| .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk) |
| .\" |
| .\" %%%LICENSE_START(VERBATIM) |
| .\" Permission is granted to make and distribute verbatim copies of this |
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| .\" Permission is granted to copy and distribute modified versions of this |
| .\" manual under the conditions for verbatim copying, provided that the |
| .\" entire resulting derived work is distributed under the terms of a |
| .\" permission notice identical to this one. |
| .\" |
| .\" Since the Linux kernel and libraries are constantly changing, this |
| .\" manual page may be incorrect or out-of-date. The author(s) assume no |
| .\" responsibility for errors or omissions, or for damages resulting from |
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| .\" References consulted: |
| .\" Linux libc source code |
| .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991) |
| .\" 386BSD man pages |
| .\" Modified Sat Jul 24 18:55:27 1993 by Rik Faith (faith@cs.unc.edu) |
| .TH MEMCMP 3 2021-03-22 "" "Linux Programmer's Manual" |
| .SH NAME |
| memcmp \- compare memory areas |
| .SH SYNOPSIS |
| .nf |
| .B #include <string.h> |
| .PP |
| .BI "int memcmp(const void *" s1 ", const void *" s2 ", size_t " n ); |
| .fi |
| .SH DESCRIPTION |
| The |
| .BR memcmp () |
| function compares the first \fIn\fP bytes (each interpreted as |
| .IR "unsigned char" ) |
| of the memory areas \fIs1\fP and \fIs2\fP. |
| .SH RETURN VALUE |
| The |
| .BR memcmp () |
| function returns an integer less than, equal to, or |
| greater than zero if the first \fIn\fP bytes of \fIs1\fP is found, |
| respectively, to be less than, to match, or be greater than the first |
| \fIn\fP bytes of \fIs2\fP. |
| .PP |
| For a nonzero return value, the sign is determined by the sign of |
| the difference between the first pair of bytes (interpreted as |
| .IR "unsigned char" ) |
| that differ in |
| .I s1 |
| and |
| .IR s2 . |
| .PP |
| If |
| .I n |
| is zero, the return value is zero. |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .ad l |
| .nh |
| .TS |
| allbox; |
| lbx lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR memcmp () |
| T} Thread safety MT-Safe |
| .TE |
| .hy |
| .ad |
| .sp 1 |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD. |
| .SH NOTES |
| Do not use |
| .BR memcmp () |
| to compare security critical data, such as cryptographic secrets, |
| because the required CPU time depends on the number of equal bytes. |
| Instead, a function that performs comparisons in constant time is required. |
| Some operating systems provide such a function (e.g., NetBSD's |
| .BR consttime_memequal ()), |
| but no such function is specified in POSIX. |
| On Linux, it may be necessary to implement such a function oneself. |
| .SH SEE ALSO |
| .BR bcmp (3), |
| .BR bstring (3), |
| .BR strcasecmp (3), |
| .BR strcmp (3), |
| .BR strcoll (3), |
| .BR strncasecmp (3), |
| .BR strncmp (3), |
| .BR wmemcmp (3) |