jos/lib/pipe.c

#include <inc/lib.h>

#define debug 0

static ssize_t devpipe_read(struct Fd *fd, void *buf, size_t n);
static ssize_t devpipe_write(struct Fd *fd, const void *buf, size_t n);
static int devpipe_stat(struct Fd *fd, struct Stat *stat);
static int devpipe_close(struct Fd *fd);

struct Dev devpipe =
{
	.dev_id =	'p',
	.dev_name =	"pipe",
	.dev_read =	devpipe_read,
	.dev_write =	devpipe_write,
	.dev_close =	devpipe_close,
	.dev_stat =	devpipe_stat,
};

#define PIPEBUFSIZ 32		// small to provoke races

struct Pipe {
	off_t p_rpos;		// read position
	off_t p_wpos;		// write position
	uint8_t p_buf[PIPEBUFSIZ];	// data buffer
};

int
pipe(int pfd[2])
{
	int r;
	struct Fd *fd0, *fd1;
	void *va;

	// allocate the file descriptor table entries
	if ((r = fd_alloc(&fd0)) < 0
	    || (r = sys_page_alloc(0, fd0, PTE_P|PTE_W|PTE_U|PTE_SHARE)) < 0)
		goto err;

	if ((r = fd_alloc(&fd1)) < 0
	    || (r = sys_page_alloc(0, fd1, PTE_P|PTE_W|PTE_U|PTE_SHARE)) < 0)
		goto err1;

	// allocate the pipe structure as first data page in both
	va = fd2data(fd0);
	if ((r = sys_page_alloc(0, va, PTE_P|PTE_W|PTE_U|PTE_SHARE)) < 0)
		goto err2;
	if ((r = sys_page_map(0, va, 0, fd2data(fd1), PTE_P|PTE_W|PTE_U|PTE_SHARE)) < 0)
		goto err3;

	// set up fd structures
	fd0->fd_dev_id = devpipe.dev_id;
	fd0->fd_omode = O_RDONLY;

	fd1->fd_dev_id = devpipe.dev_id;
	fd1->fd_omode = O_WRONLY;

	if (debug)
		cprintf("[%08x] pipecreate %08x\n", thisenv->env_id, uvpt[PGNUM(va)]);

	pfd[0] = fd2num(fd0);
	pfd[1] = fd2num(fd1);
	return 0;

    err3:
	sys_page_unmap(0, va);
    err2:
	sys_page_unmap(0, fd1);
    err1:
	sys_page_unmap(0, fd0);
    err:
	return r;
}

static int
_pipeisclosed(struct Fd *fd, struct Pipe *p)
{
	int n, nn, ret;

	while (1) {
		n = thisenv->env_runs;
		ret = pageref(fd) == pageref(p);
		nn = thisenv->env_runs;
		if (n == nn)
			return ret;
		if (n != nn && ret == 1)
			cprintf("pipe race avoided\n", n, thisenv->env_runs, ret);
	}
}

int
pipeisclosed(int fdnum)
{
	struct Fd *fd;
	struct Pipe *p;
	int r;

	if ((r = fd_lookup(fdnum, &fd)) < 0)
		return r;
	p = (struct Pipe*) fd2data(fd);
	return _pipeisclosed(fd, p);
}

static ssize_t
devpipe_read(struct Fd *fd, void *vbuf, size_t n)
{
	uint8_t *buf;
	size_t i;
	struct Pipe *p;

	p = (struct Pipe*)fd2data(fd);
	if (debug)
		cprintf("[%08x] devpipe_read %08x %d rpos %d wpos %d\n",
			thisenv->env_id, uvpt[PGNUM(p)], n, p->p_rpos, p->p_wpos);

	buf = vbuf;
	for (i = 0; i < n; i++) {
		while (p->p_rpos == p->p_wpos) {
			// pipe is empty
			// if we got any data, return it
			if (i > 0)
				return i;
			// if all the writers are gone, note eof
			if (_pipeisclosed(fd, p))
				return 0;
			// yield and see what happens
			if (debug)
				cprintf("devpipe_read yield\n");
			sys_yield();
		}
		// there's a byte.  take it.
		// wait to increment rpos until the byte is taken!
		buf[i] = p->p_buf[p->p_rpos % PIPEBUFSIZ];
		p->p_rpos++;
	}
	return i;
}

static ssize_t
devpipe_write(struct Fd *fd, const void *vbuf, size_t n)
{
	const uint8_t *buf;
	size_t i;
	struct Pipe *p;

	p = (struct Pipe*) fd2data(fd);
	if (debug)
		cprintf("[%08x] devpipe_write %08x %d rpos %d wpos %d\n",
			thisenv->env_id, uvpt[PGNUM(p)], n, p->p_rpos, p->p_wpos);

	buf = vbuf;
	for (i = 0; i < n; i++) {
		while (p->p_wpos >= p->p_rpos + sizeof(p->p_buf)) {
			// pipe is full
			// if all the readers are gone
			// (it's only writers like us now),
			// note eof
			if (_pipeisclosed(fd, p))
				return 0;
			// yield and see what happens
			if (debug)
				cprintf("devpipe_write yield\n");
			sys_yield();
		}
		// there's room for a byte.  store it.
		// wait to increment wpos until the byte is stored!
		p->p_buf[p->p_wpos % PIPEBUFSIZ] = buf[i];
		p->p_wpos++;
	}

	return i;
}

static int
devpipe_stat(struct Fd *fd, struct Stat *stat)
{
	struct Pipe *p = (struct Pipe*) fd2data(fd);
	strcpy(stat->st_name, "<pipe>");
	stat->st_size = p->p_wpos - p->p_rpos;
	stat->st_isdir = 0;
	stat->st_dev = &devpipe;
	return 0;
}

static int
devpipe_close(struct Fd *fd)
{
	(void) sys_page_unmap(0, fd);
	return sys_page_unmap(0, fd2data(fd));
}