Simple string comparison function

Question

A small, simple yet always needed function to compare two strings for equality.

int strequal (const char *str1, const char *str2) 
{
    while(1)
    {
        if((*str1) != (*str2))
        {
            return 1; /* Not Equal*/
        }
        else if((*str1) == '\0')
        {
            return 0; /* Equal */
        }

        str1++;
        str2++;
    }

    return 0; /* Equal*/
}

int main ()
{
    const char *str1 = "Hello";
    const char *str2 = "Hi";

    printf("Strings are equal? %s\n", strequal(str1, str2)? "NO":"YES");

    return 0;
}

The performance is the main focus.

Answer 1

If you want better performance, you should be using the standard library function strcmp():

#include <string.h>
int str_unequal(const char *str1, const char *str2)
{
    return strcmp(str1, str2) != 0;
}

The library function can take advantage of the target processor, possibly comparing multiple characters per iteration, which you can't easily do in a portable C program.

---

Other issues in the function:

• strequal() is a name reserved for future library extension, as is any identifier beginning str followed immediately by a letter.
• equal is misleading in the name, as it returns true only when the strings are unequal.
• while (1) is dubious practice, especially given that there's a natural terminating condition (end of one of the strings).
• Unnecessary parentheses around the result of dereference operator * - that's higher precedence than comparisons.
• The final return 0; is unreachable.

Problems with the test program:

• Uses printf without including <stdio.h>.
• Should explicitly state that main() accepts no arguments (i.e. int main(void)).
• Only tests a small portion of the functionality (no tests of two equal strings, or one that's a prefix of the other).
• Always returns a success status, even when the function is wrong.

---

Modified function and tests:

int str_equal(const char *s1, const char *s2)
{
    while (*s1) {
        if (*s1++ != *s2++) {
            return 0;
        }
    }

    return !*s2;
}

#include <stdio.h>
int test_str_equal(int expected, const char *a, const char *b)
{
    int actual = str_equal(a, b);
    if (actual == expected) { return 0; }
    fprintf(stderr, "\"%s\"==\"%s\" should return %d\n", a, b, expected);
    return 1;
}

int main(void)
{
    return test_str_equal(1, "", "")
        +  test_str_equal(0, "", "x")
        +  test_str_equal(0, "x", "")
        +  test_str_equal(1, "x", "x")
        +  test_str_equal(0, "x", "y")
        +  test_str_equal(0, "x", "xy")
        +  test_str_equal(0, "xy", "x")
        +  test_str_equal(0, "xx", "xy")
        +  test_str_equal(1, "xy", "xy");
}