man3/memcpy.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\" and Copyright 2015 Michael Kerrisk <mtk.manpages@gmail.com>
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  25 .\"
  26 .\" References consulted:
  27 .\"     Linux libc source code
  28 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  29 .\"     386BSD man pages
  30 .\" Modified Sun Jul 25 10:41:09 1993 by Rik Faith (faith@cs.unc.edu)
  31 .TH MEMCPY 3  2017-09-15 "" "Linux Programmer's Manual"
  32 .SH NAME
  33 memcpy \- copy memory area
  34 .SH SYNOPSIS
  35 .nf
  36 .B #include <string.h>
  37 .PP
  38 .BI "void *memcpy(void *" dest ", const void *" src ", size_t " n );
  39 .fi
  40 .SH DESCRIPTION
  41 The
  42 .BR memcpy ()
  43 function copies \fIn\fP bytes from memory area
  44 \fIsrc\fP to memory area \fIdest\fP.
  45 The memory areas must not overlap.
  46 Use
  47 .BR memmove (3)
  48 if the memory areas do overlap.
  49 .SH RETURN VALUE
  50 The
  51 .BR memcpy ()
  52 function returns a pointer to \fIdest\fP.
  53 .SH ATTRIBUTES
  54 For an explanation of the terms used in this section, see
  55 .BR attributes (7).
  56 .TS
  57 allbox;
  58 lb lb lb
  59 l l l.
  60 Interface       Attribute       Value
  61 T{
  62 .BR memcpy ()
  63 T}      Thread safety   MT-Safe
  64 .TE
  65 .SH CONFORMING TO
  66 POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
  67 .SH NOTES
  68 Failure to observe the requirement that the memory areas
  69 do not overlap has been the source of significant bugs.
  70 (POSIX and the C standards are explicit that employing
  71 .BR memcpy ()
  72 with overlapping areas produces undefined behavior.)
  73 Most notably, in glibc 2.13
  74 .\" glibc commit 6fb8cbcb58a29fff73eb2101b34caa19a7f88eba
  75 a performance optimization of
  76 .BR memcpy ()
  77 on some platforms (including x86-64) included changing the order
  78 .\" From forward copying to backward copying
  79 in which bytes were copied from
  80 .I src
  81 to
  82 .IR dest .
  83 .PP
  84 This change revealed breakages in a number of applications that performed
  85 copying with overlapping areas.
  86 .\" Adobe Flash player was the highest profile example:
  87 .\"   https://bugzilla.redhat.com/show_bug.cgi?id=638477
  88 .\"   Reported: 2010-09-29 02:35 EDT by JCHuynh
  89 .\"   Bug 638477 - Strange sound on mp3 flash website
  90 .\"
  91 .\"   https://sourceware.org/bugzilla/show_bug.cgi?id=12518
  92 .\"   Bug 12518 - memcpy acts randomly (and differently) with overlapping areas
  93 .\"   Reported:       2011-02-25 02:26 UTC by Linus Torvalds
  94 .\"
  95 Under the previous implementation,
  96 the order in which the bytes were copied had fortuitously hidden the bug,
  97 which was revealed when the copying order was reversed.
  98 In glibc 2.14,
  99 .\" glibc commit 0354e355014b7bfda32622e0255399d859862fcd
 100 a versioned symbol was added so that old binaries
 101 (i.e., those linked against glibc versions earlier than 2.14)
 102 employed a
 103 .BR memcpy ()
 104 implementation that safely handles the overlapping buffers case
 105 (by providing an "older"
 106 .BR memcpy ()
 107 implementation that was aliased to
 108 .BR memmove (3)).
 109 .SH SEE ALSO
 110 .BR bcopy (3),
 111 .BR bstring (3),
 112 .BR memccpy (3),
 113 .BR memmove (3),
 114 .BR mempcpy (3),
 115 .BR strcpy (3),
 116 .BR strncpy (3),
 117 .BR wmemcpy (3)