strcmp, strncmp - compare two strings

#include <string.h>
int strcmp(const char *s1, const char *s2);
int strncmp(const char s1[.n], const char s2[.n], size_t n);

The strcmp() function compares the two strings s1 and s2. The locale is not taken into account (for a locale-aware comparison, see strcoll(3)). The comparison is done using unsigned characters.

strcmp() returns an integer indicating the result of the comparison, as follows:

• 0, if the s1 and s2 are equal;

• a negative value if s1 is less than s2;

• a positive value if s1 is greater than s2.

The strncmp() function is similar, except it compares only the first (at most) n bytes of s1 and s2.

The strcmp() and strncmp() functions return an integer less than, equal to, or greater than zero if s1 (or the first n bytes thereof) is found, respectively, to be less than, to match, or be greater than s2.

The program below can be used to demonstrate the operation of strcmp() (when given two arguments) and strncmp() (when given three arguments). First, some examples using strcmp():

$ ./string_comp ABC ABC
<str1> and <str2> are equal
$ ./string_comp ABC AB      # 'C' is ASCII 67; 'C' - '\0' = 67
<str1> is greater than <str2> (67)
$ ./string_comp ABA ABZ     # 'A' is ASCII 65; 'Z' is ASCII 90
<str1> is less than <str2> (-25)
$ ./string_comp ABJ ABC
<str1> is greater than <str2> (7)
$ ./string_comp $'\201' A   # 0201 - 0101 = 0100 (or 64 decimal)
<str1> is greater than <str2> (64)

The last example uses bash(1)-specific syntax to produce a string containing an 8-bit ASCII code; the result demonstrates that the string comparison uses unsigned characters.

And then some examples using strncmp():

$ ./string_comp ABC AB 3
<str1> is greater than <str2> (67)
$ ./string_comp ABC AB 2
<str1> and <str2> are equal in the first 2 bytes

Program source

/* string_comp.c
   Licensed under GNU General Public License v2 or later.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(int argc, char *argv[])
{
    int res;
    if (argc < 3) {
        fprintf(stderr, "Usage: %s <str1> <str2> [<len>]\n", argv[0]);
        exit(EXIT_FAILURE);
    }
    if (argc == 3)
        res = strcmp(argv[1], argv[2]);
    else
        res = strncmp(argv[1], argv[2], atoi(argv[3]));
    if (res == 0) {
        printf("<str1> and <str2> are equal");
        if (argc > 3)
            printf(" in the first %d bytes\n", atoi(argv[3]));
        printf("\n");
    } else if (res < 0) {
        printf("<str1> is less than <str2> (%d)\n", res);
    } else {
        printf("<str1> is greater than <str2> (%d)\n", res);
    }
    exit(EXIT_SUCCESS);
}