Do {... } While (0)

Why do I encounter many # defines... do {...} while (0) in the kernel )?

There are several reasons:

· Empty statements will be warned during compilation, so it is necessary to use # define Foo do {} while (0 ).

· This is done to define local variables in it

· This is to be able to use complex macro definitions in conditional statements. For example, the following code:

# Define Foo (x )\

Printf ("Arg is % s \ n", x );\

Do_something_useful (X );

If the following code is used:

If (blah = 2)

Foo (blah );

Will be expanded:

If (blah = 2)

Printf ("Arg is % s \ n", blah );

Do_something_useful (blah );;

In this way, the IF condition contains the printf () statement, while the do_something_useful () call cannot work as expected. When do {...} while (0) is defined, the following statement is displayed:

If (blah = 2)

Do {

Printf ("Arg is % s \ n", blah );

Do_something_useful (blah );

} While (0 );

This is the expected situation.

· If you want to define a multi-line statement and some local variables, the general definition can only be as follows:

# Define exch (x, y) {int TMP; TMP = x; X = y; y = TMP ;}

However, in some cases, this does not work normally. The following is an if statement that contains two branches:

If (x> Y)

Exch (x, y); // branch 1

Else

Do_something (); // branch 2

However, this can only be expanded into an if statement with a single branch, as shown below:

If (x> Y) {// single branch if

Int TMP;

TMP = X;

X = y;

Y = TMP;

}

; // Empty statement

Else // Error !!! "Parse error before else"

Do_something ();

The problem is caused by adding a semicolon (;) directly after the statement block. the solution is to place the statement block in the middle of DO and while (0. in this way, a single statement is obtained, instead of being judged as a statement block by the compiler. the IF statement is as follows:

If (x> Y)

Do {

Int TMP;

TMP = X;

X = y;

Y = TMP;

} While (0 );

Else

Do_something ();