#500 #pragma pack(push,n) does not work when n is a macro

[vitaly_s]

GCC 5.2.0.

pragma pack(push,n) does not work when n is a macro

Example code produces:

sizeof(struct A) = 16
sizeof(struct B) = 12
sizeof(struct C) = 16

while sizeof(struct C) must be 12.

Also, #pragma pack(n) does not work with macro. See example in file b.c.

gcc -O2 -o b.exe b.c

[Kai Tietz]

Thanks for reporting. I tested it with current trunk and older gcc
(4.9, 4.8), and things look pretty similar. So this isn't actually a
regression, but it is a missing feature for gcc. So you might want to
report this missing feature to gcc's bz for discussion.

Thanks,
Kai

2015-10-08 16:45 GMT+02:00 vitaly_s vitalysxii@users.sf.net:

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[bugs:#500] #pragma pack(push,n) does not work when n is a macro

Status: open
Group: v1.0 (example)
Created: Thu Oct 08, 2015 02:45 PM UTC by vitaly_s
Last Updated: Thu Oct 08, 2015 02:45 PM UTC
Owner: nobody
Attachments:

b.c (843 Bytes; text/plain)

GCC 5.2.0.

pragma pack(push,n) does not work when n is a macro

Example code produces:

sizeof(struct A) = 16
sizeof(struct B) = 12
sizeof(struct C) = 16

while sizeof(struct C) must be 12.

Also, #pragma pack(n) does not work with macro. See example in file b.c.

gcc -O2 -o b.exe b.c

---

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[vitaly_s]

Thanks for answer. After I have found it, I didn’t test it with previous releases. It is good to have this information now.

So, to conclude... While my own code can use something like this

#pragma pack(push)

#if     defined(_WIN64)
#pragma pack(8)
#elif   defined(_WIN32)
#pragma pack(4)
#else
#error Win32 and Win64 targets are currently supported
#endif

...

#pragma pack(pop)

although it would be better to use simple '#pragma pack(push, _PLATFORM_PACKING)' the bug still has much more consequences. Almost every include file supplied with GCC also has '#pragma pack(push,_CRT_PACKING)' directive. Thus, it is certainly a bug, not a 'missing feature', because GCC itself heavily relies on it.