Comparing two string literals using memcmp

Question

I have compared two string literals using the memcmp function.

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

int main() 
{
  char str1[] = "abcd";
  char str2[] = "ab";

  if (memcmp(str1, str2, 4) == 0) 
  {
    printf("equal string\n");
  }
  return 0;
}

In the above program, str2 is shorter than the str1. It means string str2 is accessed out of bounds.

So, is this undefined behavior?

Answer 1

The behaviour of your code is undefined. The C standard does not require that memcmp returns as soon as the result is known; that is it doesn't necessarily have to return when \0 is compared with 'c' despite the value of 'c' == '\0' being 0 for any character encoding supported by the language. The standard also doesn't specify the order in which the lexicographical comparisons are to be made (although it would be tricky for an implementation not to start from the beginning).

str2 is a char[3] type. It's possible that an attempt to access the 4th element is made.

Reference: http://en.cppreference.com/w/c/string/byte/memcmp