C Programming/string.h/strcmp

In POSIX and in the programming language C, strcmp is a function in the C standard library (declared in string.h) that compares two C strings.

The prototype according ISO/IEC 9899:1999, 7.21.4.2

int strcmp(const char *s1, const char *s2);

strcmp returns 0 when the strings are equal, a negative integer when s1 is less than s2, or a positive integer if s1 is greater than s2, according to the lexicographical order.

A variant of strcmp exists called strncmp that only compares the strings up to a certain offset.

Another variant, strcasecmp, conforming to POSIX.1-2001, works like strcmp, but is case-insensitive. Some systems instead provide this functionality with functions named stricmp or strcmpi. To compare a subset of both strings with case-insensitivity, various systems may provide strncasecmp, strnicmp or strncmpi.

Example

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

int main (int argc, char **argv)
{
    int v;

    if (argc < 3)
    {
        fprintf (stderr, "Compares two strings\nUsage: %s string1 string2\n",argv[0]);
        return EXIT_FAILURE;
    }

    v = strcmp (argv[1], argv[2]);

    if (v < 0)
        printf ("'%s' is less than '%s'.\n", argv[1], argv[2]);
    else if (v == 0)
        printf ("'%s' equals '%s'.\n", argv[1], argv[2]);
    else if (v > 0)
        printf ("'%s' is greater than '%s'.\n", argv[1], argv[2]);

    return 0;
}

The above code is a working sample that prints whether the first argument is less than, equal to or greater than the second.

A possible implementation is (P.J. Plauger, The Standard C Library, 1992):

int strcmp (const char * s1, const char * s2)
{
    for(; *s1 == *s2; ++s1, ++s2)
        if(*s1 == 0)
            return 0;
    return *(unsigned char *)s1 < *(unsigned char *)s2 ? -1 : 1;
}

However, most real-world implementations will have various optimization tricks to reduce the execution time of the function. One will notice, that strcmp not only returns -1, 0 and +1, but also other negative or positive values, resulting from optimizing away the branching introduced by the ?: operator:

return *(const unsigned char *)s1 - *(const unsigned char *)s2;

External links

• C++ reference for std::strcmp