Re: return value buffer malloc()'ed vs alloca()'ed

[Anthony Green <green at moxielogic dot com>]

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
>