Segmentation Fault on Dividing the Input from fgets and Re-storing

Question

In this program, I am trying to get an input by using fgets and split this input by two halves when there is a " " - assumed that the user inputs in a "%d %d" fashion. Then, I am trying to store the first half in "keyPart" and the second half in "valuePart". Finally, I am aiming to convert these parts into int by using the atoi function.

However, when I execute the program, it is resulted with "Segmentation Fault".

Any problems you can spot?

Thank you.

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

#define M 31

int main(void)
{
    char unixRead[21];
    int key;
    int value;

    int i = 0;
    int j = 0;
    int splitIndex;
    char *keyPart;
    char *valuePart;
    int loopCondition = 1;
    puts("Please enter a key and a value for that key, leaving a single space in between (%d %d).");
    puts("The inputs which will not match the given template will cause problems and the program will not work.");
    while (loopCondition == 1)
    {
        fgets(unixRead, 20, stdin);
        while (unixRead[j] != ' ')
        { 
            keyPart = strcat(keyPart, unixRead[j]);
            j++; 
        }

        j++;
        while (unixRead[j] != '\0')
        {
            valuePart = strcat(valuePart, unixRead[j]);
            j++;
        }

        key = atoi(keyPart);
        value = atoi(valuePart);

        printf("Key: %d, Value: %d", key, value);
    }

    return 0;
}