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

i think, this is no “best practice”, one should keep the defines (for values) near the struct, but not inside the struct.

(Even better would be to enum and typedef the constants, so a compiler could warn if not typed properly).

The TH_OFF() macro is another case, where it “hides” another element, so maybe it could be put at this position (with an appropriate comment)