
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about the resulting errors. If a "logging" function calls another
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"logging" function, then it should not generate log messages, so
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that log messages are not generated twice for the same errors.
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- If possible, do a combined log & return operation:
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```c
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r = operation(...);
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if (r < 0)
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return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m");
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```
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If the error value is "synthetic", i.e. it was not received from
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the called function, use `SYNTHETIC_ERRNO` wrapper to tell the logging
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system to not log the errno value, but still return it:
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```c
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n = read(..., s, sizeof s);
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if (n != sizeof s)
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return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ...");
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```
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- Avoid static variables, except for caches and very few other
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cases. Think about thread-safety! While most of our code is never
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used in threaded environments, at least the library code should make
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sure it works correctly in them. Instead of doing a lot of locking
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for that, we tend to prefer using TLS to do per-thread caching (which
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only works for small, fixed-size cache objects), or we disable
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caching for any thread that is not the main thread. Use
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`is_main_thread()` to detect whether the calling thread is the main
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thread.
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- Command line option parsing:
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- Do not print full `help()` on error, be specific about the error.
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- Do not print messages to stdout on error.
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- Do not POSIX_ME_HARDER unless necessary, i.e. avoid `+` in option string.
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- Do not write functions that clobber call-by-reference variables on
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failure. Use temporary variables for these cases and change the
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passed in variables only on success.
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- When you allocate a file descriptor, it should be made `O_CLOEXEC`
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right from the beginning, as none of our files should leak to forked
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binaries by default. Hence, whenever you open a file, `O_CLOEXEC` must
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be specified, right from the beginning. This also applies to
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sockets. Effectively, this means that all invocations to:
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- `open()` must get `O_CLOEXEC` passed,
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- `socket()` and `socketpair()` must get `SOCK_CLOEXEC` passed,
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- `recvmsg()` must get `MSG_CMSG_CLOEXEC` set,
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- `F_DUPFD_CLOEXEC` should be used instead of `F_DUPFD`, and so on,
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- invocations of `fopen()` should take `e`.
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- We never use the POSIX version of `basename()` (which glibc defines it in
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`libgen.h`), only the GNU version (which glibc defines in `string.h`).
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The only reason to include `libgen.h` is because `dirname()`
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is needed. Every time you need that please immediately undefine
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`basename()`, and add a comment about it, so that no code ever ends up
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using the POSIX version!
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- Use the bool type for booleans, not integers. One exception: in public
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headers (i.e those in `src/systemd/sd-*.h`) use integers after all, as `bool`
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is C99 and in our public APIs we try to stick to C89 (with a few extension).
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- When you invoke certain calls like `unlink()`, or `mkdir_p()` and you
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know it is safe to ignore the error it might return (because a later
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call would detect the failure anyway, or because the error is in an
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error path and you thus couldn't do anything about it anyway), then
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make this clear by casting the invocation explicitly to `(void)`. Code
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checks like Coverity understand that, and will not complain about
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ignored error codes. Hence, please use this:
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```c
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(void) unlink("/foo/bar/baz");
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```
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instead of just this:
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```c
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unlink("/foo/bar/baz");
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```
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Don't cast function calls to `(void)` that return no error
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conditions. Specifically, the various `xyz_unref()` calls that return a `NULL`
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object shouldn't be cast to `(void)`, since not using the return value does not
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hide any errors.
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- Don't invoke `exit()`, ever. It is not replacement for proper error
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handling. Please escalate errors up your call chain, and use normal
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`return` to exit from the main function of a process. If you
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`fork()`ed off a child process, please use `_exit()` instead of `exit()`,
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so that the exit handlers are not run.
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- Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)`
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instead. For two reason: first, you want `O_CLOEXEC` set on the new `fd`
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(see above). Second, `dup()` will happily duplicate your `fd` as 0, 1,
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2, i.e. stdin, stdout, stderr, should those `fd`s be closed. Given the
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special semantics of those `fd`s, it's probably a good idea to avoid
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them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them.
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- When you define a destructor or `unref()` call for an object, please
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accept a `NULL` object and simply treat this as NOP. This is similar
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to how libc `free()` works, which accepts `NULL` pointers and becomes a
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NOP for them. By following this scheme a lot of `if` checks can be
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removed before invoking your destructor, which makes the code
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substantially more readable and robust.
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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
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return `NULL` from it. This allows writing code like this:
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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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- Use `alloca()`, but never forget that it is not OK to invoke `alloca()`
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within a loop or within function call parameters. `alloca()` memory is
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released at the end of a function, and not at the end of a `{}`
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block. Thus, if you invoke it in a loop, you keep increasing the
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stack pointer without ever releasing memory again. (VLAs have better
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behavior in this case, so consider using them as an alternative.)
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Regarding not using `alloca()` within function parameters, see the
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BUGS section of the `alloca(3)` man page.
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- Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)`
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- Instead of using `memzero()`/`memset()` to initialize structs allocated
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on the stack, please try to use c99 structure initializers. It's
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short, prettier 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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- When returning a return code from `main()`, please preferably use
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`EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc.
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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
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it is safe to include them in any order possible.
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However, to not clutter global includes, and to make sure internal
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definitions will not affect global headers, please always include the
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headers of external components first (these are all headers enclosed
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in <>), followed by our own exported headers (usually everything
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that's prefixed by `sd-`), and then followed by internal headers.
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Furthermore, in all three groups, order all includes alphabetically
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so duplicate includes can easily be detected.
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- To implement an endless loop, use `for (;;)` rather than `while (1)`.
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The latter is a bit ugly anyway, since you probably really
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meant `while (true)`. To avoid the discussion what the right
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always-true expression for an infinite while loop is, our
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recommendation is to simply write it without any such expression by
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using `for (;;)`.
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- Never use the `off_t` type, and particularly avoid it in public
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APIs. It's really weirdly defined, as it usually is 64-bit and we
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don't support it any other way, but it could in theory also be
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32-bit. Which one it is depends on a compiler switch chosen by the
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compiled program, which hence corrupts APIs using it unless they can
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also follow the program's choice. Moreover, in systemd we should
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parse values the same way on all architectures and cannot expose
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`off_t` values over D-Bus. To avoid any confusion regarding conversion
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and ABIs, always use simply `uint64_t` directly.
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- Commit message subject lines should be prefixed with an appropriate
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component name of some kind. For example "journal: ", "nspawn: " and
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so on.
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- Do not use "Signed-Off-By:" in your commit messages. That's a kernel
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thing we don't do in the systemd project.
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- Avoid leaving long-running child processes around, i.e. `fork()`s that
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are not followed quickly by an `execv()` in the child. Resource
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management is unclear in this case, and memory CoW will result in
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unexpected penalties in the parent much, much later on.
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- Don't block execution for arbitrary amounts of time using `usleep()`
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or a similar call, unless you really know what you do. Just "giving
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something some time", or so is a lazy excuse. Always wait for the
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proper event, instead of doing time-based poll loops.
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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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- If you want to concatenate two or more strings, consider using `strjoina()`
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or `strjoin()` rather than `asprintf()`, as the latter is a lot slower. This
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matters particularly in inner loops (but note that `strjoina()` cannot be
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used there).
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- Please avoid using global variables as much as you can. And if you
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do use them make sure they are static at least, instead of
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exported. Especially in library-like code it is important to avoid
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global variables. Why are global variables bad? They usually hinder
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generic reusability of code (since they break in threaded programs,
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and usually would require locking there), and as the code using them
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has side-effects make programs non-transparent. That said, there are
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many cases where they explicitly make a lot of sense, and are OK to
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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
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in many cases we cache data in global variables. If you add more
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caches like this, please be careful however, and think about
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threading. Only use static variables if you are sure that
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thread-safety doesn't matter in your case. Alternatively, consider
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using TLS, which is pretty easy to use with gcc's `thread_local`
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concept. It's also OK to store data that is inherently global in
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global variables, for example data parsed from command lines, see
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below.
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- If you parse a command line, and want to store the parsed parameters
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in global variables, please consider prefixing their names with
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`arg_`. We have been following this naming rule in most of our
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tools, and we should continue to do so, as it makes it easy to
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identify command line parameter variables, and makes it clear why it
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is OK that they are global variables.
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- When exposing public C APIs, be careful what function parameters you make
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`const`. For example, a parameter taking a context object should probably not
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be `const`, even if you are writing an otherwise read-only accessor function
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for it. The reason is that making it `const` fixates the contract that your
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call won't alter the object ever, as part of the API. However, that's often
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quite a promise, given that this even prohibits object-internal caching or
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lazy initialization of object variables. Moreover, it's usually not too useful
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for client applications. Hence, please be careful and avoid `const` on object
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parameters, unless you are very sure `const` is appropriate.
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- Make sure to enforce limits on every user controllable resource. If the user
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can allocate resources in your code, your code must enforce some form of
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limits after which it will refuse operation. It's fine if it is hard-coded (at
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least initially), but it needs to be there. This is particularly important
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for objects that unprivileged users may allocate, but also matters for
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everything else any user may allocated.
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- `htonl()`/`ntohl()` and `htons()`/`ntohs()` are weird. Please use `htobe32()` and
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`htobe16()` instead, it's much more descriptive, and actually says what really
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is happening, after all `htonl()` and `htons()` don't operate on `long`s and
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`short`s as their name would suggest, but on `uint32_t` and `uint16_t`. Also,
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"network byte order" is just a weird name for "big endian", hence we might
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want to call it "big endian" right-away.
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|
|
|
- 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/`
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|
- 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.
|
|
|
|
- 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.
|
|
|
|
- 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.
|
|
|
|
- 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).
|
|
|
|
- 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.
|