Thread: printf problem

I am making a simple program that outputs a random number but when I put a printf in it triggers a segment fault and I don't understand why. here is the code:

Code:

#include <stdio.h>
#include <time.h>

int main(int argc, char *argv[]) {
    if(argc < 1 || argc > 2) {
        printf("Usage: %s [minimum value (default 0)] [maximum value]\n", argv[0]);
        return 0;
    }

    srand(time(0));

    if(argc == 1)
        printf("%d", atoi(argv[1]) % rand());
    else
        printf("%d", (atoi(argv[2])-atoi(argv[1])) % rand() + atoi(argv[1]));

    return 0;
}

You're a little confused on the value of argc. The name of the program is always the first thing in argv (argv[0]), making argc always at least 1. If I call a program with no parameters, argc is 1, if I call it with 2 parameters, argc is 3:

Code:

$ ./foo
argc = 1
$ ./foo 10 20
argc = 3

That means, when you call it with no parameters, and argc is 1, and you try to atoi(argv[1]), which doesn't exist. When you call it with one parameter, it tries to atoi(argv[2]), which doesn't exist, and crashes. Fix your if check at the top to be argc < 2 || argc > 3, and fix your statement below to be argc == 2.

Also, your modulus operations are backwards. You want:

Code:

rand() % atoi(argv[1])
// and
rand() % (atoi(argv[2]) - atoi(argv[1])) + atoi(argv[1])

ok, that fixed the problem. I should have realized that. thanks.