atoi function causing segmentation (core dump) error-CodePudding

I am trying to recreate the bash "head" and "tail" commands (which print the contents of files in different ways) using c programs. I figured that the getopt function would be useful for parsing the command-line options from the files to be printed. Here's what I have so far:

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

const int BUFFER_SIZE = 100;

int main(int argc, char *argv[]) {
    int c = 10;
    int n = 0;
    int opt;
    while ((opt = getopt(argc, argv, "cn:")) != -1) {
        switch(opt) {
        case 'c':
            c = atoi(optarg);
            break;
        case 'n':
            n = atoi(optarg);
            break;
        }
    }
    printf("c = %d & n = %d\n", c, n);
}

The program compiles. However, when I run the executable with -c 5 as its command-line arguments, the program crashes. I don't understand what the problem is.

CodePudding user response：

Your optstring ("cn:") says that c does not take an argument, so in this block:

case 'c':
    c = atoi(optarg);
    printf("worked\n");
    break;

optarg is NULL, which is why atoi(optarg) leads to a segmentation fault.

If option -c takes an argument, then you want:

while ((opt = getopt(argc, argv, "c:n:")) != -1) {