This is weird...

[chrisname]

I was just reading through the Linux kernel source (I just wanted to read through some of it, to learn) and I noticed something that Linus Torvalds does that looked a little weird to me. So I tried it and it compiles:

#include <stdio.h>

int main() {
    {
        printf("Hello, world!");
    }
    char buf[512];
    {
        fgets(buf, 512, stdin);
    }
    {
        return 0;
    }
}

What? We can make blocks that have no function but to provide easy reading? I didn't know that!

Here's the actual snippet, from panic.c:

#ifdef __sparc__
	{
		extern int stop_a_enabled;
		/* Make sure the user can actually press Stop-A (L1-A) */
		stop_a_enabled = 1;
		printk(KERN_EMERG "Press Stop-A (L1-A) to return to the boot prom\n");
	}
#endif
#if defined(CONFIG_S390)
	{
		unsigned long caller;

		caller = (unsigned long)__builtin_return_address(0);
		disabled_wait(caller);
	}
#endif
	local_irq_enable();
	for (i = 0; ; ) {
		touch_softlockup_watchdog();
		i += panic_blink(i);
		mdelay(1);
		i++;
	}
	bust_spinlocks(0);

Can anyone shed some light on this? Is there any scoping? It seems that things within the brackets can access things without.

Edit: This doesn't compile:

#include <stdio.h>

int main() {
    {
        printf("Hello, world!");
        char buf[512];
    }
    {
        fgets(buf, 512, stdin);
    }
    {
        return 0;
    }
}

So obviously there is scoping. But can someone explain what the purpose of this is? I didn't know you could do it; I'm going to start using that -- it makes for easier reading imo.

[Bazzy]

That makes smaller scope chunks
eg:

int main()
{
    {
       int x;
       //use x
    } // x out of scope

    {
          float *x; // this is a completely different variable respect to the prevoious 'x'
          // use x
    }
}

[chrisname]

Ohhh. Thanks

What about this:
void warn_slowpath_fmt(const char *file, int line, const char *fmt, ...)
What is the purpose of the "..."?

I'm glad I decided to actually read through the source. Alot of it is far too complicated for me to really understand but I'm still learning from it, so that's good.

[Bazzy]

It's called ellipsis, that allows you to have any number of argument of any type
( see http://en.wikipedia.org/wiki/Variadic_function#Variadic_functions_in_C.2C_Objective-C.2C_C.2B.2B.2C_and_D )
printf works with that http://www.cplusplus.com/reference/clibrary/cstdio/printf/

Notice, variadic functions ( functions which have an ellipsis ) aren't really good -they don't check the argument types- but C++0x will introduce variadic templates which will be better

[chrisname]

Ohhhh... I was wondering about that. Thanks :)