1 /* vla.h - variable length arrays 2 3 Copyright 2014-2023 Free Software Foundation, Inc. 4 5 This program is free software: you can redistribute it and/or modify 6 it under the terms of the GNU General Public License as published by 7 the Free Software Foundation, either version 3 of the License, or 8 (at your option) any later version. 9 10 This program is distributed in the hope that it will be useful, 11 but WITHOUT ANY WARRANTY; without even the implied warranty of 12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 13 GNU General Public License for more details. 14 15 You should have received a copy of the GNU General Public License 16 along with this program. If not, see <https://www.gnu.org/licenses/>. 17 18 Written by Paul Eggert. */ 19 20 /* The VLA_ELEMS macro does not allocate variable-length arrays (VLAs), 21 so it does not have the security or performance issues commonly 22 associated with VLAs. VLA_ELEMS is for exploiting a C11 feature 23 where a function can start like this: 24 25 double scan_array (int n, double v[static n]) 26 27 to require a caller to pass a vector V with at least N elements; 28 this allows better static checking and performance in some cases. 29 In C11 this feature means that V is a VLA, so the feature is 30 supported only if __STDC_NO_VLA__ is defined, and for compatibility 31 to platforms that do not support VLAs, VLA_ELEMS (n) expands to 32 nothing when __STDC_NO_VLA__ is not defined. */ 33 34 /* A function's argument must point to an array with at least N elements. 35 Example: 'int main (int argc, char *argv[VLA_ELEMS (argc)]);'. */ 36 37 #ifdef __STDC_NO_VLA__ 38 # define VLA_ELEMS(n) 39 #else 40 # define VLA_ELEMS(n) static n 41 #endif 42 43 /* Although C99 requires support for variable-length arrays (VLAs), 44 some C compilers never supported VLAs and VLAs are optional in C11. 45 VLAs are controversial because their allocation may be unintended 46 or awkward to support, and large VLAs might cause security or 47 performance problems. GCC can diagnose the use of VLAs via the 48 -Wvla and -Wvla-larger-than warnings options, and defining the 49 macro GNULIB_NO_VLA disables the allocation of VLAs in Gnulib code. 50 51 The VLA_ELEMS macro is unaffected by GNULIB_NO_VLA, since it does 52 not allocate VLAs. Programs that use VLA_ELEMS should be compiled 53 with 'gcc -Wvla-larger-than' instead of with 'gcc -Wvla'. */