C struct sizes inconsistence [duplicate]

Answer 1

By default, structure fields are aligned on natural boundaries. For example, a 4-byte field will start on a 4-byte boundary. The compiler inserts pad bytes to achieve this. You can avoid the padding by using #pragma pack(0) or other similar compiler directives

Answer 2

Ever heard of alignment and padding?

Basically, to ensure fast access, certain types have to be on certain bounds of memory addresses.
This is called alignment.

To achieve that, the compiler is allowed to insert bytes into your data structure to achieve that alignment.
This is called padding.

Answer 3

If you have a C99 compiler and can use the "new" fixed-width types: make an array of uint8_t and do the separation in members yourself.

uint8_t data[8];

x1 = data[0];
len = data[1];
host = data[2] * 256 + data[3]; /* big endian */
content[0] = data[4];
content[1] = data[5];
content[2] = data[6];
content[3] = data[7];

/* ... */

You can follow the same procedure in C89 if you can rely on CHAR_BIT being 8.

Answer 4

C makes no guarantees on the size of a struct. The compiler is allowed to line up the members however it wants. Usually, as in this case, it will make the size word-aligned since that's fastest on most machines.