Why isn't NULL being assigned to my array of pointers?-CodePudding

I have this C program in which the last position in the *args array must be NULL. The NULL isn't being assigned or maybe printed? Is it because "%s" doesn't work with NULL?

In the program below I'm splitting a user inputted string and assigning each token to the *args array of pointers. The last element shall be NULL.

CodePudding user response：

As noted above you don't count the NULL (unless it was the first one; bug) so this means args[counter -1 ] will be the last non-NULL entry when you print it. Here are some issues that I fixed:

Replaced run flag with a break, which eliminated the need including stdbool
args is subject to overflow
It doesn't make sense to do a bunch of work on exit so moved that to right after input
streamline strtok call, and fixed defect if first call returns NULL
Prettify output including changing message "last character" to "last string".
Replaced the two magic values of 81 with defines.

And a few issues not fixed:

You use both a terminating null and a counter to significant number of elements in args. Choose one or the other.
scanf is subject to buffer overflow, replace it with fgets() and post-processing of input to to get the result of the format string.
scanf("%[^\n]%*c", input); fails to read anything when input is "\n". It also lacks a width limit (@chux-ReinstateMonica).

#include <stdio.h>
#include <string.h>

#define MAX_INPUT 81
#define MAX_ARGS 81

int main() {
    for(;;) {
        char input[MAX_INPUT];
        scanf("%[^\n]%*c", input);
        if(!strcmp(input, "Exit")) break;

        int counter = 0;
        char *token;
        char *args[MAX_ARGS];
        do {
            token = strtok(counter ? NULL : input, " ");
            args[counter] = token;
            if(token) counter  ;
        } while(token && counter < MAX_ARGS);
        if(counter == MAX_ARGS) {       
            counter--;
            args[counter] = NULL;
        }

        printf("\nlast string: %s\n", args[counter - 1]);
        for(unsigned i=0; i < counter; i  ) {
            printf("%d %s\n", i, args[i]);
        }
        printf("\n");
    }
    return 0;
}