tools/xenstore: Check the format printf for xprintf() and barf{,_perror}()

Allow GCC to analyze the format printf for xprintf() and
barf{,_perror}().

Take the opportunity to define __noreturn to make the prototype for
barf{,_perror})() easier to read.

Also document why 'extern' is used for xprintf().

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

tools/xenstore/utils.h

@@ -29,10 +29,13 @@ const char *dump_state_align(FILE *fp);
 
 #define PRINTF_ATTRIBUTE(a1, a2) __attribute__((format (printf, a1, a2)))
 
-void barf(const char *fmt, ...) __attribute__((noreturn));
-void barf_perror(const char *fmt, ...) __attribute__((noreturn));
+#define __noreturn __attribute__((noreturn))
 
-extern void (*xprintf)(const char *fmt, ...);
+void barf(const char *fmt, ...) __noreturn PRINTF_ATTRIBUTE(1, 2);
+void barf_perror(const char *fmt, ...) __noreturn PRINTF_ATTRIBUTE(1, 2);
+
+/* Function pointer as xprintf() can be configured at runtime. */
+extern void (*xprintf)(const char *fmt, ...) PRINTF_ATTRIBUTE(1, 2);
 
 #define eprintf(_fmt, _args...) xprintf("[ERR] %s" _fmt, __FUNCTION__, ##_args)