- Do not write `foo ()`, write `foo()`.
+## Code Organization and Semantics
+
+- Please name structures in `PascalCase` (with exceptions, such as public API
+ structs), variables and functions in `snake_case`.
+
+- Avoid static variables, except for caches and very few other cases. Think
+ about thread-safety! While most of our code is never used in threaded
+ environments, at least the library code should make sure it works correctly
+ in them. Instead of doing a lot of locking for that, we tend to prefer using
+ TLS to do per-thread caching (which only works for small, fixed-size cache
+ objects), or we disable caching for any thread that is not the main
+ thread. Use `is_main_thread()` to detect whether the calling thread is the
+ main thread.
+
+- Do not write functions that clobber call-by-reference variables on
+ failure. Use temporary variables for these cases and change the passed in
+ variables only on success.
+
+- The order in which header files are included doesn't matter too
+ much. systemd-internal headers must not rely on an include order, so it is
+ safe to include them in any order possible. However, to not clutter global
+ includes, and to make sure internal definitions will not affect global
+ headers, please always include the headers of external components first
+ (these are all headers enclosed in <>), followed by our own exported headers
+ (usually everything that's prefixed by `sd-`), and then followed by internal
+ headers. Furthermore, in all three groups, order all includes alphabetically
+ so duplicate includes can easily be detected.
+
+- Please avoid using global variables as much as you can. And if you do use
+ them make sure they are static at least, instead of exported. Especially in
+ library-like code it is important to avoid global variables. Why are global
+ variables bad? They usually hinder generic reusability of code (since they
+ break in threaded programs, and usually would require locking there), and as
+ the code using them has side-effects make programs non-transparent. That
+ said, there are many cases where they explicitly make a lot of sense, and are
+ OK to use. For example, the log level and target in `log.c` is stored in a
+ global variable, and that's OK and probably expected by most. Also in many
+ cases we cache data in global variables. If you add more caches like this,
+ please be careful however, and think about threading. Only use static
+ variables if you are sure that thread-safety doesn't matter in your
+ case. Alternatively, consider using TLS, which is pretty easy to use with
+ gcc's `thread_local` concept. It's also OK to store data that is inherently
+ global in global variables, for example data parsed from command lines, see
+ below.
+
+- You might wonder what kind of common code belongs in `src/shared/` and what
+ belongs in `src/basic/`. The split is like this: anything that is used to
+ implement the public shared object we provide (sd-bus, sd-login, sd-id128,
+ nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must
+ be located in `src/basic` (those objects are not allowed to link to
+ libsystemd-shared.so). Conversely, anything which is shared between multiple
+ components and does not need to be in `src/basic/`, should be in
+ `src/shared/`.
+
+ To summarize:
+
+ `src/basic/`
+ - may be used by all code in the tree
+ - may not use any code outside of `src/basic/`
+
+ `src/libsystemd/`
+ - may be used by all code in the tree, except for code in `src/basic/`
+ - may not use any code outside of `src/basic/`, `src/libsystemd/`
+
+ `src/shared/`
+ - may be used by all code in the tree, except for code in `src/basic/`,
+ `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
+ `src/journal/` that end up in `libjournal-client.a` convenience library.
+ - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/`
+
+- Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are
+ incompatible with glibc it's on them. However, if there are equivalent POSIX
+ and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there
+ aren't, we are happy to use GNU or Linux APIs, and expect non-GNU
+ implementations of libc to catch up with glibc.
+
+## Using C Constructs
+
- Preferably allocate local variables on the top of the block:
```c
}
```
-## Other
-
-- structs in `PascalCase` (with exceptions, such as public API structs),
- variables and functions in `snake_case`.
-
-- To minimize strict aliasing violations, we prefer unions over casting.
-
-- 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.).
-
-- Do not mix function invocations with variable definitions in one
- line. Wrong:
+- Do not mix function invocations with variable definitions in one line. Wrong:
```c
{
}
```
-- Use `goto` for cleaning up, and only use it for that. i.e. you may
- only jump to the end of a function, and little else. Never jump
- backwards!
-
-- Avoid static variables, except for caches and very few other
- cases. Think about thread-safety! While most of our code is never
- used in threaded environments, at least the library code should make
- sure it works correctly in them. Instead of doing a lot of locking
- for that, we tend to prefer using TLS to do per-thread caching (which
- only works for small, fixed-size cache objects), or we disable
- caching for any thread that is not the main thread. Use
- `is_main_thread()` to detect whether the calling thread is the main
- thread.
-
-- Do not write functions that clobber call-by-reference variables on
- failure. Use temporary variables for these cases and change the
- passed in variables only on success.
+- Use `goto` for cleaning up, and only use it for that. i.e. you may only jump
+ to the end of a function, and little else. Never jump backwards!
+- To minimize strict aliasing violations, we prefer unions over casting.
-- Instead of using `memzero()`/`memset()` to initialize structs allocated
- on the stack, please try to use c99 structure initializers. It's
- short, prettier and actually even faster at execution. Hence:
+- Instead of using `memzero()`/`memset()` to initialize structs allocated on
+ the stack, please try to use c99 structure initializers. It's short, prettier
+ and actually even faster at execution. Hence:
```c
struct foobar t = {
t.bar = "bazz";
```
-- The order in which header files are included doesn't matter too
- much. systemd-internal headers must not rely on an include order, so
- it is safe to include them in any order possible.
- However, to not clutter global includes, and to make sure internal
- definitions will not affect global headers, please always include the
- headers of external components first (these are all headers enclosed
- in <>), followed by our own exported headers (usually everything
- that's prefixed by `sd-`), and then followed by internal headers.
- Furthermore, in all three groups, order all includes alphabetically
- so duplicate includes can easily be detected.
-
-- To implement an endless loop, use `for (;;)` rather than `while (1)`.
- The latter is a bit ugly anyway, since you probably really
- meant `while (true)`. To avoid the discussion what the right
- always-true expression for an infinite while loop is, our
- recommendation is to simply write it without any such expression by
- using `for (;;)`.
-
-- 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.
+- To implement an endless loop, use `for (;;)` rather than `while (1)`. The
+ latter is a bit ugly anyway, since you probably really meant `while
+ (true)`. To avoid the discussion what the right always-true expression for an
+ infinite while loop is, our recommendation is to simply write it without any
+ such expression by using `for (;;)`.
- To determine the length of a constant string `"foo"`, don't bother with
`sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize
array. In that case use STRLEN, which evaluates to a static constant and
doesn't force the compiler to create a VLA.
-- Please avoid using global variables as much as you can. And if you
- do use them make sure they are static at least, instead of
- exported. Especially in library-like code it is important to avoid
- global variables. Why are global variables bad? They usually hinder
- generic reusability of code (since they break in threaded programs,
- and usually would require locking there), and as the code using them
- has side-effects make programs non-transparent. That said, there are
- many cases where they explicitly make a lot of sense, and are OK to
- use. For example, the log level and target in `log.c` is stored in a
- global variable, and that's OK and probably expected by most. Also
- in many cases we cache data in global variables. If you add more
- caches like this, please be careful however, and think about
- threading. Only use static variables if you are sure that
- thread-safety doesn't matter in your case. Alternatively, consider
- using TLS, which is pretty easy to use with gcc's `thread_local`
- concept. It's also OK to store data that is inherently global in
- global variables, for example data parsed from command lines, see
- below.
-
-- Make sure to enforce limits on every user controllable resource. If the user
- can allocate resources in your code, your code must enforce some form of
- limits after which it will refuse operation. It's fine if it is hard-coded (at
- least initially), but it needs to be there. This is particularly important
- for objects that unprivileged users may allocate, but also matters for
- everything else any user may allocated.
-
-- You might wonder what kind of common code belongs in `src/shared/` and what
- belongs in `src/basic/`. The split is like this: anything that is used to
- implement the public shared object we provide (sd-bus, sd-login, sd-id128,
- nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must
- be located in `src/basic` (those objects are not allowed to link to
- libsystemd-shared.so). Conversely, anything which is shared between multiple
- components and does not need to be in `src/basic/`, should be in
- `src/shared/`.
-
- To summarize:
-
- `src/basic/`
- - may be used by all code in the tree
- - may not use any code outside of `src/basic/`
-
- `src/libsystemd/`
- - may be used by all code in the tree, except for code in `src/basic/`
- - may not use any code outside of `src/basic/`, `src/libsystemd/`
-
- `src/shared/`
- - may be used by all code in the tree, except for code in `src/basic/`,
- `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
- `src/journal/` that end up in `libjournal-client.a` convenience library.
- - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/`
-
-- Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are
- incompatible with glibc it's on them. However, if there are equivalent POSIX
- and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there
- aren't, we are happy to use GNU or Linux APIs, and expect non-GNU
- implementations of libc to catch up with glibc.
-
-- 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.
-
## Destructors
- The destructors always deregister the object from the next bigger object, not
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 fist, followed by the specifier
+ expansion. When doing the reverse, make sure to escape `%` in specifier-style
+ first (i.e. `%` → `%%`), and then do C-style escaping where necessary.
+
+- Be exceptionally careful when formatting and parsing floating point
+ numbers. Their syntax is locale dependent (i.e. `5.000` in en_US is generally
+ understood as 5, while in de_DE as 5000.).
+
+- Make sure to enforce limits on every user controllable resource. If the user
+ can allocate resources in your code, your code must enforce some form of
+ limits after which it will refuse operation. It's fine if it is hard-coded
+ (at least initially), but it needs to be there. This is particularly
+ important for objects that unprivileged users may allocate, but also matters
+ for everything else any user may allocated.
+
## Types
- Think about the types you use. If a value cannot sensibly be negative, do not
projects / thirdparty / systemd.git / blobdiff