wait.c

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/* wait.c - wait */

#include <xinu.h>

/*------------------------------------------------------------------------
 *  wait  -  Cause current process to wait on a semaphore
 *------------------------------------------------------------------------
 */
syscall wait(
      sid32     sem     /* Semaphore on which to wait  */
    )
{
    intmask mask;           /* Saved interrupt mask     */
    struct  procent *prptr;     /* Ptr to process's table entry */
    struct  sentry *semptr;     /* Ptr to sempahore table entry */

    mask = disable();
    if (isbadsem(sem)) {
        restore(mask);
        return SYSERR;
    }

    semptr = &semtab[sem];
    if (semptr->sstate == S_FREE) {
        restore(mask);
        return SYSERR;
    }

    if (--(semptr->scount) < 0) {       /* If caller must block */
        prptr = &proctab[currpid];
        prptr->prstate = PR_WAIT;   /* Set state to waiting */
        prptr->prsem = sem;     /* Record semaphore ID  */
        enqueue(currpid,semptr->squeue);/* Enqueue on semaphore */
        resched();          /*   and reschedule */
    }

    restore(mask);
    return OK;
}