15.2.12 Format_arg Function Attribute

The format_arg attribute specifies that a function manipulates a printf style format string and that the compiler should check the format string for consistency. The index parameter gives the position of the format string in the parameter list of the function, numbered from the left beginning at index 1. For example:

extern char * __attribute__ ((format_arg (2))) my_foo (char *my_str, const char *my_format);