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Linuxhelp _ Programming in Linux _ Usage Of Do{ }while(0)

Posted by: vasanth Mar 8 2005, 04:18 AM

Hello,
In the linux source code,we can see code which are enclosed by do{ statements; }while(0) .what is the use of this construct? will it not work without this do ..while construct.

Posted by: Hemant Mar 11 2005, 08:12 AM

I dont know under what circustances this has been used,but certainly this has been used to execute something once.

Inside the loop some magic is done,which you missed probably.

Posted by: vasanth Mar 12 2005, 05:33 AM

HI,
iam including the code from the kernel where linux circular linked lists are initialized(include/linux/list.h)

#define INIT_LIST_HEAD(ptr) do {
(ptr)->next = (ptr); (ptr)->prev = (ptr);
} while (0)

why is do { }while(0) used here? what purpose does it serve?

Posted by: DS2K3 Mar 12 2005, 09:43 AM

do{}while is like suing while(), except that the statement in the while() brackets is only evaluated AFTER the first loop. I'm not a C programmer, but I tihnk 0 would make the evaluation false, and therefore the loop would only go round once.

D

Posted by: Hemant Mar 12 2005, 10:37 AM

Ok..i think i got it.

The code snippet you have given is clearly used to initialise a doubly linked list.Now the whole thing is defined as a macro.

But as you know macros are quite tricky and nasty stuff...and that is the reason why.

CODE
#define MUL(a,B) a*b

and

#define MUL(a,B) (a)*(B)


can give quite different results,depending upon the arguments passed to it.If you have got your lesson in C,you can easily figure out how?

So sometimes it becomes quite necessary to have parenthesis around your statements.Now in our case the whole macro is a bit complex and thus just as a workaround and to make sure that statements stay together for any kind of argument,they have used do{}while().

Otherwise for normal arguments,the do{}while() is redundant.It is just like making the code bulletproof.If you remove the do{}while() then you will have two statements for a single macro and it is quite obvious that only one will be used.of course they could have done something like this..
CODE
#define INIT_LIST_HEADER(ptr) {
(ptr)->next = (ptr); (ptr)->prev=(ptr);
}


But i think having workarounds is a matter of choice ,the guy who has written that preferred the earlier method.And may be it is more bulletproof.

Posted by: imransadat Mar 30 2005, 10:20 AM

The do { } while(0) enclosure helps in making the group of statements in the macro into a single statement. e.g.

#define FOO(x) { function1(x); function(2); }
.
.
.

if (cond)
FOO(1);
else
FOO(2);

it will fail cause of the extra semi colon at the end. Also the do { } while(0) statement gives the macro the additional break statement which otherwise would need complex if else to implement.

try this page for more help http://c2.com/cgi/wiki?TrivialDoWhileLoop

Imran Sadat.

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