memcpy(3) Library Functions Manual memcpy(3)
NAME
memcpy - copy memory area
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <string.h>
void *memcpy(void dest[restrict .n], const void src[restrict .n],
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 attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ memcpy() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
C11, POSIX.1-2008.
HISTORY
POSIX.1-2001, C89, SVr4, 4.3BSD.
CAVEATS
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)).
SEE ALSO
bcopy(3), bstring(3), memccpy(3), memmove(3), mempcpy(3), strcpy(3),
strncpy(3), wmemcpy(3)
Linux man-pages 6.7 2023-10-31 memcpy(3)
Generated by dwww version 1.16 on Tue Dec 16 06:20:10 CET 2025.