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5.3.6 Overlapping source and destination blocks

The following C library functions copy some data from one memory block to another (or something similar): memcpy(), strcpy(), strncpy(), strcat(), strncat(). The blocks pointed to by their ‘src’ and ‘dst’ pointers aren't allowed to overlap. Memcheck checks for this.

For example:

==27492== Source and destination overlap in
            memcpy(0xbffff294, 0xbffff280, 21)
==27492==    at 0x40026CDC: memcpy
               (mc_replace_strmem.c:71)
==27492==    by 0x804865A: main (overlap.c:40)

You don't want the two blocks to overlap because one of them could get partially overwritten by the copying.

You might think that Memcheck is being overly pedantic reporting this in the case where ‘dst’ is less than ‘src’. For example, the obvious way to implement memcpy() is by copying from the first byte to the last. However, the optimisation guides of some architectures recommend copying from the last byte down to the first. Also, some implementations of memcpy() zero ‘dst’ before copying, because zeroing the destination's cache line(s) can improve performance.

In addition, for many of these functions, the POSIX standards have wording along the lines “If copying takes place between objects that overlap, the behavior is undefined.” Hence overlapping copies violate the standard.

The moral of the story is: if you want to write truly portable code, don't make any assumptions about the language implementation.