try yourself |run program

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

void main(){ 
 char str[50];
 int length;
 clrscr();
  printf("write some thing:-");
  gets(str);
  length = strlen(str);
  printf("\nlength of string: %d",length);
 getch();
}

Program Output

write some thing:-That is moon. 
length of string: 13