This attribute causes the compiler to perform placeholder type checking on the specified argument when the output of the function is used in calls to a printf-style function.

Example

The following example declares two functions, myFormatText1() and myFormatText2(), that provide format strings to printf().

The first function, myFormatText1(), does not specify the format_arg attribute. The compiler does not check the types of the printf arguments for consistency with the format string.

The second function, myFormatText2(), specifies the format_arg attribute. In the subsequent calls to printf(), the compiler checks that the types of the supplied arguments a and b are consistent with the format string argument to myFormatText2(). The compiler produces a warning when a float is provided where an int is expected.

#include <stdio.h>

// Function used by printf. No format type checking.
extern char *myFormatText1 (const char *);

// Function used by printf. Format type checking on argument 1.
extern char *myFormatText2 (const char *) __attribute__((format_arg(1)));


int main(void) {
  int a;
  float b;

  a = 5;
  b = 9.099999;

  printf(myFormatText1("Here is an integer: %d\n"), a); // No type checking. Types match anyway.
  printf(myFormatText1("Here is an integer: %d\n"), b); // No type checking. Type mismatch, but no warning                            

  printf(myFormatText2("Here is an integer: %d\n"), a); // Type checking. Types match.
  printf(myFormatText2("Here is an integer: %d\n"), b); // Type checking. Type mismatch results in warning
}
   

$ armclang --target=aarch64-arm-none-eabi -mcpu=cortex-a53 -c format_arg_test.c 
format_arg_test.c:21:53: warning: format specifies type 'int' but the argument has type 'float' [-Wformat]
  printf(myFormatText2("Here is an integer: %d\n"), b); // Type checking. Type mismatch results in warning
                                            ~~      ^
                                            %f
1 warning generated.