what's the point of "do {} while(0)"

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1.00/5 (1 vote)

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Hi,
I'm studying select in network programming. I have a question when encountering the implementation of FD_SET in Windows(as follow). My question is why redundantly use do-while ?
Thanks for your attention.

C++

#define FD_SET (fd, set) do { \
    u_int __i; \
    for ( __i = 0; __i < ((fd_set FAR *)(set ))->fd_count; __i++) { \
        if (((fd_set FAR *)( set))->fd_array [__i] == ( fd)) { \
            break; \
        } \
    } \
    if ( __i == ((fd_set FAR *)( set))->fd_count ) { \
        if (((fd_set FAR *)( set))->fd_count < FD_SETSIZE) { \
            (( fd_set FAR *)(set))-> fd_array[__i ] = (fd); \
            (( fd_set FAR *)(set))-> fd_count++; \
        } \
    } \
} while(0)

Posted 25-Feb-15 14:43pm

cxxxxf

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Richard MacCutchan 26-Feb-15 4:13am

It just forces the code to be run once. It has no practical use, except within a complex macro such as you have shown.

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PIEBALDconsult · Answer 1 · 2015-02-25T14:50:00

Solution 1

It's nonsense, I've never used it.

http://stackoverflow.com/questions/257418/do-while-0-what-is-it-good-for[^]

http://www.quora.com/What-is-the-purpose-of-using-do-while-0-in-macros[^]

http://www.codeofhonor.com/blog/error-handling-using-forever-loop[^]

http://www.osronline.com/showThread.CFM?link=232772[^]

Posted 25-Feb-15 14:50pm

PIEBALDconsult

Comments

Sergey Alexandrovich Kryukov 25-Feb-15 22:41pm

"Nonsense" is a good word here, a 5.
—SA

cxxxxf 26-Feb-15 1:05am

Sorry, My English is poor. Why "nonsense" is a good word and what is the meaning of "a 5".

Sergey Alexandrovich Kryukov 26-Feb-15 8:42am

5 is my vote for this answer (means "excellent" on the 1 to 5 scale). I agree that using "while(0)" makes no sense ("nonsense") or ridiculously little sense, inadequate. This is what this word means. Not bad, I think.
—SA