Find whether the given sub-string present in the string, & print the index if it does else print -1( without using library function )

Find whether the given sub-string present in the string, & print the index if it does else print -1( without using library function )



For example:

If the given main string is : "coderegister" and the sub-string is "reg", 

Then the output should be the index of the first occurrence of the sub-string in the main string which is '4' here.

If the given main string is: "coderegister" and the sub-string is "service"

Then the output should be '-1'

Implementation:

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int main()
{
  char *m,*s;
  int flag=1,fla=1;
  int i,j,k,index;
  
  /* Allocate memory for input string in which we have to search for a substring */
  m=(char*)malloc(sizeof(char)*256);
  
  /* Allocate memory for sub-string to search in the main string */
  s=(char*)malloc(sizeof(char)*256);
  
  /* Get the main string from the user */
  scanf("%s",m);
  
  /* Get the sub-string from the user */
  scanf("%s",s);
  
  /* Loop until we reach end of the main string */
  for(i=0;*(m+i);i++)
  {
   
   flag=1;
   fla=1;
   /* Find the first character match of the sub-string and the main string */
    if(*(m+i)==*s)
    {
     
     /* If there is a match, store the current index in 'index' variable */
      index=i;
      
      /* Temporary counter variable to start from the first character matching
      index till end of the sub-string */
      k=i;
      
      /* Loop until the end of sub-string */
      for(j=0;*(s+j);j++,k++)
      {
       /* If any one character doesn't match, set the flag to zero and break the inner loop */
       if(*(m+k)!=*(s+j))
       {
         flag=0;
         break;
       }
      }
      /* If the flag is still set to '1' then, all the characters of the sub-string matches
      a part of string in main string */
      if(flag)
      {
       /* Print the starting index of such a match */
       printf("%d",index);
       
       /* Set fla varaiable to '0' to mark that we found atleast one match */
       fla=0;
       break;
      }
    }
  }
  
  /* If there are no matches of sub-string in the main string, print '-1' */
  if(fla)
   printf("-1");
   
  return 0;
}