Re: [9fans] fun and scary evil C code

[Bakul Shah <bakul+plan9@bitblocks.com>]

> > #define TRIO_DOUBLE_INDEX(x) (((unsigned char *)&internalEndianMagic)[7-(x)
> ])
> 
> this is actually done in /sys/src/9/port/devcons.c too:
> 
> 	static uvlong uvorder = 0x0001020304050607ULL;
> 	
> 	static uchar*
> 	le2vlong(vlong *to, uchar *f)
> 	{
> 		uchar *t, *o;
> 		int i;
> 	
> 		t = (uchar*)to;
> 		o = (uchar*)&uvorder;
> 		for(i = 0; i < sizeof(vlong); i++)
> 			t[o[i]] = f[i];
> 		return f+sizeof(vlong);
> 	}
> 	
> 	static uchar*
> 	vlong2le(uchar *t, vlong from)
> 	{
> 		uchar *f, *o;
> 		int i;
> 	
> 		f = (uchar*)&from;
> 		o = (uchar*)&uvorder;
> 		for(i = 0; i < sizeof(vlong); i++)
> 			t[i] = f[o[i]];
> 		return t+sizeof(vlong);
> 	}
> 
> presotto wrote the code but said he learned the trick from ken.
> 
> there, of course, we have a real compiler and don't have to
> write uvlong constants as floating point numbers
> (wow that seems fragile).

Most likely the TRIO_DOUBLE_INDEX macro came from some code
from the pre-long-long days, when you had no other way to
write down an 8 byte value in host endian order (even in a
less than portable way).