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MEMCPY(3) Linux Programmer's Manual MEMCPY(3)

NAME

     memcpy - copy memory area

SYNOPSIS

     #include <string.h>

     void *memcpy(void *dest, const void *src, size_t n);

DESCRIPTION

     The  memcpy()  function  copies  n bytes from memory area src to memory
     area dest.  The memory areas must not overlap.  Use memmove(3)  if  the
     memory areas do overlap.

RETURN VALUE

     The memcpy() function returns a pointer to dest.

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).

     +----------+---------------+---------+
     |Interface | Attribute     | Value   |
     +----------+---------------+---------+
     |memcpy()  | Thread safety | MT-Safe |
     +----------+---------------+---------+

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.

NOTES

     Failure to observe the requirement that the memory areas do not overlap
     has  been  the  source of significant bugs.  (POSIX and the C standards
     are explicit that employing memcpy() with  overlapping  areas  produces
     undefined  behavior.)   Most notably, in glibc 2.13 a performance opti-
     mization of memcpy() on  some  platforms  (including  x86-64)  included
     changing the order in which bytes were copied from src to dest.

     This  change  revealed  breakages in a number of applications that per-
     formed copying with overlapping areas.  Under the previous  implementa-
     tion,  the order in which the bytes were copied had fortuitously hidden
     the bug, which was revealed when the copying order  was  reversed.   In
     glibc  2.14,  a  versioned symbol was added so that old binaries (i.e.,
     those linked against glibc versions earlier than 2.14) employed a  mem-
     cpy()  implementation  that safely handles the overlapping buffers case
     (by providing an "older" memcpy() implementation that  was  aliased  to
     memmove(3)).