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doc/developer.texi: update the language feature section

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It is currently very out of touch with reality.

* declare we are using C99 fully, rather than C90 plus extensions
* mention our use of stdatomic.h
* mention forbidden C99 features, like VLAs and complex numbers
This commit is contained in:
Anton Khirnov 2022-11-14 17:55:40 +01:00
parent 256db01851
commit 9177556bb9
1 changed files with 12 additions and 37 deletions
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@ -58,54 +58,29 @@ and should try to fix issues their commit causes.
@section C language features
FFmpeg is programmed in the ISO C90 language with a few additional
features from ISO C99, namely:
FFmpeg is programmed in the ISO C99 language, extended with:
@itemize @bullet
@item
the @samp{inline} keyword;
@item
@samp{//} comments;
@item
designated struct initializers (@samp{struct s x = @{ .i = 17 @};});
@item
compound literals (@samp{x = (struct s) @{ 17, 23 @};}).
@item
for loops with variable definition (@samp{for (int i = 0; i < 8; i++)});
@item
Variadic macros (@samp{#define ARRAY(nb, ...) (int[nb + 1])@{ nb, __VA_ARGS__ @}});
@item
Implementation defined behavior for signed integers is assumed to match the
expected behavior for two's complement. Non representable values in integer
casts are binary truncated. Shift right of signed values uses sign extension.
Atomic operations from C11 @file{stdatomic.h}. They are emulated on
architectures/compilers that do not support them, so all FFmpeg-internal code
may use atomics without any extra checks. However, @file{stdatomic.h} must not
be included in public headers, so they stay C99-compatible.
@end itemize
These features are supported by all compilers we care about, so we will not
accept patches to remove their use unless they absolutely do not impair
clarity and performance.
All code must compile with recent versions of GCC and a number of other
currently supported compilers. To ensure compatibility, please do not use
additional C99 features or GCC extensions. Especially watch out for:
Compiler-specific extensions may be used with good reason, but must not be
depended on, i.e. the code must still compile and work with compilers lacking
the extension.
The following C99 features must not be used anywhere in the codebase:
@itemize @bullet
@item
mixing statements and declarations;
variable-length arrays;
@item
@samp{long long} (use @samp{int64_t} instead);
complex numbers;
@item
@samp{__attribute__} not protected by @samp{#ifdef __GNUC__} or similar;
@item
GCC statement expressions (@samp{(x = (@{ int y = 4; y; @})}).
mixed statements and declarations.
@end itemize
@section Code formatting conventions