mirror of
https://git.savannah.gnu.org/git/emacs.git
synced 2024-11-23 07:19:15 +00:00
54 lines
2.3 KiB
C
54 lines
2.3 KiB
C
/* vla.h - variable length arrays
|
|
|
|
Copyright 2014-2021 Free Software Foundation, Inc.
|
|
|
|
This program is free software: you can redistribute it and/or modify
|
|
it under the terms of the GNU General Public License as published by
|
|
the Free Software Foundation; either version 3 of the License, or
|
|
(at your option) any later version.
|
|
|
|
This program is distributed in the hope that it will be useful,
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
GNU General Public License for more details.
|
|
|
|
You should have received a copy of the GNU General Public License
|
|
along with this program. If not, see <https://www.gnu.org/licenses/>.
|
|
|
|
Written by Paul Eggert. */
|
|
|
|
/* The VLA_ELEMS macro does not allocate variable-length arrays (VLAs),
|
|
so it does not have the security or performance issues commonly
|
|
associated with VLAs. VLA_ELEMS is for exploiting a C11 feature
|
|
where a function can start like this:
|
|
|
|
double scan_array (int n, double v[static n])
|
|
|
|
to require a caller to pass a vector V with at least N elements;
|
|
this allows better static checking and performance in some cases.
|
|
In C11 this feature means that V is a VLA, so the feature is
|
|
supported only if __STDC_NO_VLA__ is defined, and for compatibility
|
|
to platforms that do not support VLAs, VLA_ELEMS (n) expands to
|
|
nothing when __STDC_NO_VLA__ is not defined. */
|
|
|
|
/* A function's argument must point to an array with at least N elements.
|
|
Example: 'int main (int argc, char *argv[VLA_ELEMS (argc)]);'. */
|
|
|
|
#ifdef __STDC_NO_VLA__
|
|
# define VLA_ELEMS(n)
|
|
#else
|
|
# define VLA_ELEMS(n) static n
|
|
#endif
|
|
|
|
/* Although C99 requires support for variable-length arrays (VLAs),
|
|
some C compilers never supported VLAs and VLAs are optional in C11.
|
|
VLAs are controversial because their allocation may be unintended
|
|
or awkward to support, and large VLAs might cause security or
|
|
performance problems. GCC can diagnose the use of VLAs via the
|
|
-Wvla and -Wvla-larger-than warnings options, and defining the
|
|
macro GNULIB_NO_VLA disables the allocation of VLAs in Gnulib code.
|
|
|
|
The VLA_ELEMS macro is unaffected by GNULIB_NO_VLA, since it does
|
|
not allocate VLAs. Programs that use VLA_ELEMS should be compiled
|
|
with 'gcc -Wvla-larger-than' instead of with 'gcc -Wvla'. */
|