2019-02-02 Paul Eggert <eggert@cs.ucla.edu>
+ vla: add commentary about VLA_ELEMS
+ * lib/vla.h (VLA_ELEMS): Add commentary,
+ some inspired by Bruno Haible’s proposal in:
+ https://lists.gnu.org/r/bug-gnulib/2019-01/msg00109.html
+
dtoastr,ftoastr,ldtoastr: port to c-strtod changes
Decouple these modules from c-strtod. Nowadays it’s reasonable to
assume the C99 signatures for strtod and strtold. Programs that
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)]);'. */
#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'. */