On Tue, Apr 23, 2002 at 01:08:41PM -0400, Dan Erickson wrote:
> I have asked this question before, but I didnt get a response from
> somebody who understood my question correctly. So.. I will try showing a
> couple more examples.
Bummer. I thought I did a good job describing my answer before. :)
> static ssize_t wdt_write(struct file *file, const char *buf, size_t count,
> loff_t *ppos)
> {
/* stuff */
> }
>
>
> What has me confussed is.... what gives this function the arguments so it
> can function properly. Ie) as bar is the arg to atoi() earlier.
Dan, with this particular function, wdt_write() isn't called directly
anywhere in kernel code. (At least, not in the kernel tree I'm using, a
2.4.17 fork.) Instead, the write: wdt_write; line in the wdt.c file
causes gcc to put a pointer to the wdt_write() function in the wdt_fops
structure (of type struct file_operations).
Somewhere in kernel code, someone else is going to wind up calling
wdt_fops->write(), and they will pass that function the struct file*,
char *, size_t, and loff_t that the wdt_write() function expects.
Perhaps a simpler example would help bridge the gap. Consider the
following (untested and uncompiled) code:
#include <stdio.h>
int func(int);
int (*fp)(int);
int func(int a) {
return a;
}
int main(int argc, char *argv[]) {
int value;
fp = &func;
value = fp(314159); /* line a */
printf("%d\n", value); /* this will print 314159\n to the console */
}
The kernel code is similar, but different; a function pointer to
wdt_write() is stored in a struct, and then somewhere along the line
someone will call wdt_write() using the function pointer in the struct.
For more details, check out pages 118--121 in K&R 2. (They don't mention
in those pages that the function can be called through the function
pointer without de-referencing the pointer; they would have written line
a as: value = (*fp)(314159); I recall reading that function pointers
don't need to be dereferenced in this manner, and that no amount of
indirection will require dereferencing, but I don't recall now where I
read that. Sorry.)
I hope this has helped. If it hasn't, we'll need a whiteboard to try
again. :)
--
http://immunix.org/
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