# HG changeset patch
# User Paul Eggert <eggert@cs.ucla.edu>
# Date 1549147199 28800
# Node ID a8eeb883e3771706305b77d118a2f8087ce2dc72
# Parent  dcd889592b62637829d6c51c2a70d1fa90a3cff3
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

diff -r dcd889592b62 -r a8eeb883e377 ChangeLog
--- a/ChangeLog	Sat Feb 02 13:01:55 2019 -0800
+++ b/ChangeLog	Sat Feb 02 14:39:59 2019 -0800
@@ -1,5 +1,10 @@
 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
diff -r dcd889592b62 -r a8eeb883e377 lib/vla.h
--- a/lib/vla.h	Sat Feb 02 13:01:55 2019 -0800
+++ b/lib/vla.h	Sat Feb 02 14:39:59 2019 -0800
@@ -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'.  */