From: Anthony Green <green@moxielogic.com>
To: Igor Bogomazov <ygrex@ygrex.ru>
Cc: "libffi-discuss@sourceware.org" <libffi-discuss@sourceware.org>
Subject: Re: return value buffer malloc()'ed vs alloca()'ed
Date: Tue, 08 Oct 2013 17:30:00 -0000 [thread overview]
Message-ID: <CACxje58LTEpBiaUHQn6Bp=Puvmz-Sg7gReGrWfz2Nz-diZG4GA@mail.gmail.com> (raw)
In-Reply-To: <20131008101354.43baeeeb@ygrex-mac>
Igor,
You've hit one of the oddities of the libffi API. The result
buffer needs to be the largest native integral type on your system.
Use a 64 bit long for rc instead of mallocing the exact return type
size. You can pass it into ffi_call as &rc and simply cast it to an
int at the end. I just checked the docs and it's definitely not
clear. I'll fix this. libffi is just a tiny bit faster when it can
make this size assumption, but I agree that it isn't pretty.
AG
On Tue, Oct 8, 2013 at 1:13 PM, Igor Bogomazov <ygrex@ygrex.ru> wrote:
> Hello,
>
> I've been trying to investigate valgrind warnings for a while and found
> an undocumented feature, please let me know if it is well-known.
>
> What I did.
>
> I modified a code given in «2.2 Simple Example» so that return value
> (rc), originally declared as (int), became an (int *)malloc(sizeof(int))
> so that it is resident in heap since that. Of cource, (&rc) replaced
> with (rc) later in the code.
>
> What I get.
>
> valgrind complaints about «Invalid write of size 8» while «Address
> 0x55ec040 is 0 bytes inside a block of size 4 alloc'd», it is exactly
> that allocated (rc) buffer.
>
> Notes.
>
> Allocating buffer for the return value using alloca() does the trick and
> makes valgrind silent.
>
> Further.
>
> I looked at x86/unix64.S, it is exactly the line:
> movq %rax, (%rdi)
> that causes the valgrind's warning (at .Lst_uint32)
>
> That is my question: is it necessary to allocate a buffer for the return
> value with alloca() and never with malloc()?
>
> --
> Sincerely yours,
>
> Igor Bogomazov
>
next prev parent reply other threads:[~2013-10-08 17:30 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-08 17:14 Igor Bogomazov
2013-10-08 17:30 ` Anthony Green [this message]
2013-10-08 17:36 ` Andrew Haley
2013-10-08 17:40 ` Anthony Green
2013-10-08 17:45 ` Andrew Haley
2013-10-09 5:09 ` Igor Bogomazov
2013-11-15 16:18 ` Broken tests in libffi testsuite Andrew Haley
2013-11-16 11:24 ` Alan Modra
2013-10-08 17:33 ` return value buffer malloc()'ed vs alloca()'ed Andrew Haley
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