strncmp(3)                 Library Functions Manual                 strncmp(3)

NAME
       strncmp - nonstrings compare

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #include <string.h>  // see memory.h(3head)

       int strncmp(const char s1[], const char s2[], size_t n);

DESCRIPTION
       The strncmp() function is similar to strcmp(3), except it compares only
       the first (at most) n bytes of s1 and s2.

       It is equivalent to

           memcmp(s1, s2, MIN(MIN(strnlen(s1,n),strnlen(s2,n))+1, n))

RETURN VALUE
       See memcmp(3).

ATTRIBUTES
       For an explanation of the terms used in this section, see
       attributes(7).

       +--------------------------------------------+---------------+---------+
       |Interface                                   | Attribute     | Value   |
       +--------------------------------------------+---------------+---------+
       |strncmp ()                                  | Thread safety | MT-Safe |
       +--------------------------------------------+---------------+---------+
STANDARDS
       C11, POSIX.1-2008.

HISTORY
       POSIX.1-2001, C89, SVr4, 4.3BSD.

EXAMPLES
       The program below can be used to demonstrate the operation of
       strncmp().

           $ ./strncmp ABC AB 3;
           <strn1> is greater than <strn2> (67)
           $ ./strncmp ABC AB 2;
           <strn1> and <str2> are equal in the first 2 bytes

   Program source
       // strncmp.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 != 4) {
               fprintf(stderr, "Usage: %s <strn1> <strn2> <n>\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           res = strncmp(argv[1], argv[2], atoi(argv[3]));

           if (res == 0) {
               printf("<strn1> and <strn2> are equal");
               printf(" in the first %d bytes\n", atoi(argv[3]));
               printf("\n");
           } else if (res < 0) {
               printf("<strn1> is less than <strn2> (%d)\n", res);
           } else {
               printf("<strn1> is greater than <strn2> (%d)\n", res);
           }

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       memory.h(3head), memcmp(3), strncasecmp(3), wcsncmp(3)

Linux man-pages 6.19              2026-08-10                        strncmp(3)