Mail Archives: djgpp/2015/05/13/16:27:55
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On 5/13/15, Eli Zaretskii (eliz AT gnu DOT org) <djgpp AT delorie DOT com> wrote:
>> Date: Wed, 13 May 2015 22:14:58 +0300
>> From: "Ozkan Sezer (sezeroz AT gmail DOT com)" <djgpp AT delorie DOT com>
>>
>> >> > Wasn't that already fixed in the past?
>> >>
>> >> Well, obviously not. that pragma pack(4) is there in the cvs
>> >
>> > People make mistakes when they commit, so it could well be that the
>> > problem was fixed at some point and then broken again in a subsequent
>> > commit.
>> >
>>
>> Alright then. OK to commit the following?
>>
>> Index: dir.h
>> ===================================================================
>> RCS file: /cvs/djgpp/djgpp/include/dir.h,v
>> retrieving revision 1.6
>> diff -U 5 -r1.6 dir.h
>> --- dir.h 2 May 2015 07:31:44 -0000 1.6
>> +++ dir.h 13 May 2015 19:12:02 -0000
>> @@ -52,11 +52,11 @@
>> unsigned long long fd_reserved __attribute__((packed));
>> char fd_longname[260] __attribute__((packed));
>> char fd_name[14] __attribute__((packed));
>> };
>>
>> -#pragma pack(4)
>> +#pragma pack()
>
> "pack()" or "pack(0)"?
>
pack(), without the 0. Actually it may even be better to use the
push/pop variant of the pragma, like:
#pragma pack(push,1)
...
#pragma pack(pop)
which have different semantics
http://gcc.gnu.org/onlinedocs/gcc/Structure-Packing-Pragmas.html
Ideas?
>> > However, note that the latter will only work when you compile it as a
>> > C
>> > source; C++ doesn't allow such syntax, and you will have to fall back
>> > to
>> > declaring each struct member with the packed attribute. Therefore,
>> > it's
>> > best
>> > to only use declarations such as above if you are *certain* it won't
>> > be
>> > ever
>> > compiled as a C++ source.
>> >
>> > Once again, a lot has changed in C++ since then, so if all the
>> > versions of GCC we want to support allow the above, there's no reason
>> > to keep the attribute on each member of those structs.
>>
>> IIRC, never had such a problem with C++, although I haven't used
>> g++ older than v3.0
>
> If you (or someone else) can verify that the size of the structure and
> the offsets of each member are the same in both C and C++ programs,
> then I see no reason to keep the attribute on every member.
I did the following as a foo.c and foo.cc:
#define COMPILE_TIME_ASSERT(name, x) \
typedef int _djchk_ ## name[(x) * 2 - 1]
#include "dos.h"
COMPILE_TIME_ASSERT(find_t,sizeof(struct _find_t)==286);
#include "dir.h"
COMPILE_TIME_ASSERT(ffblk,sizeof(struct ffblk)==290);
COMPILE_TIME_ASSERT(ffblklfn,sizeof(struct ffblklfn)==318);
#include "coff.h"
COMPILE_TIME_ASSERT(exln,sizeof(struct external_lineno)==6);
COMPILE_TIME_ASSERT(exse,sizeof(struct external_syment)==18);
COMPILE_TIME_ASSERT(exau,sizeof(union external_auxent)==18);
COMPILE_TIME_ASSERT(exrl,sizeof(struct external_reloc)==10);
Compiled using existing and modified headers for djgpp using gcc/g++
v3.4.6 and 5.1.0, no errors. I also compiled its coff.h-only parts
for linux (i686 gcc-4.3.0 and clang-3.4.2), osx (ppc gcc-4.0.1 and
x86_64 gcc-4.2.1), mingw (gcc-3.4.5), mingw-w64 (x86_64 gcc-4.5.4):
no errors.
Diff for the modified headers is attached (1.patch)
Ideas?
On 5/13/15, Andris Pavenis (andris DOT pavenis AT iki DOT fi) <djgpp AT delorie DOT com> wrote:
> I guess it would be a good idea to have small test in build process to
> verify that offsets of
> members are correct for structures intended for DOS calls. So build would
> fail if something is
> wrong with member packing
>
> Andris
We can add compile-time asserts like the above to the headers: ideas?
--
O.S.
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