strerror_r-posix: fix on MacOS MacOS X 10.5 strerror(0) is "Unknown error: 0", which is not distinguished from "Unknown error: -1" for out-of-range. Worse, strerror_r(0,,) is "Undefined error: 0", although strerror_r for all other out-of-range values matches strerror. * m4/strerror.m4 (gl_FUNC_STRERROR): Flush out MacOS bug. * m4/strerror_r.m4 (gl_FUNC_STRERROR_R_WORKS): Likewise, and fix logic bug. * lib/strerror_r.c (strerror_r): Fix the bug. * lib/strerror.c (strerror): Likewise. * doc/posix-functions/strerror_r.texi (strerror_r): Document the problem. * doc/posix-functions/strerror.texi (strerror): Likewise. * doc/posix-functions/perror.texi (perror): Likewise. * tests/test-strerror.c (main): Enhance test. * tests/test-strerror_r.c (main): Likewise. Signed-off-by: Eric Blake <eblake@redhat.com>

--- a/doc/posix-functions/strerror.texi
+++ b/doc/posix-functions/strerror.texi
@@ -13,10 +13,11 @@
 but not defined by the system, on some platforms:
 OpenBSD 4.0, OSF/1 5.1, NonStop Kernel, Cygwin 1.5.x, mingw.
 @item
-This function reports failure (by setting @code{errno}) for
-@code{strerror(0)}, although POSIX requires this to leave @code{errno}
-unchanged and report success, on some platforms:
-FreeBSD 8.2
+This function reports failure for @code{strerror(0)} (by setting
+@code{errno} or using a string similar to out-of-range values),
+although POSIX requires this to leave @code{errno} unchanged and
+report success, on some platforms:
+FreeBSD 8.2, MacOS X 10.5.
 @item
 This function fails to return a string for out-of-range integers on
 some platforms: