queue.c

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#include <xinu.h>

struct qentry queuetab[NQENT];

pid32 enqueue(pid32 pid, qid16 q)
{
    qid16 tail, prev; /* Tail & previous node indexes   */

    if (isbadqid(q) || isbadpid(pid))
    {
        return SYSERR;
    }

    tail = queuetail(q);
    prev = queuetab[tail].qprev;

    queuetab[pid].qnext = tail; /* Insert just before tail node */
    queuetab[pid].qprev = prev;
    queuetab[prev].qnext = pid;
    queuetab[tail].qprev = pid;
    return pid;
}

pid32 dequeue(
    qid16 q /* ID of queue to use       */
)
{
    pid32 pid; /* ID of process removed */

    if (isbadqid(q))
    {
        return SYSERR;
    }
    else if (isempty(q))
    {
        return EMPTY;
    }

    pid = getfirst(q);
    queuetab[pid].qprev = EMPTY;
    queuetab[pid].qnext = EMPTY;
    return pid;
}