Dynamically adding to char array with unknown size

Question

I am currently working on an assignment where I have to extract text one char at a time from each file and then concatenate them and print it out. I have found a way to extract my characters and save them in variable a-d. How do I add these char to my char array to print out to the end? I initially malloc it to the size of a char and at each if statement I realloc. All the guides I found online recommend I use strcat but I cannot pass in the a-d variables to append. How would I fix this?

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


pthread_mutex_t thread1;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
char *output;

void* processing(void* file) {
    FILE *test = fopen("drive1.data", "r");
    FILE *drive1 = fopen("drive1.data", "r");
    FILE *drive2 = fopen("drive2.data", "r");
    FILE *drive3 = fopen("drive3.data", "r");
    FILE *drive4 = fopen("drive4.data", "r");
    FILE *drive5 = fopen("drive5.data", "r");
    int c, a, b, e, d;
    int control = 0;

    //Initializingn the char array with initial size 1
    output = (char *)malloc(sizeof(char));

    while(!feof(drive1)) {
        if(control == 0) {
            pthread_mutex_lock(&thread1);
            //printf("Mutex lock\n");
            c = getc(drive1);
            output = realloc(output, sizeof(output) + sizeof(char));
            printf("%c\n", (char)c);
            strcat(output, (char)c);
            control++;
            pthread_mutex_unlock(&thread1);
            //printf("Mutex unlock\n");
        } else if(control == 1) {
            pthread_mutex_lock(&thread1);
            //printf("Mutex lock\n");
            a = getc(drive2);
            printf("%c\n", (char)a);
            control++;
            pthread_mutex_unlock(&thread1);
            //printf("Mutex unlock\n");
        } else if(control == 2) {
            pthread_mutex_lock(&thread1);
            //printf("Mutex lock\n");
            b = getc(drive3);
            printf("%c\n", (char)b);
            control++;
            pthread_mutex_unlock(&thread1);
            //printf("Mutex unlock\n");
        } else if(control == 3) {
            pthread_mutex_lock(&thread1);
            //printf("Mutex lock\n");
            d = getc(drive4);
            printf("%c\n", (char)d);
            control++;
            pthread_mutex_unlock(&thread1);
            //printf("Mutex unlock\n");
        } else if(control == 4) {
            pthread_mutex_lock(&thread1);
            //printf("Mutex lock\n");
            e = getc(drive5);
            printf("%c\n", (char)e);
            control = 0;
            pthread_mutex_unlock(&thread1);
            //printf("Mutex unlock\n");
        }
    }
}

int main() {
    pthread_t th1;
    pthread_create(&th1, NULL, processing, NULL);
    pthread_join(th1, NULL);

}