what does it mean if fopen function returns NULL?

Please help! There is a bug in my program that causes it to jump to the notfound function before checking the entire substring The program should determine whether the second string is a substring of the first. If it is, then the program should print out the first index in which the second string appears in the first. As an example, if the first string is “Hello World” and the second string is “lo”, then the program should print out 3, i.e. the starting index of “lo” in “Hello World.” If the second string is not contained in the first string, then the program should print out -1. Here is the code:  .datastr1: .space 51 #space for str1 (max50 characters including null terminator)str2: .space 51 #space for str2 (max50 characters incl null terminator)msg1: .asciiz "Please enter the first string: " #prompt user to enter str1msg2: .asciiz "Please enter the second string: " #prompt user to enter str2newline: .asciiz "\n" .text.globl mainmain:    # Print sttr1    li $v0, 4    la $a0, msg1…

Write a function that returns the greatest common divisor of 2 integers.

Refactor the following code using the concept of functions. a = 2 b = 3 C= 4 d = 5 a = 2 b = 3 C= 4 d = 5