/* pipe_getc.c - pipe_getc */

#include 

/*------------------------------------------------------------------------
 *  pipe_getc  -  read one character from a pipe device
 *------------------------------------------------------------------------
 */
devcall pipe_getc(
          struct dentry *devptr         /* Entry in device switch table */
        )
{
        char    ch;                     /* Byte of data from the buffer */
        struct  pipecblk *piptr;        /* Pointer to pipe control block*/

        /* Get a pointer to the control block for this pipe */

        piptr = &pipetab[devptr->dvminor];

        /* Check if pipe is not in use or at EOF */

        if (piptr->pstate == PIPE_FREE) {
                /* The pipe is not available */
                return SYSERR;
        }
        if (piptr->pstate == PIPE_EOF) {

                /* The writer closed the pipe, so return bytes while    */
                /*  any remain in the buffer.                           */

                if (piptr->pavail > 0) {
                        ch = piptr->pbuf[piptr->phead++];
                        if (piptr->phead >= PIPE_BUF_SIZE) {
                                piptr->phead = 0;
                        }
                        piptr->pavail--;
                        return 0xff & ch;
                }
                return EOF;
        }

        /* State is OPEN -- Wait for a byte to be available or a close  */

        wait(piptr->pcsem);

        /* If the state changed while we were blocked, the producer must*/
        /*  have called close, possibly after writing bytes to the pipe.*/

        if (piptr->pstate == PIPE_EOF) {
                if (piptr->pavail <= 0) {
                        /* The buffer is empty */
                        return EOF;
                }
        }

        /* A byte is available to be read -- pick up and return the byte*/

        ch = piptr->pbuf[piptr->phead++];
        if (piptr->phead >= PIPE_BUF_SIZE) {
                piptr->phead = 0;
        }
        piptr->pavail--;

        /* Signal the producer and return the byte */

        signal(piptr->ppsem);
        return 0xff & ch;
}