'\" t
.\" Copyright 1993, David Metcalfe <david@prism.demon.co.uk>
.\" Copyright 2020, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH strncmp 3 2026-08-10 "Linux man-pages 6.19"
.SH NAME
strncmp \- nonstrings compare
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.BR #include\~<string.h> "  // see memory.h(3head)"
.P
.BR "int strncmp(const char " s1 "[], const char " s2 "[], size_t " n );
.fi
.SH DESCRIPTION
The
.BR strncmp ()
function is similar to
.BR strcmp (3),
except it compares
only the first (at most)
.I n
bytes of
.I s1
and
.IR s2 .
.P
It is equivalent to
.P
.in +4n
.EX
memcmp(s1, s2, MIN(MIN(strnlen(s1,n),strnlen(s2,n))+1, n))
.EE
.in
.SH RETURN VALUE
See
.BR memcmp (3).
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR strncmp ()
T}	Thread safety	MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, C89, SVr4, 4.3BSD.
.SH EXAMPLES
The program below can be used to demonstrate the operation of
.BR strncmp ().
.P
.in +4n
.EX
.RB $ " ./strncmp ABC AB 3" ;
<strn1> is greater than <strn2> (67)
.RB $ " ./strncmp ABC AB 2" ;
<strn1> and <str2> are equal in the first 2 bytes
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (strncmp.c)
.EX
// 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>\[rs]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\[rs]n", atoi(argv[3]));
        printf("\[rs]n");
    } else if (res < 0) {
        printf("<strn1> is less than <strn2> (%d)\[rs]n", res);
    } else {
        printf("<strn1> is greater than <strn2> (%d)\[rs]n", res);
    }
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR memory.h (3head),
.BR memcmp (3),
.BR strncasecmp (3),
.BR wcsncmp (3)