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Merge pull request #12296 from poettering/coding-style-sections

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split CODING_STYLE document into multiple thematic sections
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@ -4,14 +4,16 @@ title: Coding Style
# Coding Style
## Formatting
- 8ch indent, no tabs, except for files in `man/` which are 2ch indent, and
still no tabs, and shell scripts, which are 4ch indent, and no tabs either.
- We prefer `/* comments */` over `// comments` in code you commit, please. This
way `// comments` are left for developers to use for local, temporary
commenting of code for debug purposes (i.e. uncommittable stuff), making such
comments easily discernible from explanatory, documenting code comments
(i.e. committable stuff).
- We prefer `/* comments */` over `// comments` in code you commit,
please. This way `// comments` are left for developers to use for local,
temporary commenting of code for debug purposes (i.e. uncommittable stuff),
making such comments easily discernible from explanatory, documenting code
comments (i.e. committable stuff).
- Don't break code lines too eagerly. We do **not** force line breaks at 80ch,
all of today's screens should be much larger than that. But then again, don't
@ -21,69 +23,7 @@ title: Coding Style
note that emacs loads `.dir-locals.el` automatically, but vim needs to be
configured to load `.vimrc`, see that file for instructions.
- Variables and functions **must** be static, unless they have a
prototype, and are supposed to be exported.
- structs in `PascalCase` (with exceptions, such as public API structs),
variables and functions in `snake_case`.
- The destructors always deregister the object from the next bigger
object, not the other way around.
- To minimize strict aliasing violations, we prefer unions over casting.
- For robustness reasons, destructors should be able to destruct
half-initialized objects, too.
- Error codes are returned as negative `Exxx`. e.g. `return -EINVAL`. There
are some exceptions: for constructors, it is OK to return `NULL` on
OOM. For lookup functions, `NULL` is fine too for "not found".
Be strict with this. When you write a function that can fail due to
more than one cause, it *really* should have an `int` as the return value
for the error code.
- Do not bother with error checking whether writing to stdout/stderr
worked.
- Do not log errors from "library" code, only do so from "main
program" code. (With one exception: it is OK to log with DEBUG level
from any code, with the exception of maybe inner loops).
- Always check OOM. There is no excuse. In program code, you can use
`log_oom()` for then printing a short message, but not in "library" code.
- Do not issue NSS requests (that includes user name and host name
lookups) from PID 1 as this might trigger deadlocks when those
lookups involve synchronously talking to services that we would need
to start up.
- Do not synchronously talk to any other service from PID 1, due to
risk of deadlocks.
- Avoid fixed-size string buffers, unless you really know the maximum
size and that maximum size is small. They are a source of errors,
since they possibly result in truncated strings. It is often nicer
to use dynamic memory, `alloca()` or VLAs. If you do allocate fixed-size
strings on the stack, then it is probably only OK if you either
use a maximum size such as `LINE_MAX`, or count in detail the maximum
size a string can have. (`DECIMAL_STR_MAX` and `DECIMAL_STR_WIDTH`
macros are your friends for this!)
Or in other words, if you use `char buf[256]` then you are likely
doing something wrong!
- Stay uniform. For example, always use `usec_t` for time
values. Do not mix `usec` and `msec`, and `usec` and whatnot.
- Make use of `_cleanup_free_` and friends. It makes your code much
nicer to read (and shorter)!
- Be exceptionally careful when formatting and parsing floating point
numbers. Their syntax is locale dependent (i.e. `5.000` in en_US is
generally understood as 5, while in de_DE as 5000.).
- Try to use this:
- Try to write this:
```c
void foo() {
@ -98,9 +38,7 @@ title: Coding Style
}
```
But it is OK if you do not.
- Single-line `if` blocks should not be enclosed in `{}`. Use this:
- Single-line `if` blocks should not be enclosed in `{}`. Write this:
```c
if (foobar)
@ -117,347 +55,51 @@ title: Coding Style
- Do not write `foo ()`, write `foo()`.
- Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where
applicable (i.e. wherever you just care about equality/inequality, not about
the sorting order).
## Code Organization and Semantics
- Preferably allocate stack variables on the top of the block:
- Please name structures in `PascalCase` (with exceptions, such as public API
structs), variables and functions in `snake_case`.
```c
{
int a, b;
a = 5;
b = a;
}
```
- 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.
- Do not mix function invocations with variable definitions in one
line. Wrong:
```c
{
int a = foobar();
uint64_t x = 7;
}
```
Right:
```c
{
int a;
uint64_t x = 7;
a = foobar();
}
```
- Use `goto` for cleaning up, and only use it for that. i.e. you may
only jump to the end of a function, and little else. Never jump
backwards!
- Think about the types you use. If a value cannot sensibly be
negative, do not use `int`, but use `unsigned`.
- Use `char` only for actual characters. Use `uint8_t` or `int8_t`
when you actually mean a byte-sized signed or unsigned
integers. When referring to a generic byte, we generally prefer the
unsigned variant `uint8_t`. Do not use types based on `short`. They
*never* make sense. Use `int`, `long`, `long long`, all in
unsigned and signed fashion, and the fixed-size types
`uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`, `int8_t`, `int16_t`, `int32_t` and so on,
as well as `size_t`, but nothing else. Do not use kernel types like
`u32` and so on, leave that to the kernel.
- Public API calls (i.e. functions exported by our shared libraries)
must be marked `_public_` and need to be prefixed with `sd_`. No
other functions should be prefixed like that.
- In public API calls, you **must** validate all your input arguments for
programming error with `assert_return()` and return a sensible return
code. In all other calls, it is recommended to check for programming
errors with a more brutal `assert()`. We are more forgiving to public
users than for ourselves! Note that `assert()` and `assert_return()`
really only should be used for detecting programming errors, not for
runtime errors. `assert()` and `assert_return()` by usage of `_likely_()`
inform the compiler that he should not expect these checks to fail,
and they inform fellow programmers about the expected validity and
range of parameters.
- Never use `strtol()`, `atoi()` and similar calls. Use `safe_atoli()`,
`safe_atou32()` and suchlike instead. They are much nicer to use in
most cases and correctly check for parsing errors.
- For every function you add, think about whether it is a "logging"
function or a "non-logging" function. "Logging" functions do logging
on their own, "non-logging" function never log on their own and
expect their callers to log. All functions in "library" code,
i.e. in `src/shared/` and suchlike must be "non-logging". Every time a
"logging" function calls a "non-logging" function, it should log
about the resulting errors. If a "logging" function calls another
"logging" function, then it should not generate log messages, so
that log messages are not generated twice for the same errors.
- 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 ...");
```
- Avoid static variables, except for caches and very few other
cases. Think about thread-safety! While most of our code is never
used in threaded environments, at least the library code should make
sure it works correctly in them. Instead of doing a lot of locking
for that, we tend to prefer using TLS to do per-thread caching (which
only works for small, fixed-size cache objects), or we disable
caching for any thread that is not the main thread. Use
`is_main_thread()` to detect whether the calling thread is the main
thread.
- Command line option parsing:
- Do not print full `help()` on error, be specific about the error.
- Do not print messages to stdout on error.
- Do not POSIX_ME_HARDER unless necessary, i.e. avoid `+` in option string.
- 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.
- 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`.
- We never use the POSIX version of `basename()` (which glibc defines it in
`libgen.h`), only the GNU version (which glibc defines in `string.h`).
The only reason to include `libgen.h` is because `dirname()`
is needed. Every time you need that please immediately undefine
`basename()`, and add a comment about it, so that no code ever ends up
using the POSIX version!
- Use the bool type for booleans, not integers. One exception: in public
headers (i.e those in `src/systemd/sd-*.h`) use integers after all, as `bool`
is C99 and in our public APIs we try to stick to C89 (with a few extension).
- 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");
```
Don't cast function calls to `(void)` that return no error
conditions. Specifically, the various `xyz_unref()` calls that return a `NULL`
object shouldn't be cast to `(void)`, since not using the return value does not
hide any errors.
- Don't invoke `exit()`, ever. It is not replacement for proper error
handling. Please escalate errors up your call chain, and use normal
`return` to exit from the main function of a process. If you
`fork()`ed off a child process, please use `_exit()` instead of `exit()`,
so that the exit handlers are not run.
- Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)`
instead. For two reason: first, you want `O_CLOEXEC` set on the new `fd`
(see above). Second, `dup()` will happily duplicate your `fd` as 0, 1,
2, i.e. stdin, stdout, stderr, should those `fd`s be closed. Given the
special semantics of those `fd`s, it's probably a good idea to avoid
them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them.
- When you define a destructor or `unref()` call for an object, please
accept a `NULL` object and simply treat this as NOP. This is similar
to how libc `free()` works, which accepts `NULL` pointers and becomes a
NOP for them. By following this scheme a lot of `if` checks can be
removed before invoking your destructor, which makes the code
substantially more readable and robust.
- Related to this: when you define a destructor or `unref()` call for an
object, please make it return the same type it takes and always
return `NULL` from it. This allows writing code like this:
```c
p = foobar_unref(p);
```
which will always work regardless if `p` is initialized or not, and
guarantees that `p` is `NULL` afterwards, all in just one line.
- Use `alloca()`, but never forget that it is not OK to invoke `alloca()`
within a loop or within function call parameters. `alloca()` memory is
released at the end of a function, and not at the end of a `{}`
block. Thus, if you invoke it in a loop, you keep increasing the
stack pointer without ever releasing memory again. (VLAs have better
behavior in this case, so consider using them as an alternative.)
Regarding not using `alloca()` within function parameters, see the
BUGS section of the `alloca(3)` man page.
- Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)`
- Instead of using `memzero()`/`memset()` to initialize structs allocated
on the stack, please try to use c99 structure initializers. It's
short, prettier and actually even faster at execution. Hence:
```c
struct foobar t = {
.foo = 7,
.bar = "bazz",
};
```
instead of:
```c
struct foobar t;
zero(t);
t.foo = 7;
t.bar = "bazz";
```
- When returning a return code from `main()`, please preferably use
`EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc.
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
much. systemd-internal headers must not rely on an include order, so it is
safe to include them in any order possible. However, to not clutter global
includes, and to make sure internal definitions will not affect global
headers, please always include the headers of external components first
(these are all headers enclosed in <>), followed by our own exported headers
(usually everything that's prefixed by `sd-`), and then followed by internal
headers. Furthermore, in all three groups, order all includes alphabetically
so duplicate includes can easily be detected.
- To implement an endless loop, use `for (;;)` rather than `while (1)`.
The latter is a bit ugly anyway, since you probably really
meant `while (true)`. To avoid the discussion what the right
always-true expression for an infinite while loop is, our
recommendation is to simply write it without any such expression by
using `for (;;)`.
- Never use the `off_t` type, and particularly avoid it in public
APIs. It's really weirdly defined, as it usually is 64-bit and we
don't support it any other way, but it could in theory also be
32-bit. Which one it is depends on a compiler switch chosen by the
compiled program, which hence corrupts APIs using it unless they can
also follow the program's choice. Moreover, in systemd we should
parse values the same way on all architectures and cannot expose
`off_t` values over D-Bus. To avoid any confusion regarding conversion
and ABIs, always use simply `uint64_t` directly.
- Commit message subject lines should be prefixed with an appropriate
component name of some kind. For example "journal: ", "nspawn: " and
so on.
- Do not use "Signed-Off-By:" in your commit messages. That's a kernel
thing we don't do in the systemd project.
- 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 determine the length of a constant string `"foo"`, don't bother with
`sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize
the call away for fixed strings). The only exception is when declaring an
array. In that case use STRLEN, which evaluates to a static constant and
doesn't force the compiler to create a VLA.
- If you want to concatenate two or more strings, consider using `strjoina()`
or `strjoin()` rather than `asprintf()`, as the latter is a lot slower. This
matters particularly in inner loops (but note that `strjoina()` cannot be
used there).
- Please avoid using global variables as much as you can. And if you
do use them make sure they are static at least, instead of
exported. Especially in library-like code it is important to avoid
global variables. Why are global variables bad? They usually hinder
generic reusability of code (since they break in threaded programs,
and usually would require locking there), and as the code using them
has side-effects make programs non-transparent. That said, there are
many cases where they explicitly make a lot of sense, and are OK to
use. For example, the log level and target in `log.c` is stored in a
global variable, and that's OK and probably expected by most. Also
in many cases we cache data in global variables. If you add more
caches like this, please be careful however, and think about
threading. Only use static variables if you are sure that
thread-safety doesn't matter in your case. Alternatively, consider
using TLS, which is pretty easy to use with gcc's `thread_local`
concept. It's also OK to store data that is inherently global in
global variables, for example data parsed from command lines, see
- 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.
- 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.
- 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.
- Make sure to enforce limits on every user controllable resource. If the user
can allocate resources in your code, your code must enforce some form of
limits after which it will refuse operation. It's fine if it is hard-coded (at
least initially), but it needs to be there. This is particularly important
for objects that unprivileged users may allocate, but also matters for
everything else any user may allocated.
- `htonl()`/`ntohl()` and `htons()`/`ntohs()` are weird. Please use `htobe32()` and
`htobe16()` instead, it's much more descriptive, and actually says what really
is happening, after all `htonl()` and `htons()` don't operate on `long`s and
`short`s as their name would suggest, but on `uint32_t` and `uint16_t`. Also,
"network byte order" is just a weird name for "big endian", hence we might
want to call it "big endian" right-away.
- 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,
@ -489,25 +131,348 @@ title: Coding Style
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).
## Using C Constructs
- Preferably allocate local variables on the top of the block:
```c
{
int a, b;
a = 5;
b = a;
}
```
- Do not mix function invocations with variable definitions in one line. Wrong:
```c
{
int a = foobar();
uint64_t x = 7;
}
```
Right:
```c
{
int a;
uint64_t x = 7;
a = foobar();
}
```
- Use `goto` for cleaning up, and only use it for that. i.e. you may only jump
to the end of a function, and little else. Never jump backwards!
- To minimize strict aliasing violations, we prefer unions over casting.
- Instead of using `memzero()`/`memset()` to initialize structs allocated on
the stack, please try to use c99 structure initializers. It's short, prettier
and actually even faster at execution. Hence:
```c
struct foobar t = {
.foo = 7,
.bar = "bazz",
};
```
instead of:
```c
struct foobar t;
zero(t);
t.foo = 7;
t.bar = "bazz";
```
- To implement an endless loop, use `for (;;)` rather than `while (1)`. The
latter is a bit ugly anyway, since you probably really meant `while
(true)`. To avoid the discussion what the right always-true expression for an
infinite while loop is, our recommendation is to simply write it without any
such expression by using `for (;;)`.
- To determine the length of a constant string `"foo"`, don't bother with
`sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize
the call away for fixed strings). The only exception is when declaring an
array. In that case use STRLEN, which evaluates to a static constant and
doesn't force the compiler to create a VLA.
## Destructors
- The destructors always deregister the object from the next bigger object, not
the other way around.
- For robustness reasons, destructors should be able to destruct
half-initialized objects, too.
- When you define a destructor or `unref()` call for an object, please accept a
`NULL` object and simply treat this as NOP. This is similar to how libc
`free()` works, which accepts `NULL` pointers and becomes a NOP for them. By
following this scheme a lot of `if` checks can be removed before invoking
your destructor, which makes the code substantially more readable and robust.
- Related to this: when you define a destructor or `unref()` call for an
object, please make it return the same type it takes and always return `NULL`
from it. This allows writing code like this:
```c
p = foobar_unref(p);
```
which will always work regardless if `p` is initialized or not,x and
guarantees that `p` is `NULL` afterwards, all in just one line.
## Error Handling
- Error codes are returned as negative `Exxx`. e.g. `return -EINVAL`. There are
some exceptions: for constructors, it is OK to return `NULL` on OOM. For
lookup functions, `NULL` is fine too for "not found".
Be strict with this. When you write a function that can fail due to more than
one cause, it *really* should have an `int` as the return value for the error
code.
- 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 he should
not expect these checks to fail, and they inform fellow programmers about the
expected validity and range of parameters.
- When you invoke certain calls like `unlink()`, or `mkdir_p()` and you know it
is safe to ignore the error it might return (because a later call would
detect the failure anyway, or because the error is in an error path and you
thus couldn't do anything about it anyway), then make this clear by casting
the invocation explicitly to `(void)`. Code checks like Coverity understand
that, and will not complain about ignored error codes. Hence, please use
this:
```c
(void) unlink("/foo/bar/baz");
```
instead of just this:
```c
unlink("/foo/bar/baz");
```
Don't cast function calls to `(void)` that return no error
conditions. Specifically, the various `xyz_unref()` calls that return a
`NULL` object shouldn't be cast to `(void)`, since not using the return value
does not hide any errors.
- 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 logging on their own,
"non-logging" function never log on their own and expect their callers to
log. All functions in "library" code, i.e. in `src/shared/` and suchlike must
be "non-logging". Every time a "logging" function calls a "non-logging"
function, it should log about the resulting errors. If a "logging" function
calls another "logging" function, then it should not generate log messages,
so that log messages are not generated twice for the same errors.
- 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. They are a source of errors, since they possibly
result in truncated strings. It is often nicer to use dynamic memory,
`alloca()` or VLAs. If you do allocate fixed-size strings on the stack, then
it is probably only OK if you either use a maximum size such as `LINE_MAX`,
or count in detail the maximum size a string can have. (`DECIMAL_STR_MAX` and
`DECIMAL_STR_WIDTH` macros are your friends for this!)
Or in other words, if you use `char buf[256]` then you are likely doing
something wrong!
- Make use of `_cleanup_free_` and friends. It makes your code much nicer to
read (and shorter)!
- Use `alloca()`, but never forget that it is not OK to invoke `alloca()`
within a loop or within function call parameters. `alloca()` memory is
released at the end of a function, and not at the end of a `{}` block. Thus,
if you invoke it in a loop, you keep increasing the stack pointer without
ever releasing memory again. (VLAs have better behavior in this case, so
consider using them as an alternative.) Regarding not using `alloca()`
within function parameters, see the BUGS section of the `alloca(3)` man page.
- If you want to concatenate two or more strings, consider using `strjoina()`
or `strjoin()` rather than `asprintf()`, as the latter is a lot slower. This
matters particularly in inner loops (but note that `strjoina()` cannot be
used there).
## 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.
- 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
- 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
use `int`, but use `unsigned`.
- Use `char` only for actual characters. Use `uint8_t` or `int8_t` when you
actually mean a byte-sized signed or unsigned integers. When referring to a
generic byte, we generally prefer the unsigned variant `uint8_t`. Do not use
types based on `short`. They *never* make sense. Use `int`, `long`, `long
long`, all in unsigned and signed fashion, and the fixed-size types
`uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`, `int8_t`, `int16_t`, `int32_t`
and so on, as well as `size_t`, but nothing else. Do not use kernel types
like `u32` and so on, leave that to the kernel.
- Stay uniform. For example, always use `usec_t` for time values. Do not mix
`usec` and `msec`, and `usec` and whatnot.
- Never use the `off_t` type, and particularly avoid it in public APIs. It's
really weirdly defined, as it usually is 64-bit and we don't support it any
other way, but it could in theory also be 32-bit. Which one it is depends on
a compiler switch chosen by the compiled program, which hence corrupts APIs
using it unless they can also follow the program's choice. Moreover, in
systemd we should parse values the same way on all architectures and cannot
expose `off_t` values over D-Bus. To avoid any confusion regarding conversion
and ABIs, always use simply `uint64_t` directly.
- Unless you allocate an array, `double` is always a better choice than
`float`. Processors speak `double` natively anyway, so there is no speed
benefit, and on calls like `printf()` `float`s get promoted to `double`s
anyway, so there is no point.
- Use the bool type for booleans, not integers. One exception: in public
headers (i.e those in `src/systemd/sd-*.h`) use integers after all, as `bool`
is C99 and in our public APIs we try to stick to C89 (with a few extension).
## Deadlocks
- Do not issue NSS requests (that includes user name and host name lookups)
from PID 1 as this might trigger deadlocks when those lookups involve
synchronously talking to services that we would need to start up.
- 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
where specified by the user and hence might actually refer to other types of
file system objects. This is a good idea so that we don't end up blocking on
'strange' file nodes, for example if the user pointed us to a FIFO or device
node which may block when opening. Moreover even for actual regular files
`O_NONBLOCK` has a benefit: it bypasses any mandatory lock that might be in
effect on the regular file. If in doubt consider turning off `O_NONBLOCK` again
after opening.
effect on the regular file. If in doubt consider turning off `O_NONBLOCK`
again after opening.
## Command Line
- If you parse a command line, and want to store the parsed parameters in
global variables, please consider prefixing their names with `arg_`. We have
been following this naming rule in most of our tools, and we should continue
to do so, as it makes it easy to identify command line parameter variables,
and makes it clear why it is OK that they are global variables.
- Command line option parsing:
- Do not print full `help()` on error, be specific about the error.
- Do not print messages to stdout on error.
- Do not POSIX_ME_HARDER unless necessary, i.e. avoid `+` in option string.
## Exporting Symbols
- Variables and functions **must** be static, unless they have a prototype, and
are supposed to be exported.
- Public API calls (i.e. functions exported by our shared libraries)
must be marked `_public_` and need to be prefixed with `sd_`. No
other functions should be prefixed like that.
- When exposing public C APIs, be careful what function parameters you make
`const`. For example, a parameter taking a context object should probably not
be `const`, even if you are writing an otherwise read-only accessor function
for it. The reason is that making it `const` fixates the contract that your
call won't alter the object ever, as part of the API. However, that's often
quite a promise, given that this even prohibits object-internal caching or
lazy initialization of object variables. Moreover, it's usually not too
useful for client applications. Hence, please be careful and avoid `const` on
object parameters, unless you are very sure `const` is appropriate.
## Referencing Concepts
- When referring to a configuration file option in the documentation and such,
please always suffix it with `=`, to indicate that it is a configuration file
@ -521,6 +486,52 @@ title: Coding Style
suffix it with `/`, to indicate that it is a directory, not a regular file
(or other file system object).
## Functions to Avoid
- Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)`
- Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where
applicable (i.e. wherever you just care about equality/inequality, not about
the sorting order).
- Never use `strtol()`, `atoi()` and similar calls. Use `safe_atoli()`,
`safe_atou32()` and suchlike instead. They are much nicer to use in most
cases and correctly check for parsing errors.
- `htonl()`/`ntohl()` and `htons()`/`ntohs()` are weird. Please use `htobe32()`
and `htobe16()` instead, it's much more descriptive, and actually says what
really is happening, after all `htonl()` and `htons()` don't operate on
`long`s and `short`s as their name would suggest, but on `uint32_t` and
`uint16_t`. Also, "network byte order" is just a weird name for "big endian",
hence we might want to call it "big endian" right-away.
- Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)` instead. For
two reason: first, you want `O_CLOEXEC` set on the new `fd` (see
above). Second, `dup()` will happily duplicate your `fd` as 0, 1, 2,
i.e. stdin, stdout, stderr, should those `fd`s be closed. Given the special
semantics of those `fd`s, it's probably a good idea to avoid
them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them.
- Don't use `fgets()`, it's too hard to properly handle errors such as overly
long lines. Use `read_line()` instead, which is our own function that handles
this much nicer.
- Don't invoke `exit()`, ever. It is not replacement for proper error
handling. Please escalate errors up your call chain, and use normal `return`
to exit from the main function of a process. If you `fork()`ed off a child
process, please use `_exit()` instead of `exit()`, so that the exit handlers
are not run.
- We never use the POSIX version of `basename()` (which glibc defines it in
`libgen.h`), only the GNU version (which glibc defines in `string.h`). The
only reason to include `libgen.h` is because `dirname()` is needed. Every
time you need that please immediately undefine `basename()`, and add a
comment about it, so that no code ever ends up using the POSIX version!
# Committing to git
- Commit message subject lines should be prefixed with an appropriate component
name of some kind. For example "journal: ", "nspawn: " and so on.
- Do not use "Signed-Off-By:" in your commit messages. That's a kernel thing we
don't do in the systemd project.