Comment 129 for bug 727064

(In reply to comment #79)

If you prefer, the POSIX man page for memcpy states: "If copying takes place between objects that overlap, the behavior is undefined." The change history is: "First released in Issue 1. Derived from Issue 1 of the SVID."

http://www.opengroup.org/onlinepubs/009695399/functions/memcpy.html

A quick Google search shows this to be the preferred language. I'm not sure why it differs in the Linux man pages. 'Should' is semantically meaningless - it is either predictable or it is not.

The BSD man page [1993] corroborates:

http://www.unix.com/man-page/FreeBSD/3/memcpy/

BUGS
     In this implementation memcpy() is implemented using bcopy(3), and there-
     fore the strings may overlap. On other systems, copying overlapping
     strings may produce surprises. Programs intended to be portable should
     use memmove(3) when src and dst may overlap.

And here the text from an old version of the GNU C library documentation:

http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_5.html#SEC61

Function: void * memcpy (void *to, const void *from, size_t size)

The memcpy function copies size bytes from the object beginning at from into the object beginning at to. The behavior of this function is undefined if the two arrays to and from overlap; use memmove instead if overlapping is possible.

For others looking back, I also have to note that you omitted the rest of the text from comment #37, which states: "Use memmove(3) if the memory areas do overlap."