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On Wednesday 30 June 2004 10:25, Momchil Velikov wrote:
<snip>
> Borislav> <from include/asm-i386/thread_info.h>
> Borislav> 86 /* how to get the thread information struct from C */
> Borislav> 87 static inline struct thread_info *current_thread_info(void)
> Borislav> 88 {
> Borislav> 89 struct thread_info *ti;
> Borislav> 90 __asm__("andl %%esp,%0; ":"=r" (ti) : ""
> (~(THREAD_SIZE - 1))); Borislav> 91 return ti;
> Borislav> 92 }
>
> Borislav> I've read several tutorials on inline asm but what is peculiar
> here is, that Borislav> the input operand's constraint is "" (nothing)
>
> What version of the kernel is that? There's no such thing in 2.6.6
> and both gcc 3.3 and gcc 3.5 report an error compiling your example.
> The constraint must be "0", so initially the constant is loaded into a
> register and the result is obtained in the same register ("0" means
> "the same constraint as the one of operand 0").
You're right. I use lxr - a cross reference tool to browse the kernel code,
so, to my dismay, the program indexed the source files wrong - in the
original source file <include/asm-i386/thread_info.h> the input operand is
"0" but in the browsable version the zero is gone and i have only "" ..
strange, i'll have to check that.
>
> Borislav> I also have another question:
>
> Borislav> <from include/linux/kernel.h>
> Borislav> 203 /**
> Borislav> 204 * container_of - cast a member of a structure out to the
> containing Borislav> structure
> Borislav> 205 *
> Borislav> 206 * @ptr: the pointer to the member.
> Borislav> 207 * @type: the type of the container struct this is
> embedded in. Borislav> 208 * @member: the name of the member within
> the struct. Borislav> 209 *
> Borislav> 210 */
> Borislav> 211 #define container_of(ptr, type, member) ({
> \ Borislav> 212 const typeof( ((type *)0)->member ) *__mptr =
> (ptr); \ Borislav> 213 (type *)( (char *)__mptr -
> offsetof(type,member) );})
>
> Borislav> I can't understand the ((type *)0) part - type is passed as an
> argument and it Borislav> is some struct pointer but the trailing 0 ...
> what does it actually do?
>
> No type is passed as an argument as this is a macro.
Actually, type is an argument to the macro:
#define container_of(ptr, type, member)
the second argument, but this is irrelevant since I still got what i wanted to
know for which i blagodarja :))
This was _the_ explanation I was looking for - i couldn't be more indepth :))
I'll have to brush up my C knowledge but the 0 in the expression really got
me sweatin',
Thanks again.
Boris.
<snip>
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