Re: Basic C question

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On 03-08-08 16:06, Rene Herman wrote:

On 03-08-08 09:58, Manish Katiyar wrote:

ok... I know i am going to be embarrassed but I am confused by the below function.

I'd not be embarrased...

The last parameter in do_mpage_readpage() is of type get_block_t, but
when passed to block_read_full_page() it gets passed as get_block_t *

The exact nature of get_block_t is important here. Variables of type get_block_t are function pointers -- have function type:

typedef int (get_block_t)(...);

static struct bio * do_mpage_readpage(............., get_block_t get_block) {
       ................
       ................
           if (page_has_buffers(page))
        goto confused;
        .................
confused:
          if (!PageUptodate(page))
            block_read_full_page(page, get_block);
       .........
}

int block_read_full_page(struct page *page, get_block_t *get_block) {
        .........
       ..........
}


How does this work ? :-( ....... Isn't gcc supposed to catch this and
flag warning unless I am missing something very obvious.

In C, except as an operand of sizeof or unary &, an expression having function type is automatically converted from function type to pointer to function.

Or put differently: except for sizeof and unary &, "function_designator" and "&function_designator" are interchangeable as expressions. You can therefore write the call as:

    block_read_full_page(page, &get_block);

without changing anything, after which I suppose you'd be fine with the declarations.

Note -- I put that wrong. You could not do that, but you could if get_block was an actual (direct) function designator, which is the point I was trying to make.

Now, granted, this specific example is still not nice. I haven't checked other uses but it appears that block_read_full_page() should've been declared not with a get_block_t *, but simply with a get_block_t. If you can convince yourself of such, you could submit it...

Rene.

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