sh-5.0$ cat 1.c
#include <stdio.h>
#include <stdlib.h>

#define MAXLEN 100

void strcat2(char *destination, char *source);

int main()
{
    // create a character array. All of these C strings are terminated with a '\0'
    char start[MAXLEN] = "This is a sentence looking for";
    char end[] = " an ending!";

    // strcat2 destination, source
    strcat2(start, end);

    // str is the same as &str[0] which is handed to printf and prints all the
    // up to '\0'
    printf("%s\n", start);
}

void strcat2(char *start, char *end)
{
    // move pointer to end of start.
    while (*start++)
        ;

    // allows me to overwrite the null character
    start--;

    while ((*start++ = *end++))
        ;
}
sh-5.0$ gcc 1.c
sh-5.0$ ./a.out
This is a sentence looking for an ending!
sh-5.0$