Why is macro declared this way? [duplicate]

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?

Answer 1

It allows things like

if (condition) 
   test_errno(...);

to work properly with or without braces.