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

char *stringcat(const char str1[],  char str2[], int n){
    int str1_length = strlen(str1);
    int str2_length = strlen(str2);
    char *p,*k;


    if (n > str1_length)
        return NULL;

    char *f = (char *)malloc((str1_length + n)* sizeof(char));
    p = &f[str1_length];
    k = str2;
    strcpy(f,str1);

    for (int i=0; i<n; i++)
        *p++ = *k++;

    return f;
}

int main(){

    printf("%s",stringcat("coffee","cup",3));

    return 0;
}