Re: [9fans] a pair nec bugs

[erik quanstrom <quanstro@quanstro.net>]

here's what i settled on for now.  i believe it to
be correct in all cases, but if you are sharing file
descriptors among many processes, you may have
/dev/bintime open many more times that necessary.

i put off solving other instances of fd-group private
until later, since the solution in hand is not very
satisfying.

in the future, i believe it might make sense to make
file descriptor groups, environment groups, etc. available
via /dev/fdid to allow for sharing without false sharing.

- erik
----

#include <u.h>
#include <libc.h>
#include <tos.h>

typedef	struct	Nfd	Nfd;
struct Nfd {
	int	pid;
	int	fd;
};

static	void	**nsecpriv;
#define	Fd	((Nfd*)nsecpriv[0])


static uvlong order = 0x0001020304050607ULL;

static void
be2vlong(vlong *to, uchar *f)
{
	uchar *t, *o;
	int i;

	t = (uchar*)to;
	o = (uchar*)&order;
	for(i = 0; i < sizeof order; i++)
		t[o[i]] = f[i];
}

static Nfd*
getfd(void)
{
	Nfd *p;

	if(nsecpriv != nil && Fd->pid == _tos->pid)
		return Fd;
	if(nsecpriv == nil){
		/*
		 * privalloc's allocates slots on a shared
		 * basis, even though the memory slots
		 * themselves are proc-private.
		 */
		nsecpriv = privalloc();
		if(nsecpriv == nil)
			return nil;
		*nsecpriv = p = malloc(sizeof *p);
		if(p == nil)
			return nil;
	}else
		p = Fd;
	p->fd = -1;
	return p;
}

vlong
nsec(void)
{
	uchar b[8];
	vlong t;
	Nfd *p;

	if((p = getfd()) == nil)
		return 0;
	if(p->fd == -1){
		p->fd = open("/dev/bintime", OREAD|OCEXEC);
		p->pid = _tos->pid;
	}
	if(pread(p->fd, b, sizeof b, 0) == sizeof b){
		be2vlong(&t, b);
		return t;
	}
	return 0;
}