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c3e270f4 FB |
1 | --- |
2 | title: Coding Style | |
3 | --- | |
4 | ||
c1d3483d | 5 | # Coding Style |
82143987 | 6 | |
3b69b18f ZJS |
7 | - 8ch indent, no tabs, except for files in `man/` which are 2ch indent, and |
8 | still no tabs, and shell scripts, which are 4ch indent, and no tabs either. | |
82143987 FA |
9 | |
10 | - We prefer `/* comments */` over `// comments` in code you commit, please. This | |
11 | way `// comments` are left for developers to use for local, temporary | |
12 | commenting of code for debug purposes (i.e. uncommittable stuff), making such | |
3cc306e6 | 13 | comments easily discernible from explanatory, documenting code comments |
82143987 FA |
14 | (i.e. committable stuff). |
15 | ||
16 | - Don't break code lines too eagerly. We do **not** force line breaks at 80ch, | |
17 | all of today's screens should be much larger than that. But then again, don't | |
c90ee834 | 18 | overdo it, ~109ch should be enough really. The `.editorconfig`, `.vimrc` and |
82143987 | 19 | `.dir-locals.el` files contained in the repository will set this limit up for |
3b69b18f ZJS |
20 | you automatically, if you let them (as well as a few other things). Please |
21 | note that emacs loads `.dir-locals.el` automatically, but vim needs to be | |
22 | configured to load `.vimrc`, see that file for instructions. | |
82143987 FA |
23 | |
24 | - Variables and functions **must** be static, unless they have a | |
25 | prototype, and are supposed to be exported. | |
26 | ||
27 | - structs in `PascalCase` (with exceptions, such as public API structs), | |
28 | variables and functions in `snake_case`. | |
29 | ||
30 | - The destructors always deregister the object from the next bigger | |
31 | object, not the other way around. | |
32 | ||
33 | - To minimize strict aliasing violations, we prefer unions over casting. | |
34 | ||
35 | - For robustness reasons, destructors should be able to destruct | |
36 | half-initialized objects, too. | |
37 | ||
38 | - Error codes are returned as negative `Exxx`. e.g. `return -EINVAL`. There | |
39 | are some exceptions: for constructors, it is OK to return `NULL` on | |
40 | OOM. For lookup functions, `NULL` is fine too for "not found". | |
41 | ||
42 | Be strict with this. When you write a function that can fail due to | |
43 | more than one cause, it *really* should have an `int` as the return value | |
44 | for the error code. | |
45 | ||
46 | - Do not bother with error checking whether writing to stdout/stderr | |
47 | worked. | |
48 | ||
49 | - Do not log errors from "library" code, only do so from "main | |
50 | program" code. (With one exception: it is OK to log with DEBUG level | |
51 | from any code, with the exception of maybe inner loops). | |
52 | ||
53 | - Always check OOM. There is no excuse. In program code, you can use | |
54 | `log_oom()` for then printing a short message, but not in "library" code. | |
55 | ||
56 | - Do not issue NSS requests (that includes user name and host name | |
57 | lookups) from PID 1 as this might trigger deadlocks when those | |
58 | lookups involve synchronously talking to services that we would need | |
59 | to start up. | |
60 | ||
61 | - Do not synchronously talk to any other service from PID 1, due to | |
62 | risk of deadlocks. | |
63 | ||
64 | - Avoid fixed-size string buffers, unless you really know the maximum | |
65 | size and that maximum size is small. They are a source of errors, | |
66 | since they possibly result in truncated strings. It is often nicer | |
67 | to use dynamic memory, `alloca()` or VLAs. If you do allocate fixed-size | |
68 | strings on the stack, then it is probably only OK if you either | |
69 | use a maximum size such as `LINE_MAX`, or count in detail the maximum | |
70 | size a string can have. (`DECIMAL_STR_MAX` and `DECIMAL_STR_WIDTH` | |
71 | macros are your friends for this!) | |
72 | ||
73 | Or in other words, if you use `char buf[256]` then you are likely | |
74 | doing something wrong! | |
75 | ||
76 | - Stay uniform. For example, always use `usec_t` for time | |
77 | values. Do not mix `usec` and `msec`, and `usec` and whatnot. | |
78 | ||
79 | - Make use of `_cleanup_free_` and friends. It makes your code much | |
80 | nicer to read (and shorter)! | |
81 | ||
82 | - Be exceptionally careful when formatting and parsing floating point | |
83 | numbers. Their syntax is locale dependent (i.e. `5.000` in en_US is | |
84 | generally understood as 5, while in de_DE as 5000.). | |
85 | ||
86 | - Try to use this: | |
87 | ||
88 | ```c | |
89 | void foo() { | |
90 | } | |
91 | ``` | |
92 | ||
93 | instead of this: | |
94 | ||
95 | ```c | |
96 | void foo() | |
97 | { | |
98 | } | |
99 | ``` | |
100 | ||
101 | But it is OK if you do not. | |
102 | ||
103 | - Single-line `if` blocks should not be enclosed in `{}`. Use this: | |
104 | ||
105 | ```c | |
106 | if (foobar) | |
107 | waldo(); | |
108 | ``` | |
109 | ||
110 | instead of this: | |
111 | ||
112 | ```c | |
113 | if (foobar) { | |
114 | waldo(); | |
115 | } | |
116 | ``` | |
117 | ||
118 | - Do not write `foo ()`, write `foo()`. | |
119 | ||
120 | - Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where | |
121 | applicable (i.e. wherever you just care about equality/inequality, not about | |
122 | the sorting order). | |
123 | ||
124 | - Preferably allocate stack variables on the top of the block: | |
125 | ||
126 | ```c | |
127 | { | |
128 | int a, b; | |
129 | ||
130 | a = 5; | |
131 | b = a; | |
132 | } | |
133 | ``` | |
134 | ||
f36712b7 CM |
135 | - Unless you allocate an array, `double` is always a better choice |
136 | than `float`. Processors speak `double` natively anyway, so there is | |
82143987 FA |
137 | no speed benefit, and on calls like `printf()` `float`s get promoted |
138 | to `double`s anyway, so there is no point. | |
139 | ||
140 | - Do not mix function invocations with variable definitions in one | |
141 | line. Wrong: | |
142 | ||
143 | ```c | |
144 | { | |
145 | int a = foobar(); | |
146 | uint64_t x = 7; | |
147 | } | |
148 | ``` | |
149 | ||
150 | Right: | |
151 | ||
152 | ```c | |
153 | { | |
154 | int a; | |
155 | uint64_t x = 7; | |
156 | ||
157 | a = foobar(); | |
158 | } | |
159 | ``` | |
160 | ||
161 | - Use `goto` for cleaning up, and only use it for that. i.e. you may | |
162 | only jump to the end of a function, and little else. Never jump | |
163 | backwards! | |
164 | ||
165 | - Think about the types you use. If a value cannot sensibly be | |
166 | negative, do not use `int`, but use `unsigned`. | |
167 | ||
168 | - Use `char` only for actual characters. Use `uint8_t` or `int8_t` | |
169 | when you actually mean a byte-sized signed or unsigned | |
170 | integers. When referring to a generic byte, we generally prefer the | |
171 | unsigned variant `uint8_t`. Do not use types based on `short`. They | |
172 | *never* make sense. Use `int`, `long`, `long long`, all in | |
173 | unsigned and signed fashion, and the fixed-size types | |
174 | `uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`, `int8_t`, `int16_t`, `int32_t` and so on, | |
175 | as well as `size_t`, but nothing else. Do not use kernel types like | |
176 | `u32` and so on, leave that to the kernel. | |
177 | ||
178 | - Public API calls (i.e. functions exported by our shared libraries) | |
179 | must be marked `_public_` and need to be prefixed with `sd_`. No | |
180 | other functions should be prefixed like that. | |
181 | ||
182 | - In public API calls, you **must** validate all your input arguments for | |
183 | programming error with `assert_return()` and return a sensible return | |
184 | code. In all other calls, it is recommended to check for programming | |
185 | errors with a more brutal `assert()`. We are more forgiving to public | |
186 | users than for ourselves! Note that `assert()` and `assert_return()` | |
187 | really only should be used for detecting programming errors, not for | |
188 | runtime errors. `assert()` and `assert_return()` by usage of `_likely_()` | |
189 | inform the compiler that he should not expect these checks to fail, | |
190 | and they inform fellow programmers about the expected validity and | |
191 | range of parameters. | |
192 | ||
193 | - Never use `strtol()`, `atoi()` and similar calls. Use `safe_atoli()`, | |
194 | `safe_atou32()` and suchlike instead. They are much nicer to use in | |
195 | most cases and correctly check for parsing errors. | |
196 | ||
197 | - For every function you add, think about whether it is a "logging" | |
198 | function or a "non-logging" function. "Logging" functions do logging | |
199 | on their own, "non-logging" function never log on their own and | |
200 | expect their callers to log. All functions in "library" code, | |
201 | i.e. in `src/shared/` and suchlike must be "non-logging". Every time a | |
202 | "logging" function calls a "non-logging" function, it should log | |
203 | about the resulting errors. If a "logging" function calls another | |
204 | "logging" function, then it should not generate log messages, so | |
205 | that log messages are not generated twice for the same errors. | |
206 | ||
cd7bcfa8 ZJS |
207 | - If possible, do a combined log & return operation: |
208 | ||
209 | ```c | |
210 | r = operation(...); | |
211 | if (r < 0) | |
212 | return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m"); | |
213 | ``` | |
214 | ||
215 | If the error value is "synthetic", i.e. it was not received from | |
216 | the called function, use `SYNTHETIC_ERRNO` wrapper to tell the logging | |
217 | system to not log the errno value, but still return it: | |
218 | ||
219 | ```c | |
220 | n = read(..., s, sizeof s); | |
221 | if (n != sizeof s) | |
222 | return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ..."); | |
223 | ``` | |
224 | ||
82143987 FA |
225 | - Avoid static variables, except for caches and very few other |
226 | cases. Think about thread-safety! While most of our code is never | |
227 | used in threaded environments, at least the library code should make | |
228 | sure it works correctly in them. Instead of doing a lot of locking | |
229 | for that, we tend to prefer using TLS to do per-thread caching (which | |
230 | only works for small, fixed-size cache objects), or we disable | |
231 | caching for any thread that is not the main thread. Use | |
232 | `is_main_thread()` to detect whether the calling thread is the main | |
233 | thread. | |
234 | ||
235 | - Command line option parsing: | |
236 | - Do not print full `help()` on error, be specific about the error. | |
237 | - Do not print messages to stdout on error. | |
238 | - Do not POSIX_ME_HARDER unless necessary, i.e. avoid `+` in option string. | |
239 | ||
240 | - Do not write functions that clobber call-by-reference variables on | |
241 | failure. Use temporary variables for these cases and change the | |
242 | passed in variables only on success. | |
243 | ||
244 | - When you allocate a file descriptor, it should be made `O_CLOEXEC` | |
245 | right from the beginning, as none of our files should leak to forked | |
246 | binaries by default. Hence, whenever you open a file, `O_CLOEXEC` must | |
247 | be specified, right from the beginning. This also applies to | |
248 | sockets. Effectively, this means that all invocations to: | |
249 | ||
250 | - `open()` must get `O_CLOEXEC` passed, | |
251 | - `socket()` and `socketpair()` must get `SOCK_CLOEXEC` passed, | |
252 | - `recvmsg()` must get `MSG_CMSG_CLOEXEC` set, | |
253 | - `F_DUPFD_CLOEXEC` should be used instead of `F_DUPFD`, and so on, | |
254 | - invocations of `fopen()` should take `e`. | |
255 | ||
256 | - We never use the POSIX version of `basename()` (which glibc defines it in | |
257 | `libgen.h`), only the GNU version (which glibc defines in `string.h`). | |
258 | The only reason to include `libgen.h` is because `dirname()` | |
259 | is needed. Every time you need that please immediately undefine | |
260 | `basename()`, and add a comment about it, so that no code ever ends up | |
261 | using the POSIX version! | |
262 | ||
263 | - Use the bool type for booleans, not integers. One exception: in public | |
264 | headers (i.e those in `src/systemd/sd-*.h`) use integers after all, as `bool` | |
265 | is C99 and in our public APIs we try to stick to C89 (with a few extension). | |
266 | ||
267 | - When you invoke certain calls like `unlink()`, or `mkdir_p()` and you | |
268 | know it is safe to ignore the error it might return (because a later | |
269 | call would detect the failure anyway, or because the error is in an | |
270 | error path and you thus couldn't do anything about it anyway), then | |
271 | make this clear by casting the invocation explicitly to `(void)`. Code | |
272 | checks like Coverity understand that, and will not complain about | |
273 | ignored error codes. Hence, please use this: | |
274 | ||
275 | ```c | |
276 | (void) unlink("/foo/bar/baz"); | |
277 | ``` | |
278 | ||
279 | instead of just this: | |
280 | ||
281 | ```c | |
282 | unlink("/foo/bar/baz"); | |
283 | ``` | |
284 | ||
285 | Don't cast function calls to `(void)` that return no error | |
286 | conditions. Specifically, the various `xyz_unref()` calls that return a `NULL` | |
287 | object shouldn't be cast to `(void)`, since not using the return value does not | |
288 | hide any errors. | |
289 | ||
290 | - Don't invoke `exit()`, ever. It is not replacement for proper error | |
291 | handling. Please escalate errors up your call chain, and use normal | |
292 | `return` to exit from the main function of a process. If you | |
293 | `fork()`ed off a child process, please use `_exit()` instead of `exit()`, | |
294 | so that the exit handlers are not run. | |
295 | ||
296 | - Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)` | |
297 | instead. For two reason: first, you want `O_CLOEXEC` set on the new `fd` | |
298 | (see above). Second, `dup()` will happily duplicate your `fd` as 0, 1, | |
299 | 2, i.e. stdin, stdout, stderr, should those `fd`s be closed. Given the | |
300 | special semantics of those `fd`s, it's probably a good idea to avoid | |
301 | them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them. | |
302 | ||
303 | - When you define a destructor or `unref()` call for an object, please | |
304 | accept a `NULL` object and simply treat this as NOP. This is similar | |
305 | to how libc `free()` works, which accepts `NULL` pointers and becomes a | |
306 | NOP for them. By following this scheme a lot of `if` checks can be | |
307 | removed before invoking your destructor, which makes the code | |
308 | substantially more readable and robust. | |
309 | ||
310 | - Related to this: when you define a destructor or `unref()` call for an | |
311 | object, please make it return the same type it takes and always | |
312 | return `NULL` from it. This allows writing code like this: | |
313 | ||
314 | ```c | |
315 | p = foobar_unref(p); | |
316 | ``` | |
317 | ||
318 | which will always work regardless if `p` is initialized or not, and | |
319 | guarantees that `p` is `NULL` afterwards, all in just one line. | |
320 | ||
321 | - Use `alloca()`, but never forget that it is not OK to invoke `alloca()` | |
322 | within a loop or within function call parameters. `alloca()` memory is | |
323 | released at the end of a function, and not at the end of a `{}` | |
324 | block. Thus, if you invoke it in a loop, you keep increasing the | |
325 | stack pointer without ever releasing memory again. (VLAs have better | |
326 | behavior in this case, so consider using them as an alternative.) | |
327 | Regarding not using `alloca()` within function parameters, see the | |
328 | BUGS section of the `alloca(3)` man page. | |
329 | ||
330 | - Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)` | |
331 | ||
332 | - Instead of using `memzero()`/`memset()` to initialize structs allocated | |
333 | on the stack, please try to use c99 structure initializers. It's | |
334 | short, prettier and actually even faster at execution. Hence: | |
335 | ||
336 | ```c | |
337 | struct foobar t = { | |
338 | .foo = 7, | |
339 | .bar = "bazz", | |
340 | }; | |
341 | ``` | |
342 | ||
343 | instead of: | |
344 | ||
345 | ```c | |
346 | struct foobar t; | |
347 | zero(t); | |
348 | t.foo = 7; | |
349 | t.bar = "bazz"; | |
350 | ``` | |
351 | ||
352 | - When returning a return code from `main()`, please preferably use | |
353 | `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc. | |
354 | ||
355 | - The order in which header files are included doesn't matter too | |
356 | much. systemd-internal headers must not rely on an include order, so | |
357 | it is safe to include them in any order possible. | |
358 | However, to not clutter global includes, and to make sure internal | |
359 | definitions will not affect global headers, please always include the | |
360 | headers of external components first (these are all headers enclosed | |
361 | in <>), followed by our own exported headers (usually everything | |
362 | that's prefixed by `sd-`), and then followed by internal headers. | |
363 | Furthermore, in all three groups, order all includes alphabetically | |
364 | so duplicate includes can easily be detected. | |
365 | ||
366 | - To implement an endless loop, use `for (;;)` rather than `while (1)`. | |
367 | The latter is a bit ugly anyway, since you probably really | |
368 | meant `while (true)`. To avoid the discussion what the right | |
369 | always-true expression for an infinite while loop is, our | |
370 | recommendation is to simply write it without any such expression by | |
371 | using `for (;;)`. | |
372 | ||
373 | - Never use the `off_t` type, and particularly avoid it in public | |
374 | APIs. It's really weirdly defined, as it usually is 64-bit and we | |
375 | don't support it any other way, but it could in theory also be | |
376 | 32-bit. Which one it is depends on a compiler switch chosen by the | |
377 | compiled program, which hence corrupts APIs using it unless they can | |
378 | also follow the program's choice. Moreover, in systemd we should | |
379 | parse values the same way on all architectures and cannot expose | |
380 | `off_t` values over D-Bus. To avoid any confusion regarding conversion | |
381 | and ABIs, always use simply `uint64_t` directly. | |
382 | ||
383 | - Commit message subject lines should be prefixed with an appropriate | |
384 | component name of some kind. For example "journal: ", "nspawn: " and | |
385 | so on. | |
386 | ||
387 | - Do not use "Signed-Off-By:" in your commit messages. That's a kernel | |
388 | thing we don't do in the systemd project. | |
389 | ||
390 | - Avoid leaving long-running child processes around, i.e. `fork()`s that | |
391 | are not followed quickly by an `execv()` in the child. Resource | |
392 | management is unclear in this case, and memory CoW will result in | |
393 | unexpected penalties in the parent much, much later on. | |
394 | ||
395 | - Don't block execution for arbitrary amounts of time using `usleep()` | |
396 | or a similar call, unless you really know what you do. Just "giving | |
397 | something some time", or so is a lazy excuse. Always wait for the | |
398 | proper event, instead of doing time-based poll loops. | |
399 | ||
a98dc693 ZJS |
400 | - To determine the length of a constant string `"foo"`, don't bother with |
401 | `sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize | |
402 | the call away for fixed strings). The only exception is when declaring an | |
b2454670 | 403 | array. In that case use STRLEN, which evaluates to a static constant and |
a98dc693 ZJS |
404 | doesn't force the compiler to create a VLA. |
405 | ||
406 | - If you want to concatenate two or more strings, consider using `strjoina()` | |
407 | or `strjoin()` rather than `asprintf()`, as the latter is a lot slower. This | |
408 | matters particularly in inner loops (but note that `strjoina()` cannot be | |
409 | used there). | |
82143987 FA |
410 | |
411 | - Please avoid using global variables as much as you can. And if you | |
412 | do use them make sure they are static at least, instead of | |
413 | exported. Especially in library-like code it is important to avoid | |
414 | global variables. Why are global variables bad? They usually hinder | |
415 | generic reusability of code (since they break in threaded programs, | |
416 | and usually would require locking there), and as the code using them | |
417 | has side-effects make programs non-transparent. That said, there are | |
418 | many cases where they explicitly make a lot of sense, and are OK to | |
419 | use. For example, the log level and target in `log.c` is stored in a | |
420 | global variable, and that's OK and probably expected by most. Also | |
421 | in many cases we cache data in global variables. If you add more | |
422 | caches like this, please be careful however, and think about | |
423 | threading. Only use static variables if you are sure that | |
424 | thread-safety doesn't matter in your case. Alternatively, consider | |
425 | using TLS, which is pretty easy to use with gcc's `thread_local` | |
426 | concept. It's also OK to store data that is inherently global in | |
427 | global variables, for example data parsed from command lines, see | |
428 | below. | |
429 | ||
430 | - If you parse a command line, and want to store the parsed parameters | |
431 | in global variables, please consider prefixing their names with | |
432 | `arg_`. We have been following this naming rule in most of our | |
433 | tools, and we should continue to do so, as it makes it easy to | |
434 | identify command line parameter variables, and makes it clear why it | |
435 | is OK that they are global variables. | |
436 | ||
437 | - When exposing public C APIs, be careful what function parameters you make | |
438 | `const`. For example, a parameter taking a context object should probably not | |
439 | be `const`, even if you are writing an otherwise read-only accessor function | |
440 | for it. The reason is that making it `const` fixates the contract that your | |
441 | call won't alter the object ever, as part of the API. However, that's often | |
442 | quite a promise, given that this even prohibits object-internal caching or | |
443 | lazy initialization of object variables. Moreover, it's usually not too useful | |
444 | for client applications. Hence, please be careful and avoid `const` on object | |
445 | parameters, unless you are very sure `const` is appropriate. | |
446 | ||
447 | - Make sure to enforce limits on every user controllable resource. If the user | |
448 | can allocate resources in your code, your code must enforce some form of | |
449 | limits after which it will refuse operation. It's fine if it is hard-coded (at | |
450 | least initially), but it needs to be there. This is particularly important | |
451 | for objects that unprivileged users may allocate, but also matters for | |
452 | everything else any user may allocated. | |
453 | ||
454 | - `htonl()`/`ntohl()` and `htons()`/`ntohs()` are weird. Please use `htobe32()` and | |
455 | `htobe16()` instead, it's much more descriptive, and actually says what really | |
456 | is happening, after all `htonl()` and `htons()` don't operate on `long`s and | |
457 | `short`s as their name would suggest, but on `uint32_t` and `uint16_t`. Also, | |
458 | "network byte order" is just a weird name for "big endian", hence we might | |
459 | want to call it "big endian" right-away. | |
460 | ||
461 | - You might wonder what kind of common code belongs in `src/shared/` and what | |
a527f70a ZJS |
462 | belongs in `src/basic/`. The split is like this: anything that is used to |
463 | implement the public shared object we provide (sd-bus, sd-login, sd-id128, | |
464 | nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must | |
465 | be located in `src/basic` (those objects are not allowed to link to | |
b2454670 | 466 | libsystemd-shared.so). Conversely, anything which is shared between multiple |
a527f70a ZJS |
467 | components and does not need to be in `src/basic/`, should be in |
468 | `src/shared/`. | |
82143987 FA |
469 | |
470 | To summarize: | |
471 | ||
472 | `src/basic/` | |
473 | - may be used by all code in the tree | |
474 | - may not use any code outside of `src/basic/` | |
475 | ||
476 | `src/libsystemd/` | |
477 | - may be used by all code in the tree, except for code in `src/basic/` | |
478 | - may not use any code outside of `src/basic/`, `src/libsystemd/` | |
479 | ||
480 | `src/shared/` | |
a527f70a ZJS |
481 | - may be used by all code in the tree, except for code in `src/basic/`, |
482 | `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under | |
483 | `src/journal/` that end up in `libjournal-client.a` convenience library. | |
82143987 FA |
484 | - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/` |
485 | ||
486 | - Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are | |
487 | incompatible with glibc it's on them. However, if there are equivalent POSIX | |
488 | and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there | |
489 | aren't, we are happy to use GNU or Linux APIs, and expect non-GNU | |
490 | implementations of libc to catch up with glibc. | |
491 | ||
492 | - Whenever installing a signal handler, make sure to set `SA_RESTART` for it, so | |
493 | that interrupted system calls are automatically restarted, and we minimize | |
494 | hassles with handling `EINTR` (in particular as `EINTR` handling is pretty broken | |
495 | on Linux). | |
496 | ||
497 | - When applying C-style unescaping as well as specifier expansion on the same | |
498 | string, always apply the C-style unescaping fist, followed by the specifier | |
499 | expansion. When doing the reverse, make sure to escape `%` in specifier-style | |
500 | first (i.e. `%` → `%%`), and then do C-style escaping where necessary. | |
501 | ||
502 | - It's a good idea to use `O_NONBLOCK` when opening 'foreign' regular files, i.e. | |
503 | file system objects that are supposed to be regular files whose paths where | |
504 | specified by the user and hence might actually refer to other types of file | |
505 | system objects. This is a good idea so that we don't end up blocking on | |
506 | 'strange' file nodes, for example if the user pointed us to a FIFO or device | |
507 | node which may block when opening. Moreover even for actual regular files | |
508 | `O_NONBLOCK` has a benefit: it bypasses any mandatory lock that might be in | |
509 | effect on the regular file. If in doubt consider turning off `O_NONBLOCK` again | |
510 | after opening. | |
511 | ||
512 | - When referring to a configuration file option in the documentation and such, | |
513 | please always suffix it with `=`, to indicate that it is a configuration file | |
514 | setting. | |
515 | ||
516 | - When referring to a command line option in the documentation and such, please | |
517 | always prefix with `--` or `-` (as appropriate), to indicate that it is a | |
518 | command line option. | |
519 | ||
520 | - When referring to a file system path that is a directory, please always | |
521 | suffix it with `/`, to indicate that it is a directory, not a regular file | |
522 | (or other file system object). | |
523 | ||
524 | - Don't use `fgets()`, it's too hard to properly handle errors such as overly | |
525 | long lines. Use `read_line()` instead, which is our own function that handles | |
526 | this much nicer. |