Do {} while (0) in the kernel)

Why are there so many do {} while (0) Macros in the kernel? I don't understand it at first. It doesn't seem to work. But after understanding it, you will know its benefits. The benefit lies in the macros of multiple statements.

 
# Define Foo (x) print ("Arg is % Sn", x); do_something (X );
 

InCodeUsed in:

 
If (2 = blah) Foo (blah );
 

After pre-compilation is expanded:

 
If (2 = blah) print ("Arg is % Sn", blah); do_something (blah );
 

As you can see, the do_something function is out of the IF statement control. This is not what we want. Use do {} while (0); It is foolproof.

 If (2 = blah) do {printf ("Arg is % Sn", blah); do_something (blah);} while (0 ); of course, you can also use the following form: # define exch (x, y) {int TMP; TMP = x; X = y; y = TMP ;} however, it may cause problems in the IF-else statement. For example: 
 If (x> Y) exch (x, y); // branch 1elsedo_something (); // branch 2 
 
 after expansion: 
 
 If (x> Y) {// single-branch if-statement !!! Int TMP; // The one and only branch consiststmp = x; // of the block. X = y; y = TMP; V}; // empty statementelse // Error !!! "Parse error before else" do_something (); 
 
 the syntax is incorrect. If do {} while (0) is used, this problem will not occur. 
 
 If (x> Y) do {int TMP; TMP = x; X = y; y = TMP;} while (0); elsedo_something (); 
 
 Well, remember to use your own code after you understand it!