[PATCH] flexmember: update comment

[Paul Eggert]

* m4/flexmember.m4 (AC_C_FLEXIBLE_ARRAY_MEMBER): Improve comment.
---
 ChangeLog        |  5 +++++
 m4/flexmember.m4 | 10 +++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 0c4c515a0..efa81e2ae 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2019-05-24  Paul Eggert  <address@hidden>
+
+       flexmember: update comment
+       * m4/flexmember.m4 (AC_C_FLEXIBLE_ARRAY_MEMBER): Improve comment.
+
 2019-05-20  Bruno Haible  <address@hidden>
 
        setlocale: Improve fallback on macOS.
diff --git a/m4/flexmember.m4 b/m4/flexmember.m4
index 1347068fe..ef6373df2 100644
--- a/m4/flexmember.m4
+++ b/m4/flexmember.m4
@@ -34,10 +34,14 @@ AC_DEFUN([AC_C_FLEXIBLE_ARRAY_MEMBER],
     AC_DEFINE([FLEXIBLE_ARRAY_MEMBER], [],
       [Define to nothing if C supports flexible array members, and to
        1 if it does not.  That way, with a declaration like 'struct s
-       { int n; double d@<:@FLEXIBLE_ARRAY_MEMBER@:>@; };', the struct hack
+       { int n; char d@<:@FLEXIBLE_ARRAY_MEMBER@:>@; };', the struct hack
        can be used with pre-C99 compilers.
-       When computing the size of such an object, don't use 'sizeof (struct s)'
-       as it overestimates the size.  Use 'offsetof (struct s, d)' instead.
+       Use 'FLEXSIZEOF (struct s, d, N)' to calculate the size in bytes
+       of such a struct containing an N-element array, as both
+       'sizeof (struct s) + N * sizeof (char)' and
+       'offsetof (struct s, d) + N * sizeof (char)'
+       might compute a size that can cause malloc to align storage
+       improperly, even in C11.
        Don't use 'offsetof (struct s, d@<:@0@:>@)', as this doesn't work with
        MSVC and with C++ compilers.])
   else
-- 
2.21.0