| format

Previous | Specifying Attributes of Functions | Next

Syntax: format (archetype, string-index, first-to-check)
The `format` attribute specifies that a function takes printf style arguments which should be type-checked against a format string. For example, the declaration:

    extern int
    my_printf (void *my_object, const char *my_format, ...)
          __attribute__ ((format (printf, 2, 3)));

causes the compiler to check the arguments in calls to my_printf for consistency with the printf style format string argument my_format.
The parameter archetype determines how the format string is interpreted, and should be `printf`, `scanf`, `strftime` or `strfmon` (note that only the function printf is implemented in the TIGCC Library). (You can also use `__printf__`, `__scanf__`, `__strftime__` or `__strfmon__`.) The parameter string-index specifies which argument is the format string argument (starting from 1), while first-to-check is the number of the first argument to check against the format string. For functions where the arguments are not available to be checked (such as vprintf), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. For `strftime` formats, the third parameter is required to be zero.
In the example above, the format string (my_format) is the second argument of the function my_print, and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3.
The `format` attribute allows you to identify your own functions which take format strings as arguments, so that GCC can check the calls to these functions for errors. The compiler always (unless '-ffreestanding' is used) checks formats for the standard library functions. See Options Controlling C Dialect.