fifo.c

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/*
 *  linux/fs/fifo.c
 *
 *  written by Paul H. Hargrove
 *
 *  Fixes:
 *  10-06-1999, AV: fixed OOM handling in fifo_open(), moved
 *          initialization there, switched to external
 *          allocation of pipe_inode_info.
 */

#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/sched.h>
#include <linux/pipe_fs_i.h>

static int wait_for_partner(struct inode* inode, unsigned int *cnt)
{
    int cur = *cnt; 

    while (cur == *cnt) {
        pipe_wait(inode->i_pipe);
        if (signal_pending(current))
            break;
    }
    return cur == *cnt ? -ERESTARTSYS : 0;
}

static void wake_up_partner(struct inode* inode)
{
    wake_up_interruptible(&inode->i_pipe->wait);
}

static int fifo_open(struct inode *inode, struct file *filp)
{
    struct pipe_inode_info *pipe;
    int ret;

    mutex_lock(&inode->i_mutex);
    pipe = inode->i_pipe;
    if (!pipe) {
        ret = -ENOMEM;
        pipe = alloc_pipe_info(inode);
        if (!pipe)
            goto err_nocleanup;
        inode->i_pipe = pipe;
    }
    filp->f_version = 0;

    /* We can only do regular read/write on fifos */
    filp->f_mode &= (FMODE_READ | FMODE_WRITE);

    switch (filp->f_mode) {
    case FMODE_READ:
    /*
     *  O_RDONLY
     *  POSIX.1 says that O_NONBLOCK means return with the FIFO
     *  opened, even when there is no process writing the FIFO.
     */
        filp->f_op = &read_pipefifo_fops;
        pipe->r_counter++;
        if (pipe->readers++ == 0)
            wake_up_partner(inode);

        if (!pipe->writers) {
            if ((filp->f_flags & O_NONBLOCK)) {
                /* suppress POLLHUP until we have
                 * seen a writer */
                filp->f_version = pipe->w_counter;
            } else {
                if (wait_for_partner(inode, &pipe->w_counter))
                    goto err_rd;
            }
        }
        break;
    
    case FMODE_WRITE:
    /*
     *  O_WRONLY
     *  POSIX.1 says that O_NONBLOCK means return -1 with
     *  errno=ENXIO when there is no process reading the FIFO.
     */
        ret = -ENXIO;
        if ((filp->f_flags & O_NONBLOCK) && !pipe->readers)
            goto err;

        filp->f_op = &write_pipefifo_fops;
        pipe->w_counter++;
        if (!pipe->writers++)
            wake_up_partner(inode);

        if (!pipe->readers) {
            if (wait_for_partner(inode, &pipe->r_counter))
                goto err_wr;
        }
        break;
    
    case FMODE_READ | FMODE_WRITE:
    /*
     *  O_RDWR
     *  POSIX.1 leaves this case "undefined" when O_NONBLOCK is set.
     *  This implementation will NEVER block on a O_RDWR open, since
     *  the process can at least talk to itself.
     */
        filp->f_op = &rdwr_pipefifo_fops;

        pipe->readers++;
        pipe->writers++;
        pipe->r_counter++;
        pipe->w_counter++;
        if (pipe->readers == 1 || pipe->writers == 1)
            wake_up_partner(inode);
        break;

    default:
        ret = -EINVAL;
        goto err;
    }

    /* Ok! */
    mutex_unlock(&inode->i_mutex);
    return 0;

err_rd:
    if (!--pipe->readers)
        wake_up_interruptible(&pipe->wait);
    ret = -ERESTARTSYS;
    goto err;

err_wr:
    if (!--pipe->writers)
        wake_up_interruptible(&pipe->wait);
    ret = -ERESTARTSYS;
    goto err;

err:
    if (!pipe->readers && !pipe->writers)
        free_pipe_info(inode);

err_nocleanup:
    mutex_unlock(&inode->i_mutex);
    return ret;
}

/*
 * Dummy default file-operations: the only thing this does
 * is contain the open that then fills in the correct operations
 * depending on the access mode of the file...
 */
const struct file_operations def_fifo_fops = {
    .open       = fifo_open,    /* will set read_ or write_pipefifo_fops */
    .llseek     = noop_llseek,
};