What is the logic behind defining macros inside a struct?

Question

As apparent in the title, I'm questioning the reason behind defining the macros inside a struct. I frequently see this approach in network programming for instance following snippet:

struct sniff_tcp {
    u_short th_sport;               /* source port */
    u_short th_dport;               /* destination port */
    tcp_seq th_seq;                 /* sequence number */
    tcp_seq th_ack;                 /* acknowledgement number */
    u_char  th_offx2;               /* data offset, rsvd */
#define TH_OFF(th)      (((th)->th_offx2 & 0xf0) >> 4)
    u_char  th_flags;
    #define TH_FIN  0x01
    #define TH_SYN  0x02
    #define TH_RST  0x04
    #define TH_PUSH 0x08
    #define TH_ACK  0x10
    #define TH_URG  0x20
    #define TH_ECE  0x40
    #define TH_CWR  0x80
    #define TH_FLAGS        (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG|TH_ECE|TH_CWR)
    u_short th_win;                 /* window */
    u_short th_sum;                 /* checksum */
    u_short th_urp;                 /* urgent pointer */
};

This example is from sniffex.c code in tcpdump's web site.

Is this for enhancing readability and making code clearer.

Answer 1

Well, the defined constants were relevant to the possible values of one of the fields.

So, the author decided to improve code locality, and make the API users avoid running in circles. Seems logical.

Otherwise, preprocessor is completely independent of code. You can even put a define inside an expression.

Answer 2

We have to suspect that it's attempting to convey to us that the macros are only to be used in conjunction with data in this struct, but it's a poor and convoluted way to represent this.

In C++ it could be preferable to use a nested enum and an inline function to accomplish this, but since the code is C a macro is probably the best alternative.

In my opinion it decreases readability and I'd prefer to see the macros outside of the struct with comments indicating where and how they should be used. That way there's no guessing as to what exactly the macros are for and the struct definition stays free of cruft.