Fix ctime() standard compliance bug

fixes issue2209:
ctime() was updated in 0.9.31 to call localtime_r() instead of
localtime() to
avoid using a static buffer. Unfortunately, this change replaces the
static
buffer (which is zeroed out on initialization) with an uninitialized
local
buffer.

In the common case, this has no effect. However, with a sufficiently
large
time_t value, the value returned differs from that returned by
asctime(localtime(t)), and thus violates the ANSI/ISO standard.

An example input is (on a 64-bit machine):
time_t t = 0x7ffffffffff6c600;

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

libc/misc/time/time.c

@@ -479,6 +479,7 @@ char *ctime(const time_t *t)
 	 * localtime's static buffer:
 	 */
 	struct tm xtm;
+	memset(&xtm, 0, sizeof(xtm));
 
 	return asctime(localtime_r(t, &xtm));
 }

test/time/tst-ctime.c

@@ -0,0 +1,44 @@
+/* vi: set sw=4 ts=4: */
+/* testcase for ctime(3) with large time
+ * Copyright (C) 2010 David A Ramos <daramos@gustav.stanford.edu>
+ * Licensed under the LGPL v2.1, see the file COPYING.LIB in this tarball.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+
+#define MAX_POSITIVE(type)  (~0 & ~((type) 1 << (sizeof(type)*8 - 1)))
+
+int do_test(int argc, char **argv) {
+	char *correct = 0, *s;
+	int status;
+
+	/* need a very high positive number (e.g., max - 1024) */
+	time_t test = MAX_POSITIVE(time_t) - 1024;
+
+	s = asctime(localtime(&test));
+
+	if (s) {
+		// copy static buffer to heap
+		correct = malloc(strlen(s)+1);
+		strcpy(correct, s);
+	}
+
+	s = ctime(&test);
+
+	printf("ANSI:\t%suClibc:\t%s", correct, s);
+
+	if (s != correct && strcmp(correct, s))
+		status = EXIT_FAILURE;
+	else
+		status = EXIT_SUCCESS;
+
+	if (correct)
+		free(correct);
+
+    return status;
+}
+
+#include <test-skeleton.c>