Re: the printk problem

On Saturday 05 July 2008 00:01, Andrew Morton wrote:

quoted

quoted

We also jump through hoops to print things like sector_t and
resource_size_t.  They always need to be cast to `unsiged long long',
which generates additional stack space and text in some setups.

The thing is that GCC checks types.  So it's fine to add "print this
pointer specially", but you can't in general add new printf arguments
without also hacking GCC.  Unless you use -Wno-format, and require
sparse to check special kernel types.

It would be excellent if gcc had an extension system so that you could
add new printf control chars and maybe even tell gcc how to check them.
But of course, if that were to happen, we couldn't use it for 4-5 years.

What I had initially proposed was to abuse %S, which takes a wchar_t*. 
gcc accepts `unsigned long *' for %S.

Then, we put the kernel-specific control char after the S, so we can
print an inode (rofl) with

	struct inode *inode;

	printk("here is an inode: %Si\n", (unsigned long *)inode);

Downsides are:

- there's a cast, so you could accidentally do

	printk("here is an inode: %Si\n", (unsigned long *)dentry);

- there's a cast, and they're ugly

- gcc cannot of course check that the arg matches the control string

Unfortunately (and this seems weird), gcc printf checking will not
accept a void* for %S: it _has_ to be wchar_t*, and the checker won't
permit void* substitution for that.

Repeating myself here...
We can add an alternative alias to printk:

 asmlinkage int printk(const char * fmt, ...)
         __attribute__ ((format (printf, 1, 2))) __cold;
+asmlinkage int custom_printk(const char * fmt, ...) __cold asm ("printk");

custom_printk() is actually just printk(), that is,
we won't need additional function, we need to teach
*printk* about MAC addresses, NIPQUADs etc;

and then use printk() if you use only standard %fmt (and have it
checked by gcc), or use custom_printk() if you have non-standard
%fmt in the format string.

The only downside that in second case, you lose gcc checking.
No big deal.
--
vda