[Valgrind-developers] Memcheck vs. malloc_usable_size()

[Greg P. <gp...@us...>]

Lots of Mac OS X code does the moral equivalent of this:

    void *p = malloc(1);
    size_t size = malloc_usable_size(p);
    memset(p, 0, size);

Memcheck reports "invalid write" errors for the memset(). 
The problem is that Memcheck's malloc replacement marks 
only the requested byte count as usable, but Valgrind's 
malloc_usable_size() implementation returns the actual 
allocated size, which may be bigger. In this case, Memcheck 
marks 1 byte of p as valid, but malloc_usable_size(p) returns 
8 (or 16, depending on the minimum allocation size).

One fix would be to have Memcheck wrap malloc_usable_size() 
and return the requested size only. Another would be for 
Memcheck to mark the full allocated size as usable, but 
that would tend to mask small overrun errors.


-- 
Greg Parker     gp...@us...