CERT C: Rule FIO47-C

Format string specifiers and arguments mismatch occurs when the format specifiers in the formatted output functions such as printf do not match their corresponding arguments. For example, an argument of type unsigned long must have a format specification of %lu.

Mismatch between format specifiers and the corresponding arguments result in undefined behavior.

Make sure that the format specifiers match the corresponding arguments. For instance, in this example, the %d specifier does not match the string argument message and the %s specifier does not match the integer argument err_number.

  const char *message = "License not available";
  int err_number = -4;
  printf("Error: %d (error type %s)\n", message, err_number);

If you do not want to fix the issue, add comments to your result or code to avoid another review. See:

#include <stdio.h>

void string_format(void) {

    unsigned long fst = 1;

    printf("%d\n", fst); //Noncompliant
}

In the printf statement, the format specifier, %d, does not match the data type of fst.

One possible correction is to use the %lu format specifier. This specifier matches the unsigned integer type and long size of fst.

#include <stdio.h>

void string_format(void) {

    unsigned long fst = 1;

    printf("%lu\n", fst);
}

One possible correction is to change the argument to match the format specifier. Convert fst to an integer to match the format specifier and print the value 1.

#include <stdio.h>

void string_format(void) {

    unsigned long fst = 1;

    printf("%d\n", (int)fst);
}