steveo225 steveo225 - 4 months ago 42
C++ Question

Force C++ structure to pack tightly

I am attempting to read in a binary file. The problem is that the creator of the file took no time to properly align data structures to their natural boundaries and everything is packed tight. This makes it difficult to read the data using C++ structs.

Is there a way to force a

to be packed tight?


struct {
short a;
int b;

The above structure is 8 bytes: 2 for
short a
, 2 for padding, 4 for
int b
. However, on disk, the data is only 6 bytes (not having the 2 bytes of padding for alignment)

Please be aware the actual data structures are thousands of bytes and many fields, including a couple arrays, so I would prefer not to read each field individually.


If you're using GCC, you can do struct __attribute__ ((packed)) { short a; int b; }

On VC++ you can do #pragma pack(1). This option is also supported by 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.