diff options
Diffstat (limited to 'lib/vla.h')
| -rw-r--r-- | lib/vla.h | 30 |
1 files changed, 28 insertions, 2 deletions
diff --git a/lib/vla.h b/lib/vla.h index 59de9a6bcd6..ce02428f53a 100644 --- a/lib/vla.h +++ b/lib/vla.h @@ -1,10 +1,10 @@ /* vla.h - variable length arrays - Copyright 2014-2017 Free Software Foundation, Inc. + Copyright 2014-2022 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 + 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, @@ -17,6 +17,20 @@ 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)]);'. */ @@ -25,3 +39,15 @@ #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'. */ |