165 lines
4.1 KiB
HTML
165 lines
4.1 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of MEMCPY</TITLE>
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</HEAD><BODY>
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<H1>MEMCPY</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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memcpy - copy memory area
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/string.h">string.h</A>></B>
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<B>void *memcpy(void *</B><I>dest</I><B>, const void *</B><I>src</I><B>, size_t </B><I>n</I><B>);</B>
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</PRE>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The
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<B>memcpy</B>()
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function copies <I>n</I> bytes from memory area
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<I>src</I> to memory area <I>dest</I>.
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The memory areas must not overlap.
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Use
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<B><A HREF="/cgi-bin/man/man2html?3+memmove">memmove</A></B>(3)
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if the memory areas do overlap.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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The
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<B>memcpy</B>()
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function returns a pointer to <I>dest</I>.
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<A NAME="lbAF"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>memcpy</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe<BR></TD></TR>
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</TABLE>
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<A NAME="lbAG"> </A>
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<H2>CONFORMING TO</H2>
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POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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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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<B>memcpy</B>()
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with overlapping areas produces undefined behavior.)
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Most notably, in glibc 2.13
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a performance optimization of
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<B>memcpy</B>()
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on some platforms (including x86-64) included changing the order
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in which bytes were copied from
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<I>src</I>
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to
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<I>dest</I>.
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<P>
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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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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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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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<B>memcpy</B>()
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implementation that safely handles the overlapping buffers case
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(by providing an "older"
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<B>memcpy</B>()
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implementation that was aliased to
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<B><A HREF="/cgi-bin/man/man2html?3+memmove">memmove</A></B>(3)).
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<A NAME="lbAI"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?3+bcopy">bcopy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+bstring">bstring</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+memccpy">memccpy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+memmove">memmove</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+mempcpy">mempcpy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+strcpy">strcpy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+strncpy">strncpy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+wmemcpy">wmemcpy</A></B>(3)
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<A NAME="lbAJ"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="1"><A HREF="#lbAB">NAME</A><DD>
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<DT id="2"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="3"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="4"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="5"><A HREF="#lbAF">ATTRIBUTES</A><DD>
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<DT id="6"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="7"><A HREF="#lbAH">NOTES</A><DD>
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<DT id="8"><A HREF="#lbAI">SEE ALSO</A><DD>
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<DT id="9"><A HREF="#lbAJ">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:48 GMT, March 31, 2021
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