Hi,
On Wed, 28 Jan 2009, Linus Torvalds wrote:
> On Tue, 27 Jan 2009, Johannes Schindelin wrote:
> >
> > To help ye Gods, I put together this almost minimal C program:
>
> This one is buggy.
Not exactly buggy. Underexplained.
> > out = fopen("/dev/null", "w");
> > fwrite(compressed + 51, 51, 1, out);
> > fwrite(compressed + 51, 1, 1, stderr);
> > fflush(out);
> > fclose(out);
>
> The problem is that the first argument to that first "fwrite()" is simply
> wrong. It shouldn't be "compressed + 51", it should be just "compressed".
Nope. It should be "compressed + 51" to narrow down the issue, as
valgrind does not complain about _any other_ offset.
Not even when that is _well_ after the 58 bytes deflate() says are
available.
> As it is, you're writing 51 bytes, starting at 51 bytes in, and that's
> obviously not correct (you only got 58 bytes from deflate()).
It is not, granted. But I left it in for a purpose: to show that valgrind
does not even bother to mention bytes we think should be invalid.
I thought that there might be a shortcut for /dev/null, so I changed the
outfile to a real file, and it _still_ does not complain.
> So valgrind does complain about it, but for a perfectly valid reason.
Only it does not. It complains about the write of 1 byte, not the write
of 51.
But I know why: "out" is opened buffered, so it shows the error (well
delayed, I might add, and not in a helpful manner) when fflush() is
called.
The real issue, namely that an access of offset 51 triggers a valgrind
error, is demonstrated by my small test case.
Ciao,
Dscho
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