include/curl/mprintf.h: add __attribute__ for the prototypes

- if gcc or clang is used
- if __STDC_VERSION__ >= 199901L, which means greater than C90
- if not using mingw
- if CURL_NO_FMT_CHECKS is not defined

Closes curl#11589

Author: bagder

include/curl/mprintf.h

@@ -32,18 +32,36 @@
 extern "C" {
 #endif
 
-CURL_EXTERN int curl_mprintf(const char *format, ...);
-CURL_EXTERN int curl_mfprintf(FILE *fd, const char *format, ...);
-CURL_EXTERN int curl_msprintf(char *buffer, const char *format, ...);
+#if (defined(__GNUC__) || defined(__clang__)) &&                        \
+  defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) &&         \
+  !defined(__MINGW32__) && !defined(CURL_NO_FMT_CHECKS)
+#define CURL_TEMP_PRINTF(a,b) __attribute__ ((format(printf, a, b)))
+#else
+#define CURL_TEMP_PRINTF(a,b)
+#endif
+
+CURL_EXTERN int curl_mprintf(const char *format, ...) CURL_TEMP_PRINTF(1, 2);
+CURL_EXTERN int curl_mfprintf(FILE *fd, const char *format, ...)
+  CURL_TEMP_PRINTF(2, 3);
+CURL_EXTERN int curl_msprintf(char *buffer, const char *format, ...)
+  CURL_TEMP_PRINTF(2, 3);
 CURL_EXTERN int curl_msnprintf(char *buffer, size_t maxlength,
-                               const char *format, ...);
-CURL_EXTERN int curl_mvprintf(const char *format, va_list args);
-CURL_EXTERN int curl_mvfprintf(FILE *fd, const char *format, va_list args);
-CURL_EXTERN int curl_mvsprintf(char *buffer, const char *format, va_list args);
+                               const char *format, ...) CURL_TEMP_PRINTF(3, 4);
+CURL_EXTERN int curl_mvprintf(const char *format, va_list args)
+  CURL_TEMP_PRINTF(1, 0);
+CURL_EXTERN int curl_mvfprintf(FILE *fd, const char *format, va_list args)
+  CURL_TEMP_PRINTF(2, 0);
+CURL_EXTERN int curl_mvsprintf(char *buffer, const char *format, va_list args)
+  CURL_TEMP_PRINTF(2, 0);
 CURL_EXTERN int curl_mvsnprintf(char *buffer, size_t maxlength,
-                                const char *format, va_list args);
-CURL_EXTERN char *curl_maprintf(const char *format, ...);
-CURL_EXTERN char *curl_mvaprintf(const char *format, va_list args);
+                                const char *format, va_list args)
+  CURL_TEMP_PRINTF(3, 0);
+CURL_EXTERN char *curl_maprintf(const char *format, ...)
+  CURL_TEMP_PRINTF(1, 2);
+CURL_EXTERN char *curl_mvaprintf(const char *format, va_list args)
+  CURL_TEMP_PRINTF(1, 0);
+
+#undef CURL_TEMP_PRINTF
 
 #ifdef  __cplusplus
 } /* end of extern "C" */