memcmp(3)		    Library Functions Manual		       memcmp(3)

NAME
     memcmp - compare memory areas

LIBRARY
     Standard C library (libc, -lc)

SYNOPSIS
     #include <string.h>

     int memcmp(size_t n;
		const void s1[n], const void s2[n], size_t n);

DESCRIPTION
     The  memcmp()  function compares the first n bytes (each interpreted as un-
     signed char) of the memory areas s1 and s2.

RETURN VALUE
     The memcmp() function returns an integer less than, equal	to,  or	 greater
     than  zero	 if  the  first n bytes of s1 is found, respectively, to be less
     than, to match, or be greater than the first n bytes of s2.

     For a nonzero return value, the sign is determined by the sign of the  dif-
     ference between the first pair of bytes (interpreted as unsigned char) that
     differ in s1 and s2.

     If n is zero, the return value is zero.

ATTRIBUTES
     For  an  explanation  of the terms used in this section, see attributes(7).
     allbox; lbx lb lb l l l.  Interface Attribute Value T{ memcmp() T}	  Thread
     safety  MT-Safe

STANDARDS
     C11, POSIX.1-2008.

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

CAVEATS
     Do not use memcmp() to compare confidential data, such as cryptographic se-
     crets, because the CPU time required for the comparison depends on the con-
     tents of the addresses compared, this function is subject	to  timing-based
     side-channel  attacks.  In such cases, a function that performs comparisons
     in deterministic time, depending only on n (the quantity of bytes compared)
     is required.   Some  operating  systems  provide  such  a	function  (e.g.,
     NetBSD's consttime_memequal()), but no such function is specified in POSIX.
     On Linux, you may need to implement such a function yourself.

SEE ALSO
     bstring(3),    strcasecmp(3),    strcmp(3),   strcoll(3),	 strncasecmp(3),
     strncmp(3), wmemcmp(3)

Linux man-pages 6.18		   2026-02-08			       memcmp(3)