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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | SEE ALSO | COLOPHON |
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memcpy(3) Library Functions Manual memcpy(3)
memcpy - copy memory area
Standard C library (libc, -lc)
#include <string.h>
void *memcpy(size_t n;
void dest[restrict n], const void src[restrict n],
size_t n);
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.
The memcpy() function returns a pointer to dest.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ memcpy() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C11, POSIX.1-2008.
POSIX.1-2001, C89, SVr4, 4.3BSD.
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 optimization 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
performed copying with overlapping areas. Under the previous
implementation, 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 memcpy() implementation that safely
handles the overlapping buffers case (by providing an "older"
memcpy() implementation that was aliased to memmove(3)).
bcopy(3), bstring(3), memccpy(3), memmove(3), mempcpy(3),
strcpy(3), strncpy(3), wmemcpy(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 memcpy(3)
Pages that refer to this page: bcopy(3), bstring(3), cmsg(3), CPU_SET(3), memccpy(3), memmove(3), mempcpy(3), size_t(3type), void(3type), wmemcpy(3), feature_test_macros(7), signal-safety(7), string_copying(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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