Why is macro declared this way?

Question

What is the reason to define macro this way:

#define test_errno(msg) do{if (errno) {perror(msg); exit(EXIT_FAILURE);}} while(0)

I mean what is the reason behind do{}while(0)? Of course it will be done once only, zero is constant, cannot change to nonzero somehow, so why to use such construction?

DrC · Accepted Answer · 2013-06-28T02:08:59.017

3

It allows things like

if (condition) 
   test_errno(...);

to work properly with or without braces.

edited Jun 28 '13 at 02:08

answered Jun 28 '13 at 00:54

DrC

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1

a missing `;`. the macro considers this and it needs an ending `;`, so it looks like a real function. – vvy Jun 28 '13 at 01:51

score 2 · Answer 2 · answered Jun 28 '13 at 00:57

There is no good reason in this case. In similar constructs, it is useful for the statements inside to have continue and break to do useful things.

#define test_errno(msg) do {if (cond1) break;   \
                            if (cond2) break;   \
                            if (cond3) break;   \
               do_something_if_all_condtions_met();} while(0)

Why is macro declared this way?

2 Answers2