Steps to reproduce:
1. call g_vsnprintf with %I64 format and single __uint64 with value > 2^32
2. 
3. 


Stack trace:


Other information:
Test program (thanks to Joerg Mayer) below, followed by proposed patch to vasnprintf.c

/* Testprogram for http://bugs.wireshark.org/bugzilla/show_bug.cgi?id=1025
 *
 * Compile with:
 * gcc -Wall `pkg-config --cflags glib-2.0` `pkg-config --libs glib-2.0` -o gtest64  gtest64.c
 * which expands on my system to
 * gcc -Wall -I/opt/gnome/include/glib-2.0 -I/opt/gnome/lib/glib-2.0/include -L/opt/gnome/lib -lglib-2.0 -o gtest64  gtest64.c

 * Finally, run with ./gtest64 or something like that.
 */

#include <glib.h>
#include <glib/gprintf.h>

/* Formats for printing 64-bit unsigned hexadecimal numbers */
#ifndef PRIx64
#ifdef _MSC_EXTENSIONS
#define PRIx64  "I64x"
#else /* _MSC_EXTENSIONS */
#define PRIx64  "llx"
#endif /* _MSC_EXTENSIONS */
#endif /* PRIx64 */

#define BUFLEN 100
char buffer[BUFLEN];

guint64 test1 = G_GINT64_CONSTANT(0x11223344U);
guint64 test2 = G_GINT64_CONSTANT(0x11223344556677U);
guint64 test3 = G_GINT64_CONSTANT(0x1122334455667788U);
guint64 test4 = G_GINT64_CONSTANT(0xFFEEDDCC11223344U);

void
printit(const char *format, ...)
{
	va_list ap;

	va_start(ap, format);
	g_vsnprintf(buffer, BUFLEN, format, ap);
	g_printf("%s\n", buffer);
	va_end(ap);
}



int main() {
	printit("Test1: %" PRIx64 , test1);
	printit("Test2: %" PRIx64 , test2);
	printit("Test3: %" PRIx64 , test3);
	printit("Test4: %" PRIx64 , test4);
	return 0;
}



Proposed patch:

--- vasnprintf.c.orig	2004-05-14 05:58:19.000000000 +0100
+++ vasnprintf.c	2006-08-12 12:26:39.234375000 +0100
@@ -480,6 +480,16 @@
 			  + 2; /* account for leading sign or alternate form */
 		      else
 # endif
+# ifdef HAVE_INT64_AND_I64
+		      if (type == TYPE_INT64 || type == TYPE_UINT64)
+			tmp_length =
+			  (unsigned int) (sizeof (unsigned __int64) * CHAR_BIT
+					  * 0.25 /* binary -> hexadecimal */
+					 )
+			  + 1 /* turn floor into ceil */
+			  + 2; /* account for leading sign or alternate form */
+		      else
+# endif
 		      if (type == TYPE_LONGINT || type == TYPE_ULONGINT)
 			tmp_length =
 			  (unsigned int) (sizeof (unsigned long) * CHAR_BIT