[Bug libstdc++/65114] New: char_traits::copy violates memcpy constraints, own postcondition

[Bug libstdc++/65114] New: char_traits::copy violates memcpy constraints, own postcondition

[msebor at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65114

            Bug ID: 65114
           Summary: char_traits::copy violates memcpy constraints, own
                    postcondition
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gmail dot com

The precondition on char_traits::copy(s, p, n), namely that p not be in the
range [s, s + n), is weaker than the precondition on a call to memcpy(s, p, n).
 The latter requires that none of the characters accessed via s is also
accessed via p.  The program below shows an example of a call to
char_traits<char>::copy that satisfies the C++ precondition but violates the C
precondition (as indicated by the Valgrind error).  The element a[1] is
accessed by both s and p.  As a result, the program also fails the C++
postcondition.

$ cat t.cpp && g++ -Wall t.cpp && valgrind ./a.out
#include <assert.h>
#include <stdio.h>
#include <string>

struct Traits: std::char_traits<char> {
    static char* copy (char *s, const char *p, size_t n) {
        assert (p < s || s + n < p);
        return std::char_traits<char>::copy (s, p, n);
    }
};

int main ()
{
    char a[] = "abc";
    const size_t n = 2;
    const char *p = a;
    char *s = a + 1;

    for (size_t i = 0; i != n; ++i)
        Traits::assign (s[i], p[i]);

    char b[] = "abc";
    p = b;
    s = b + 1;

    Traits::copy (s, p, n);

    printf ("%s == %s\n", a, b);

    assert (0 == Traits::compare (a, b, sizeof a));
}

==15497== Memcheck, a memory error detector
==15497== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==15497== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==15497== Command: ./a.out
==15497== 
==15497== Source and destination overlap in memcpy(0xffefffe41, 0xffefffe40, 2)
==15497==    at 0x4C2E13D: memcpy@@GLIBC_2.14 (in
/usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==15497==    by 0x400859: std::char_traits<char>::copy(char*, char const*,
unsigned long) (in /home/msebor/tmp/a.out)
==15497==    by 0x4008BA: Traits::copy(char*, char const*, unsigned long) (in
/home/msebor/tmp/a.out)
==15497==    by 0x40078A: main (in /home/msebor/tmp/a.out)
==15497== 
aaa == aab
a.out: t.cpp:30: int main(): Assertion `0 == Traits::compare (a, b, sizeof a)'
failed.
==15497== 
==15497== HEAP SUMMARY:
==15497==     in use at exit: 0 bytes in 0 blocks
==15497==   total heap usage: 2 allocs, 2 frees, 188 bytes allocated
==15497== 
==15497== All heap blocks were freed -- no leaks are possible
==15497== 
==15497== For counts of detected and suppressed errors, rerun with: -v
==15497== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
Aborted (core dumped)

[Bug libstdc++/65114] char_traits::copy violates memcpy constraints, own postcondition

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65114

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-02-19
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
PR65049 is another case where I wonder if char_traits members should be more
consistent with the C memxxx() functions than currently specified, i.e. should
have the same preconditions as the C functions.

[Bug libstdc++/65114] char_traits::copy violates memcpy constraints, own postcondition

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65114

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |SUSPENDED

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is https://wg21.link/lwg3085