mirror of https://github.com/mkerrisk/man-pages
118 lines
3.9 KiB
Groff
118 lines
3.9 KiB
Groff
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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.\" and Copyright 2015 Michael Kerrisk <mtk.manpages@gmail.com>
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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.\" manual page may be incorrect or out-of-date.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 Sun Jul 25 10:41:09 1993 by Rik Faith (faith@cs.unc.edu)
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.TH MEMCPY 3 2017-09-15 "" "Linux Programmer's Manual"
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.SH NAME
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memcpy \- copy memory area
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.SH SYNOPSIS
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.nf
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.B #include <string.h>
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.PP
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.BI "void *memcpy(void *" dest ", const void *" src ", size_t " n );
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.fi
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.SH DESCRIPTION
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The
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.BR memcpy ()
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function copies \fIn\fP bytes from memory area
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\fIsrc\fP to memory area \fIdest\fP.
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The memory areas must not overlap.
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Use
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.BR memmove (3)
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if the memory areas do overlap.
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.SH RETURN VALUE
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The
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.BR memcpy ()
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function returns a pointer to \fIdest\fP.
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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 memcpy ()
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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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Failure to observe the requirement that the memory areas
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do not overlap has been the source of significant bugs.
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(POSIX and the C standards are explicit that employing
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.BR memcpy ()
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with overlapping areas produces undefined behavior.)
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Most notably, in glibc 2.13
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.\" glibc commit 6fb8cbcb58a29fff73eb2101b34caa19a7f88eba
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a performance optimization of
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.BR memcpy ()
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on some platforms (including x86-64) included changing the order
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.\" From forward copying to backward copying
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in which bytes were copied from
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.I src
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to
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.IR dest .
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.PP
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This change revealed breakages in a number of applications that performed
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copying with overlapping areas.
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.\" Adobe Flash player was the highest profile example:
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.\" https://bugzilla.redhat.com/show_bug.cgi?id=638477
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.\" Reported: 2010-09-29 02:35 EDT by JCHuynh
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.\" Bug 638477 - Strange sound on mp3 flash website
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.\"
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.\" https://sourceware.org/bugzilla/show_bug.cgi?id=12518
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.\" Bug 12518 - memcpy acts randomly (and differently) with overlapping areas
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.\" Reported: 2011-02-25 02:26 UTC by Linus Torvalds
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.\"
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Under the previous implementation,
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the order in which the bytes were copied had fortuitously hidden the bug,
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which was revealed when the copying order was reversed.
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In glibc 2.14,
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.\" glibc commit 0354e355014b7bfda32622e0255399d859862fcd
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a versioned symbol was added so that old binaries
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(i.e., those linked against glibc versions earlier than 2.14)
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employed a
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.BR memcpy ()
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implementation that safely handles the overlapping buffers case
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(by providing an "older"
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.BR memcpy ()
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implementation that was aliased to
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.BR memmove (3)).
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.SH SEE ALSO
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.BR bcopy (3),
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.BR bstring (3),
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.BR memccpy (3),
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.BR memmove (3),
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.BR mempcpy (3),
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.BR strcpy (3),
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.BR strncpy (3),
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.BR wmemcpy (3)
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