alignas nie mogą zastąpić #pragma pack.. GCC akceptuje deklarację alignas, ale nadal utrzymuje prawidłowo wyrównany element: spełnienie najostrzejszego wymogu wyrównania (w tym przypadku wyrównanie long) również spełnia określone wymaganie.. Jednak GCC jest zbyt pobłażliwe jako średnia rzeczywiście wyraźnie zabrania tego w §7.6.2 ustęp 5: W kompilatorze GCC, należy dodać po deklaracji struktury przed średnikiem kończącym jedną linijkę: __attribute__ ((packed)) Działa ona dokładnie tak samo, jak makra #pragma, jednak jest ona obecna tylko w kompilatorze GCC. #pragma pack (push, 1) struct {short a; int b;} #pragma pack (pop) Other compilers may have options to do a tight packing of the structure with no padding. * Any small packed array type with a static size. The #pragma pack option allows you to change the alignment of datatypes within a struct to align to boundaries smaller than it's size. Data objects are generally aligned at specific word boundaries so that read/write operations can be performed efficiently. The #pragma pack directive cannot increase the alignment of a member, but rather can decrease the alignment. Default packing and #pragma pack. If this command line option is used, warnings will even be issued for unknown pragmas in system header files. For compatibility with Microsoft Windows compilers, GCC supports a set of #pragma directives that change the maximum alignment of members of structures (other than zero-width bit-fields), unions, and classes subsequently defined. 6.57.8 Structure-Packing Pragmas. /* Handle #pragma, system V.4 style. c - Is gcc's __attribute__((packed)) / #pragma pack unsafe? #pragma pack(n) instances and finalized by a single #pragma pack(pop). Logged In: YES user_id=477089. Some things (particularly in relation to hardware) do not have the luxury to waste bytes like this and they send their data in an exact fit. The following table shows the strictest alignment for each platform. Packed objects are read and written using unaligned accesses. Sometimes you can coerce your compiler into not using the processor’s normal alignment rules by using a pragma, usually #pragma pack. #pragma pack(2) When n is 0 or omitted, the member alignment reverts to the natural alignment values. It does not force alignment of ALL variables - it only changes the alignment of variables larger than the pack setting. Note: compilers that do not support these pragmas may provide equivalent compile-time options, such as gcc's -fcx-limited-range and -ffp-contract. By default optimizations are suppressed. Structure-Packing Pragmas¶. This means that it is not wise to read data from a hardware device directly into a normal structure. Ogólnie rzecz biorąc, celem #pragma pack polega na kontrolowaniu rozmiaru i układu struktur w c poprzez kontrolowanie maksymalnego wyrównania ich elementów członkowskich. 4: #pragma GCC dependency Checks the dates of current and other file. filter_none. If this command line option is used, warnings will even be issued for unknown pragmas in system header files. Does anyone know the story here? I am porting a C++/C project that typically is compiled with IBM VisualAge C++ compiler and is littered with #pragma pack directives to control the alignment of structures (e.g. Padding is usually used to make fields aligned to their natural size, because some architectures impose penalties for unaligned access or don't allow it at all. In C, the compiler will lay out members of a struct in the order in which they're declared, with possible padding bytes inserted between members, or after the last member, to ensure that each member is aligned properly. 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