# Define x do {...} while (0) Usage

For example, define macro, # define free1 (p) if (p) Free (P)
Then, call:
If (expression)
Free1 (P );
Else
Printf ("expression was false. \ n ");

Expand:
If (expression)
If (p) Free (P );
Else
Printf ("expression was false. \ n ");

Let's see if else matches the second if. It's wrong, huh, huh!
-
Change to this definition, # define free1 (p) do {If (p) Free (p);} while (0)
Expand:
If (expression)
Do {If (p) Free (p) ;}while (0 );
Else
Printf ("expression was false. \ n ");
That's right!

That is to say, such a definition method can be used anywhere without ambiguity.