mirror of https://github.com/mkerrisk/man-pages
98 lines
3.0 KiB
Groff
98 lines
3.0 KiB
Groff
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" References consulted:
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.\" Linux libc source code
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.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
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.\" 386BSD man pages
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.\" Modified Sat Jul 24 18:55:27 1993 by Rik Faith (faith@cs.unc.edu)
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.TH MEMCMP 3 2017-03-13 "" "Linux Programmer's Manual"
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.SH NAME
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memcmp \- compare memory areas
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.SH SYNOPSIS
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.nf
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.B #include <string.h>
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.sp
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.BI "int memcmp(const void *" s1 ", const void *" s2 ", size_t " n );
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.fi
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.SH DESCRIPTION
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The
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.BR memcmp ()
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function compares the first \fIn\fP bytes (each interpreted as
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.IR "unsigned char" )
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of the memory areas \fIs1\fP and \fIs2\fP.
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.SH RETURN VALUE
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The
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.BR memcmp ()
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function returns an integer less than, equal to, or
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greater than zero if the first \fIn\fP bytes of \fIs1\fP is found,
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respectively, to be less than, to match, or be greater than the first
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\fIn\fP bytes of \fIs2\fP.
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For a nonzero return value, the sign is determined by the sign of
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the difference between the first pair of bytes (interpreted as
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.IR "unsigned char" )
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that differ in
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.I s1
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and
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.IR s2 .
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If
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.I n
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is zero, the return value is zero.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lb lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR memcmp ()
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T} Thread safety MT-Safe
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.TE
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.SH CONFORMING TO
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POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
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.SH NOTES
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.PP
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Do not use
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.BR memcmp ()
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to compare security critical data, such as cryptographic secrets,
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because the required CPU time depends on the number of equal bytes.
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Instead, a function that performs comparisons in constant time is required.
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Some operating systems provide such a function (e.g., NetBSD's
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.BR consttime_memequal ()),
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but no such function is specified in POSIX.
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On Linux, it may be necessary to implement such a function oneself.
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.SH SEE ALSO
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.BR bcmp (3),
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.BR bstring (3),
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.BR strcasecmp (3),
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.BR strcmp (3),
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.BR strcoll (3),
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.BR strncasecmp (3),
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.BR strncmp (3),
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.BR wmemcmp (3)
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