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Date: Sun, 29 Dec 2002 08:06:00 -0000
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From: Jeff Sturm <jsturm@one-point.com>
Subject: Re: libgcj/9078: libffi: problems with uint8 on powerpc
Reply-To: Jeff Sturm <jsturm@one-point.com>
X-SW-Source: 2002-12/txt/msg01366.txt.bz2
List-Id: <gcc-prs.sourceware.org>

The following reply was made to PR libgcj/9078; it has been noted by GNATS.

From: Jeff Sturm <jsturm@one-point.com>
To: 173074@bugs.debian.org
Cc: gcc-gnats@gcc.gnu.org, <debian-gcc@lists.debian.org>,
   <gcc-prs@gcc.gnu.org>, <gcc-bugs@gcc.gnu.org>, <java-prs@gcc.gnu.org>,
   David Paul BELANGER <dbelan2@CS.McGill.CA>
Subject: Re: libgcj/9078: libffi: problems with uint8 on powerpc
Date: Sun, 29 Dec 2002 11:04:41 -0500 (EST)

 On Sat, 28 Dec 2002, Matthias Klose wrote:
 > ffi_type_uint8 and other arguments shorter than one word are
 > not passed in correctly to the function called by ffi_call.
 
 For an ffi_type_uint8, ffi_call expects the corresponding value
 pointer to be a (unsigned char *).  So this is correct usage:
 
     int b = 24;
     args[0] = &ffi_type_uint;
     values[0] = &b;
 
 Also correct would be:
 
     unsigned char b = 24;
     args[0] = &ffi_type_uint8;
     values[0] = &b;
 
 > Also, returns values are not passed correctly.  For example, instead of
 > finding the returned byte value where the pointer points to, it is found
 > at an offset of 3 from the pointer.
 
 Return values are handled a little differently than arguments.
 libffi/README says:
 
     RVALUE is a pointer to a chunk of memory that is to hold the
         result of the function call. Currently, it must be
         at least one word in size (except for the n32 version
         under Irix 6.x, which must be a pointer to an 8 byte
         aligned value (a long long). It must also be at least
         word aligned (depending on the return type, and the
         system's alignment requirements). If RTYPE is
         &ffi_type_void, this is ignored. If RVALUE is NULL,
         the return value is discarded.
 
 So this cannot work:
 
     unsigned char result;
     if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
   		   &ffi_type_uint8, args) != FFI_OK) {
 
 You could use "unsigned int result" on a 32-bit target, or use ffi_arg
 which is typedef'ed to work correctly on 32 or 64-bit targets:
 
     ffi_arg result;
     if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
                    &ffi_type_uint8, args) != FFI_OK) {
 
 With those changes your example should be portable to any target supported
 by libffi.
 
 Jeff