Re: why do we use while (0)

["sahlot arvind" <asahlot@xxxxxxxxx>]

I tried this -

---------------------------------

struct list_head {
    struct list_head *next, *prev;
};

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

int main ()
{
    int i;
    struct list_head mylist;
    struct list_head *ptr = &mylist;
    printf ("hi\n");
    if (i)
    {
        INIT_LIST_HEAD(ptr);
    }
    else
    {
        printf ("jkldkas\n");
    }
    return 0;
}

--------------------------

 

And there is no compilation error.

 

On 12/26/07, Li Zefan <lizf@xxxxxxxxxxxxxx> wrote:

sahlot arvind wrote:
> Recently I started looking into linux kernel and trying to understand the
> code.
> I am working with linux-2.6.9.
> in file include/llinux/list.h - I found something like this.
>
> #define INIT_LIST_HEAD(ptr) do { \
>         (ptr)->next = (ptr); (ptr)->prev = (ptr); \
> } while (0)
>
>
> My question is why do we use a loop when we actually know that it is not
> going to execute more than once? Cannot we simply do -
>
> #define INIT_LIST_HEAD(ptr)    {(ptr)->next = (ptr); (ptr)->prev = (ptr)}
>
> Do we get some kind of optimization by using while (0)?
>

with #define INIT_LIST_HEAD(ptr)    {(ptr)->next = (ptr); (ptr)->prev = (ptr);}

try this:

if (...)
       INIT_LIST_HEAD(ptr);
else
       ...;

you will get compilation error:

error: expected _expression_ before 'else'

see why? expand the macro:

if (...)
{
       (ptr)->next = (ptr);
       (ptr)->prev = (ptr);
};   <---here
else
       ...;