Re: OSS sound

Rob Crittenden wrote:

>Yes, I understand how macros work. My question is why have a do/while loop that
>will always exit at the first test? Why not write the macros as:
>
>...
>#define CLEAR_OMR(omr) { int x = 0; read((omr)->msg_pipe[0], &x, sizeof(x)); }
>#define RESET_OMR(omr) { } 
>...
>
>It just seems like a strange thing to do.
>  
>
Rob,

When you're writing a macro with braces {}, you'll need to swallow the 
semicolon,
as Peter pointed out.
Suppose:

#define MACRO(x)  { x = 5; }

now I do :

if (abc)
    MACRO(abc);
else
    something else;

This exapands to :

if (abc)
    { abc = 5; };
else
    something else;

This will give a compilation error, since you now have an extra 
semicolon after
the braces, which makes it 2 statements after the if. The else does not 
match up
to the if!
To avoid this problem, whenever a macro contains multiple statements, it is
enclosed in a do {} while(0). (Note: without the trailing semicolon!)
This way the macro expands to :

if (abc)
    do { abc = 5; } while(0);
else
    something else;

This allows the macro to contain multiple statements in a block of code, 
while
still looking like one statement to the user.

(BTW, this does not induce any performance penalty, since the 
always-false check
is optimized out by the compiler).

--Lev



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