C

#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;
}