Why is macro declared this way? [duplicate]

Question

This question already has answers here:

how does do{} while(0) work in macro? (3 answers)

Why use apparently meaningless do-while and if-else statements in macros? (9 answers)

Closed 8 years ago.

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?

Answer 1

It allows things like

if (condition) 
   test_errno(...);

to work properly with or without braces.

Answer 2

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)