Question

I have a struct like this:

typedef struct _HEADER_IO
    {
        uint8_t field1 : 2;
        uint8_t field2 : 4;
        uint8_t field3 : 1;
        uint8_t field4 : 1;
        uint16_t field5;
        uint8_t field6;
    } HEADER_IO;

It’s basicly a message header that will be sent over tcp. The server reads this so that it knows what data follows in the buffer. However for some reason intead of the size being 4 bytes (2+4+1+1 first byte + 2 bytes from field 5 + 1 byte field 6) the size is 6 bytes.

Looking it up in memory view it is:

XX AA XX XX XX AA

Instead of:

XX XX XX XX

Where AA are never set no matter what I do. This is a problem because I am planning for the header to be send() to a server and the extra bytes are included making the server interpret the header wrong. What am I doing wrong?

Answer 1

In standard C you can’t help the fact that struct members can have padding inserted between them. You have to write a function to decode the data and store it in your struct before processing. This is because on some architectures unaligned memory access (reading from a pointer not aligned to, for example, 4 bytes) is very expensive and C will automatically pad your structures to avoid the cost. There’s no standard way to turn the feature on or off.

For example in GCC you can add __attribute__((packed)) after the struct definition and Visual Studio has some #pragma commands (see http://gcc.gnu.org/onlinedocs/gcc/Structure_002dPacking-Pragmas.html) that are also supported by GCC but beware that overall this is non-standard.

Since your comments mentioned it’s a Windows program, probably it would work if you add this before the struct definition:

#pragma pack(push,1)

And this after it:

#pragma pack(pop)

While it would be more portable to write code to more manually decode the header, the above approach should be faster.