memcpy(3) — Linux manual page

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
       attributes(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 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)).

SEE ALSO

       bcopy(3), bstring(3), memccpy(3), memmove(3), mempcpy(3),
       strcpy(3), strncpy(3), wmemcpy(3)

COLOPHON

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Linux man-pages 6.9.1          2024-05-02                      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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