On 11/9/2011 8:06 AM, Petr Pilař wrote:
Hello,
I came upon this weird behaviour, which is probably best described
using a sample, so here it is: http://codepad.org/7zSvMHGZ
I think this line alone sums it up pretty nicely:
"std::copy_backward(data + 2, data + size, data + size + 1);" (data_m
was declared as "int data_m[size + 1];")
When I compile the code like so: g++ Test.cpp -ggdb3 and then run
valgrind with --leak-check=full on it, I get this:
http://codepad.org/1fMnYMbn
Again, the line "Source and destination overlap in memcpy(0x7ff00038c,
0x7ff000388, 32)" pretty much sums it up.
What am I doing wrong if anything? Is this caused by
std::copy_backward using memcpy and not memmove? Or is this a problem
on valgrinds's side? Or perhaps none of these? :)
Best regards
Peter.
According to
http://www.cplusplus.com/reference/algorithm/copy_backward/
"If both ranges overlap in such a way that /result/ (which is the
past-the-end element in the destination range) points to an element in
the range /[first,last)/, the function copy() should be used instead."
The third edition of Stroustrup's "The C++ Programming Language":
"We use copy() when the sequences do not overlap, or if the end of the
output sequence is in the input sequence. We use copy_backward() when
the beginning of the output sequence is in the input sequence. In that
way, no element is overwritten until it has been copied."
Take a look at the equivalent template implementation in the above
link. If in fact this is how the function is implemented in
<algorithm>, the compiler could optimize it pretty heavily, and decide
to use memcpy() to copy multiple elements at a time rather than
implement the core of the loop inline copying single elements. Or maybe
the g++ implementation of the template uses memcpy() directly.
I bet that you would not get an error with copy_backward() if the number
of bytes to be copied is less than 32 (e.g. size == 3, or size == 6 for
32-bit integers).
The use of memcpy() this way when the sequences have a substantial
amount of overlap is probably a g++ or library bug. What happens if you
make your own local version of copy_backward() using the above model and
use that? What is in the implementation of copy_backward() in
<algorithm>? What version of g++ are you using?
--
David Chapman [email protected]
Chapman Consulting -- San Jose, CA
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