[PATCH] mktime: avoid infinite loop

[Eric Blake]

* m4/mktime.m4 (AC_FUNC_MKTIME): Avoid undefined arithmetic on
possibly-signed type.
Reported by Rich Felker.

Signed-off-by: Eric Blake <address@hidden>
---

Does this look like a correct port of the intprops.h principles
over to this test?

 ChangeLog    |    7 +++++++
 m4/mktime.m4 |   20 +++++++++++---------
 2 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 34aa134..09a150c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2011-01-27  Eric Blake  <address@hidden>
+
+       mktime: avoid infinite loop
+       * m4/mktime.m4 (AC_FUNC_MKTIME): Avoid undefined arithmetic on
+       possibly-signed type.
+       Reported by Rich Felker.
+
 2011-01-27  Bruno Haible  <address@hidden>

        vma-iter, get-rusage-as: Add OpenBSD support.
diff --git a/m4/mktime.m4 b/m4/mktime.m4
index f2b131b..7836b76 100644
--- a/m4/mktime.m4
+++ b/m4/mktime.m4
@@ -1,4 +1,4 @@
-# serial 17
+# serial 18
 dnl Copyright (C) 2002-2003, 2005-2007, 2009-2011 Free Software Foundation,
 dnl Inc.
 dnl This file is free software; the Free Software Foundation
@@ -165,20 +165,22 @@ main ()
   int result = 0;
   time_t t, delta;
   int i, j;
+  int time_t_signed_magnitude = (time_t) ~ (time_t) 0 < (time_t) -1;
+  int time_t_signed = ! ((time_t) 0 < (time_t) -1);

   /* This test makes some buggy mktime implementations loop.
      Give up after 60 seconds; a mktime slower than that
      isn't worth using anyway.  */
   alarm (60);

-  for (;;)
-    {
-      t = (time_t_max << 1) + 1;
-      if (t <= time_t_max)
-        break;
-      time_t_max = t;
-    }
-  time_t_min = - ((time_t) ~ (time_t) 0 == (time_t) -1) - time_t_max;
+  time_t_max = (! time_t_signed
+                ? (time_t) -1
+                : ~ (~ (time_t) 0 << (sizeof (time_t) * CHAR_BIT - 1)));
+  time_t_min = (! time_t_signed
+                ? (time_t) 0
+                : time_t_signed_magnitude
+                ? ~ (time_t) 0
+                : ~ (time_t) 0 << (sizeof (time_t) * CHAR_BIT - 1));

   delta = time_t_max / 997; /* a suitable prime number */
   for (i = 0; i < N_STRINGS; i++)
-- 
1.7.3.5