Does chmod() need leading zeroes to work properly?

Question

I'm trying to call the chmod() function from my C program, but it isn't setting the correct permissions unless the mode_t argument has leading zeroes in it. For example chmod(argv[2],00777) will work, but chmod(argv[2],777) will set the permissions to complete garbage.

The big problem comes when I try to use atoi() because it will discard any leading zeroes. Is there any way I can make the following program work even if the user enters a value with no leading zeroes in the command line?

#include <stdio.h>
#include <sys/types.h>
#include <stdlib.h>

int main(int argc, char** argv)
{  
    if (argc != 3)
        exit(EXIT_FAILURE);

    if (chmod(argv[2], (mode_t)atoi(argv[1])) != 0)
        printf("Warning: Unable to change file permissions.");

    return 0;
}

Answer 1

Yes, the zero is necessary, unless you convert the number into the correct base ahead of time. A leading zero in an integer literal makes C read the number in octal, in the same way that 0x makes it read the integer in hexadecimal. If you use strtol, you can provide a base parameter of 8 to read an integer in octal.