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__attribute__((format_arg(string-index))) function attribute

This attribute specifies that a function takes a format string as an argument. Format strings can contain typed placeholders that are intended to be passed to printf-style functions such as printf(), scanf(), strftime(), or strfmon().

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.



Where string-index specifies the argument that is the format string argument (starting from one).


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
                                            ~~      ^
1 warning generated.