C program to insert a string with in another string (sub string)

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

void insert_substring(char*, char*, int);
char* substring(char*, int, int);

void main()
{
   char text[100], substring[100];
   int position;
   clrscr();
   printf("Enter some text\n");
   gets(text);

   printf("Enter a string to insert\n");
   gets(substring);

   printf("Enter the position to insert\n");
   scanf("%d", &position);

   insert_substring(text, substring, position);

   printf("%s\n",text);

   getch();
}

void insert_substring(char *a, char *b, int position)
{
   char *f, *e;
   int length;

   length = strlen(a);

   f = substring(a, 1, position - 1 );   
   e = substring(a, position, length-position+1);

   strcpy(a, "");
   strcat(a, f);
   free(f);
   strcat(a, b);
   strcat(a, e);
   free(e);
}

char *substring(char *string, int position, int length)
{
   char *pointer;
   int c;

   pointer = malloc(length+1);

   if( pointer == NULL )
       exit(EXIT_FAILURE);

   for( c = 0 ; c < length ; c++ )
      *(pointer+c) = *((string+position-1)+c);     

   *(pointer+c) = '\0';

   return pointer;
}

OUTPUT:-