How to use the strncpy function in the 3rd array of 2d array strings

Question

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

int main(void) {
    char d2arr[6][7] = {
        { 'a', 'a', 'a', 'a', 'a', 'a' },
        { 'b', 'b', 'b', 'b', 'b', 'b' },
        { 'c', 'c', 'c', 'c', 'c', 'c' },
        { 'd', 'd', 'd', 'd', 'd', 'd' },
        { 'e', 'e', 'e', 'e', 'e', 'e' },
    };
    char d3arr[6][7][7];
    memset(d3arr, NULL, sizeof(d3arr));

    strncpy(d2arr[0], d3arr[0][0], sizeof(7));
    strncpy(d2arr[1], d3arr[0][1], sizeof(7));
    strncpy(d2arr[2], d3arr[0][2], sizeof(7));
    strncpy(d2arr[3], d3arr[0][3], sizeof(7));
    strncpy(d2arr[4], d3arr[0][4], sizeof(7));

    printf("%s\n", d3arr[0][0]);
    printf("%s\n", d3arr[0][1]);
    printf("%s\n", d3arr[0][2]);
    printf("%s\n", d3arr[0][3]);
    printf("%s\n", d3arr[0][4]);

    return 0;
}

There is no error message, and the results do not come out, so I ask questions. Please let me know why the string in the 2D array is not copied.

Answer 1

The problem is here:

strncpy(d2arr[0] , d3arr[0][0] , sizeof(7) );
strncpy(d2arr[1] , d3arr[0][1] , sizeof(7) );
strncpy(d2arr[2] , d3arr[0][2] , sizeof(7) );
strncpy(d2arr[3] , d3arr[0][3] , sizeof(7) );
strncpy(d2arr[4] , d3arr[0][4] , sizeof(7) );

The strncpy function works as under:

strncpy(destination, source, no_of_characters);

But in your code you have swapped destination and source arrays, and as a result, your code doesn't work. Also instead of using 'sizeof(7)' you should use only '7'. Also you should use strncpy_s instead of strncpy as strncpy can be unsafe.

So after all the corrections, here's the code:

strncpy_s(d3arr[0][0], d2arr[0], 7);
strncpy_s(d3arr[0][1], d2arr[1], 7);
strncpy_s(d3arr[0][2], d2arr[2], 7);
strncpy_s(d3arr[0][3], d2arr[3], 7);
strncpy_s(d3arr[0][4], d2arr[4], 7);

Just replace the part with strncpy functions with the above code, and you'll be good to go. Other part of the code is totally fine.