[src/trunk]: src/lib/libc/stdlib add warning on realloc() size de-synchroniza...

[itojun <itojun%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/1766dd9a5105
branches:  trunk
changeset: 552107:1766dd9a5105
user:      itojun <itojun%NetBSD.org@localhost>
date:      Fri Sep 19 05:36:59 2003 +0000

description:
add warning on realloc() size de-synchronization.  from openbsd

diffstat:

 lib/libc/stdlib/malloc.3 |  18 ++++++++++++------
 1 files changed, 12 insertions(+), 6 deletions(-)

diffs (52 lines):

diff -r 1b78c3e6917b -r 1766dd9a5105 lib/libc/stdlib/malloc.3
--- a/lib/libc/stdlib/malloc.3  Fri Sep 19 05:33:15 2003 +0000
+++ b/lib/libc/stdlib/malloc.3  Fri Sep 19 05:36:59 2003 +0000
@@ -1,4 +1,4 @@
-.\"    $NetBSD: malloc.3,v 1.20 2003/08/07 16:43:41 agc Exp $
+.\"    $NetBSD: malloc.3,v 1.21 2003/09/19 05:36:59 itojun Exp $
 .\"
 .\" Copyright (c) 1980, 1991, 1993
 .\"    The Regents of the University of California.  All rights reserved.
@@ -34,7 +34,7 @@
 .\"     @(#)malloc.3   8.1 (Berkeley) 6/4/93
 .\"     From FreeBSD: Id: malloc.3,v 1.18 1999/03/28 14:16:04 phk Exp
 .\"
-.Dd August 11, 2002
+.Dd September 19, 2003
 .Dt MALLOC 3
 .Os
 .Sh NAME
@@ -126,23 +126,29 @@
 one must be careful to avoid the following idiom:
 .Pp
 .Bd -literal -offset indent
+nsize += 50
 if ((p = realloc(p, nsize)) == NULL)
-       return NULL;
+       return (NULL);
 .Ed
 .Pp
-In most cases, this will result in a leak of memory.
+Do not adjust the variable describing how much memory has been allocated
+until one knows the allocation has been successful.
+This can cause aberrant program behavior if the incorrect size value is used.
+In most cases, the above sample will also result in a leak of memory.
 As stated earlier, a return value of
 .Dv NULL
 indicates that the old object still remains allocated.
 Better code looks like this:
 .Bd -literal -offset indent
-if ((p2 = realloc(p, nsize)) == NULL) {
+newsize = size + 50;
+if ((p2 = realloc(p, newsize)) == NULL) {
        if (p)
                free(p);
        p = NULL;
-       return NULL;
+       return (NULL);
 }
 p = p2;
+nsize = newsize;
 .Ed
 .\"XXX".Pp
 .\"XXX"The