Extraction of substring in c program

Program to extraction of substring program

#include <stdio.h>

#include <conio.h>

     void main(){

     int m,n,i,len;

     char str[40]="",substr[40]="";

     clrscr();

     printf("*****SubString Demo*****\n\n");

     printf("Enter string : ");

     scanf("%[^\n]s",str);

     for(m=0;str[m]!='\0';m++);

     len = m;

     printf("\nString Extracting Details\n");

     strt:

     printf("enter starting position : ");

     scanf("%d",&m);

     printf("enter ending position : ");

     scanf("%d",&n);

     if(m<=0 || m>len || n<m || n<=0 || n>len){

     printf("\n\nInvalid Input\n\n");

     goto strt; }

     else{

     for(i=0;m<=n;m++,i++)

     substr[i]=str[m-1];

     substr[i]='\0';

     printf("\nSub String is %s",substr);}

     getch();

}