Do {...} Clever Use of while (0)

In the Linux KernelCodeHas such macro definition:
# Define dump_write (ADDR, NR) do {memcpy (bufp, ADDR, NR); bufg + = nR;} while (0)

This macro definition means that the loop body will be executed once when this macro operation is referenced, but why is it defined as such a strange form?

Let's take a look at the macro definitions in several other forms:
# Define dump_write (ADDR, NR) memcpy (bufp, ADDR, NR); bufp + = nR;

Is this macro definition acceptable?

We put this macro in a context:
If (ADDR)

Dump_write (ADDR, NR );

Else

Do_something_else ();

The result shows an error occurs during compilation. Why?

Expand the macro, and the preceding statement block is changed:
If (ADDR)

Memcpy (bufp, ADDR, NR); bufp + = nR;

Else

Do_something_else ();

In this way, a statement bufp + = nR is added between the IF-else statement to form a statement similar to if-dosomething-Else. In this way, else will not be able to find the matching if, so the compiler will report an error.

Alternative reference format:
If (! ADDR)

Do_something_else ();

Else

Dump_write (ADDR, NR );

In this way, compilation is successful, but the problem is more serious, because bufp + = nR; will always be executed.

Here, we may immediately consider adding a braces in the macro definition:

# Define dump_write (ADDR, NR) {memcpy (bufp, ADDR, NR); bufp + = nR ;}

Can this work? Let's test it in context:

If (ADDR)

Dump_write (ADDR, NR );

Else

Do_something_else ();

Still compilation error. Why?

Expand the macro:

If (ADDR)

{Memcpy (bufp, ADDR, NR); bufp + = nR ;};

Else

Do_something_else ();

Here, an empty statement is inserted in the IF-else statement: If-null statement-else. The compiler cannot find the matching if statement for else. Therefore, an error will still be reported during compilation.

Using the macro definition in the following format is completely correct.

# Define dump_write (ADDR, NR) do {memcpy (bufp, ADDR, NR); bufg + = nR;} while (0)