Hacking - The Art of Exploitation, 2nd Edition

Programming 59

reader@hacking:~/booksrc $ gcc -o commandline commandline.c
reader@hacking:~/booksrc $ ./commandline
There were 1 arguments provided:
argument #0 - ./commandline
reader@hacking:~/booksrc $ ./commandline this is a test
There were 5 arguments provided:
argument #0 - ./commandline
argument #1 - this
argument #2 - is
argument #3 - a
argument #4 - test
reader@hacking:~/booksrc $

The zeroth argument is always the name of the executing binary, and

the rest of the argument array (often called an argument vector) contains the

remaining arguments as strings.
Sometimes a program will want to use a command-line argument as an

integer as opposed to a string. Regardless of this, the argument is passed in

as a string; however, there are standard conversion functions. Unlike simple

typecasting, these functions can actually convert character arrays containing

numbers into actual integers. The most common of these functions is atoi(),

which is short for ASCII to integer. This function accepts a pointer to a string

as its argument and returns the integer value it represents. Observe its usage

in convert.c.

convert.c

#include <stdio.h>

void usage(char *program_name) {
printf("Usage: %s <# of times to repeat>\n", program_name);
exit(1);
}

int main(int argc, char *argv[]) {
int i, count;

if(argc < 3) // If fewer than 3 arguments are used,
usage(argv[0]); // display usage message and exit.

count = atoi(argv[2]); // Convert the 2nd arg into an integer.
printf("Repeating %d times..\n", count);

for(i=0; i < count; i++)
printf("%3d - %s\n", i, argv[1]); // Print the 1st arg.
}

The results of compiling and executing convert.c are as follows.

reader@hacking:~/booksrc $ gcc convert.c
reader@hacking:~/booksrc $ ./a.out
Usage: ./a.out <# of times to repeat>