vprintf functions can use printf format attribute too

src/sprintf.h

@@ -7,8 +7,8 @@
 
 #define VSPRINTF_MAXSIZE		4096
 
-size_t simple_vsnprintf(char *, size_t, const char *, va_list);
-size_t simple_vsprintf(char *, const char *, va_list);
+size_t simple_vsnprintf(char *, size_t, const char *, va_list) __attribute__((format(printf, 3, 0)));
+size_t simple_vsprintf(char *, const char *, va_list) __attribute__((format(printf, 2, 0)));
 size_t simple_sprintf (char *, const char *, ...) __attribute__((format(printf, 2, 3)));
 size_t simple_snprintf2 (char *, size_t, const char *, ...);
 size_t simple_snprintf (char *, size_t, const char *, ...) __attribute__((format(printf, 3, 4)));