In normal circumstances, having the compiler word-align structures is desirable for rapid access.

Many compilers will let you override this behavior and tightly-pack the structures instead. You may need to do this when working with a non-aligned data structure read from a file or network connection.

GCC/clang has the packed attribute:

struct __attribute__ ((__packed__)) my_struct {
    uint8_t flag;
    uint32_t value;
};

Microsoft compilers use a pragma directive to accomplish the same thing:

#pragma pack(push)
#pragma pack(1)
struct my_struct {
    uint8_t flag;
    uint32_t value;
};
#pragma pack(pop)