C/C++: Force Bit Field Order and Alignment


I read that the order of bit fields within a struct is platform specific. What about if I use different compiler-specific packing options, will this guarantee data is stored in the proper order as they are written? For example:

struct Message
  unsigned int version : 3;
  unsigned int type : 1;
  unsigned int id : 5;
  unsigned int data : 6;
} __attribute__ ((__packed__));

On an Intel processor with the GCC compiler, the fields were laid out in memory as they are shown. Message.version was the first 3 bits in the buffer, and Message.type followed. If I find equivalent struct packing options for various compilers, will this be cross-platform?

8/5/2018 12:37:52 PM

Accepted Answer

No, it will not be fully-portable. Packing options for structs are extensions, and are themselves not fully portable. In addition to that, C99 ยง6.7.2.1, paragraph 10 says: "The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined."

Even a single compiler might lay the bit field out differently depending on the endianness of the target platform, for example.

9/29/2009 1:31:06 AM

Bit fields vary widely from compiler to compiler, sorry.

With GCC, big endian machines lay out the bits big end first and little endian machines lay out the bits little end first.

K&R says "Adjacent [bit-]field members of structures are packed into implementation-dependent storage units in an implementation-dependent direction. When a field following another field will not fit ... it may be split between units or the unit may be padded. An unnamed field of width 0 forces this padding..."

Therefore, if you need machine independent binary layout you must do it yourself.

This last statement also applies to non-bitfields due to padding -- however all compilers seem to have some way of forcing byte packing of a structure, as I see you already discovered for GCC.

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