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Mention 'no VLAs' in the C-standards UDP section.

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Now we're enforcing it in the build, it ought to be documented as
well.
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Simon Tatham 2019-01-02 22:04:57 +00:00
parent 8d6d7a3615
commit 2af10ee8d1
1 changed files with 20 additions and 11 deletions
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@ -42,18 +42,27 @@ to do endianness-dependent operations, and so on.
some of these portability issues. The Windows front end compiles on
both 32- and 64-bit x86 and also Arm.)
Our current choice of C standards version is C99: you can assume that
C99 features are available (in particular \cw{<stdint.h>},
\cw{<stdbool.h>} and \c{inline}), but you shouldn't use things that
are new in C11 (such as \cw{<uchar.h>} or \cw{_Generic}).
Our current choice of C standards version is \e{mostly} C99. With a
couple of exceptions, you can assume that C99 features are available
(in particular \cw{<stdint.h>}, \cw{<stdbool.h>} and \c{inline}), but
you shouldn't use things that are new in C11 (such as \cw{<uchar.h>}
or \cw{_Generic}).
One small exception to that: for historical reasons, we still build in
one configuration which lacks \cw{<inttypes.h>}. So that file is
included centrally in \c{defs.h}, and has a set of workaround
definitions for the \cw{PRIx64}-type macros we use. If you need to use
another one of those macros, you need to add a workaround definition
in \c{defs.h}, and don't casually re-include \cw{<inttypes.h>}
anywhere else in the source file.
The exceptions to that rule are due to the need for Visual Studio
compatibility:
\b Don't use variable-length arrays. Visual Studio doesn't support
them even now that it's adopted the rest of C99. We use \cw{-Wvla}
when building with gcc and clang, to make it easier to avoid
accidentally breaking that rule.
\b For historical reasons, we still build with one older VS version
which lacks \cw{<inttypes.h>}. So that file is included centrally in
\c{defs.h}, and has a set of workaround definitions for the
\cw{PRIx64}-type macros we use. If you need to use another one of
those macros, you need to add a workaround definition in \c{defs.h},
and don't casually re-include \cw{<inttypes.h>} anywhere else in the
source file.
Here are a few portability assumptions that we \e{do} currently allow
(because we'd already have to thoroughly vet the existing code if they