The C library's strcmp routine is badly named

One of those gotchas you find early in your C programming career is the strcmp string comparison routine. It takes two const char pointers to null- terminated strings and returns a integer: zero if the two strings match, -1 if the second is lexicographically less than the first, else 1.

However, it’s really easy to forget this and assume the routine compares for equality, and that the return value is effectively a boolean. I’m sure we’ve all forgotten this at some point and ended up debugging a problem which turns out to be inverted logic in a string compare.

Why can’t strcmp return true for equality? Well, its return value is a tri-state value, of which two states indicate non-equality. That means the only choice for indicating equality is zero.

A common idiom for using strcmp is this:

if (!strcmp(name, "Bob"))
    printf("Hello Bob");

What’s with the logical not operator? Of course it’s to turn the implied false (from the zero if the strings are equal) into the true needed to compare for equality (which is what the programmer actually wanted). It’s easy to forget the not operator. Another common idiom is:

if (strcmp(name, "Bob") == 0)
    printf("Hello Bob");

Again, it’s easy to forget the == 0 part. I’ve also seen code that incorrcctly combines both:

// This doesn't work...
if (!strcmp(name, "Bob") == 0)
    printf("Hello Bob");

This kind of bug is hard to spot when reading through code. You tend to see what you expect to see there, not what’s actually there.

I think it would have been much less error-prone if the function had been named strdiff. To me this better indicates that the function returns the difference between two strings. The example code reads a little more naturally:

if (!strdiff(name, "Bob"))
    printf("Hello Bob");

Of course, even this isn’t an ideal name. One might reasonably expect the difference between two strings to have a magnitude, not just a sign.

An alternative, suggested by Malcolm, is to separate the operations “are these strings the same” and “is this string greater than or less than”. Perhaps it would make more sense then to have a function streq (returning a boolean) checking for equality, in addition to the strdiff function. (The strdiff function is still useful inasmuch as its return value is compatible with sorting functions like qsort.)

The example code would then be:

if (streq(name, "Bob"))
    printf("Hello Bob");
Filed under: Coding
Posted at 12:45:00 BST on 14th April 2008.

About Matt Godbolt

Matt Godbolt is a C++ developer working in Chicago in the finance industry.