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 struct to be packed tight?
Example:
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 could get tight packing of structs (and classes) using its packed attribute:
struct __attribute__((packed)) { short a; int b; }
Since C++11† there's a standard way of specifying attributes and GCC supports the standard syntax as well:
struct [[gnu::packed]] { short a; int b; }
Clang pretty much copies GCC's syntax and hence both syntaxes above work on it too.
On VC++ you can do #pragma pack(1). This option is also supported by GCC and Clang.
#pragma pack(push, 1)
struct { short a; int b; };
#pragma pack(pop)
Other compilers may have different options to do a tight packing of the structure with no padding.
†: You've to specify -std=c++11, -std=gnu++11 or newer of the variants. If you're using C then support exists since C23.