PROGRAMMING - Hacking: The Art of Exploitation - page 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
r eader@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 <message> <# 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 <message> <# of times to repeat>