C: 2 different headers define struct udphdr. How to tell GCC to choose one?

Question

I am working with pcap code and the struct udphdr seems to be defined in two includes. How to tell GCC to use a specific one?

What happens when you include both headers? Does the compiler give an error message? Please post the message. Are the struct udphdr a variable definition, or just a struct type declaration? Do you know if they headers are otherwise different? Do you know which header *must* be included? — gbulmer, Mar 21 '12 at 02:02
There is no compiler error. One definition is like this http://www.linuxbase.org/navigator/browse/type_single.php?cmd=display&id=37079 and the other like this http://tomoyo.sourceforge.jp/cgi-bin/lxr/source/include/linux/udp.h. The compiler seems to only recognize the latter but I believe the first is the correct one. — dmz73, Mar 21 '12 at 02:59

score 3 · Answer 1 · answered Jun 06 '12 at 10:29

If you want to uh_* elements , you must use -D_BSD_SOURCE macro as parameter of gcc, despite you get the following errors:

error: ‘struct udphdr’ has no member named ‘uh_sport’
error: ‘struct udphdr’ has no member named ‘uh_dport’
error: ‘struct udphdr’ has no member named ‘uh_ulen’
error: ‘struct udphdr’ has no member named ‘uh_sum’
error: ‘struct udphdr’ has no member named ‘uh_sum’

score 3 · Answer 2 · edited Jun 12 '12 at 09:08

Upon looking at /usr/include/netinet/udp.h more carefully the problem is not that the same struct is defined in two different headers. In this file it is defined as:

/* UDP header as specified by RFC 768, August 1980. */
#ifdef __FAVOR_BSD

    struct udphdr {
        u_int16_t uh_sport;  /* source port */
        u_int16_t uh_dport;  /* destination port */
        u_int16_t uh_ulen;   /* udp length */
        u_int16_t uh_sum;    /* udp checksum */
    };

#else

    struct udphdr {
        u_int16_t source;
        u_int16_t dest;
        u_int16_t len;
        u_int16_t check;
    };

#endif

So it looks like it is using the POSIX version of the struct which I believe is ok.

score 0 · Answer 3 · answered Mar 21 '12 at 03:20

I can't see a simple way to deal with that. I've done a few tests with #undef without success.

I would re-declare the struct you are interested in somewhere else in your own code, under a different name to avoid clashing, and then use it in the code with this new name.

This approach has problems, of course: what happens if the original header get's updated and that structure changes (some new members, others get removed), you know what I mean?

score -1 · Answer 4 · answered Mar 21 '12 at 02:10

-1

Just use the right one: netinet/udp.h under Linux:

From Linux Standard Base:

http://www.linuxbase.org/navigator/browse/type_single.php?cmd=display&id=37079

answered Mar 21 '12 at 02:10

ouah

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C: 2 different headers define struct udphdr. How to tell GCC to choose one?

4 Answers4