Am Freitag, 17. Februar 2006 14:31 schrieb Mukund JB.:
> Dear All,
>
> I have seen a macro like this in the Linux kernel sources in
> include/linux/usb.h line no: 1090.
>
> #define usb_dotoggle(dev, ep, out) ((dev)->toggle[out] ^= (1 << (ep)))
>
> I did not understand how it works?
> I have written a small usermode program to test this.
>
> #define x 1
> #define y 2
> #define macro() (x = y)
> main()
> {
> int abc = macro();
> printf("abc = %d\n",abc);
> }
>
Your code is quite different from the one in the kernel. The kernel macro is a
so called "parameterized macro", meaning that
usb_dotoggle(alpha, beta, gamma)
is replaced by:
((alpha)->toggle[gamma] ^= (1 <<(beta)))
You define a macro without parameters, leading to
int abc = (1 = 2);
which obviously is not correct C, as you cannot assign something to a constant
(in other words, 1 is not a lvalue).
I recommend you read some kind of C tutorial on parametrized macros.
- Askadar
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