Write a C Program for extracting sub-string from string

/**************************************************************/
/*Programmed by : Vivek Patel**********************************/
/*For Bugs Free feel to contact********************************/
/*Website : www.vivekpatel.cjb.net*****************************/
/*Email : vivek_patel9@rediffmail.com**************************/
/**************************************************************/

#include
#include
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 || nlen){
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();
}