let gcc's -Wformat do its job; avoid "make syntax-check" failure

* src/util.c (virAsprintf): Remove trailing space.
* src/util.h (virAsprintf): Use ATTRIBUTE_FORMAT.
* HACKING (Printf-style functions): New section.

ChangeLog

+Wed Dec 17 18:10:51 +0100 2008 Jim Meyering <meyering@redhat.com>
+
+	let gcc's -Wformat do its job; avoid "make syntax-check" failure
+	* src/util.c (virAsprintf): Remove trailing space.
+	* src/util.h (virAsprintf): Use ATTRIBUTE_FORMAT.
+	* HACKING (Printf-style functions): New section.
+
 Wed Dec 17 16:43:39 GMT 2008 Daniel P. Berrange <berrange@redhat.com>
 
 	* src/libvirt.c: Add missing checks for read-only connection

HACKING

@@ -247,6 +247,22 @@ are some special reasons why you cannot include these files
 explicitly.
 
 
+Printf-style functions
+======================
+
+Whenever you add a new printf-style function, i.e., one with a format
+string argument and following "..." in its prototype, be sure to use
+gcc's printf attribute directive in the prototype.  For example, here's
+the one for virAsprintf, in util.h:
+
+    int virAsprintf(char **strp, const char *fmt, ...)
+	ATTRIBUTE_FORMAT(printf, 2, 3);
+
+This makes it so gcc's -Wformat and -Wformat-security options can do
+their jobs and cross-check format strings with the number and types
+of arguments.
+
+
 
                 Libvirt commiters guidelines
                 ============================

src/util.h

@@ -112,7 +112,8 @@ int virMacAddrCompare (const char *mac1, const char *mac2);
 
 void virSkipSpaces(const char **str);
 int virParseNumber(const char **str);
-int virAsprintf(char **strp, const char *fmt, ...);
+int virAsprintf(char **strp, const char *fmt, ...)
+    ATTRIBUTE_FORMAT(printf, 2, 3);
 
 #define VIR_MAC_BUFLEN 6
 #define VIR_MAC_PREFIX_BUFLEN 3