// ===========================================================
// File: Syscalls.c
//
// Implementation of the Syscalls system call functions.
// Theses are system calls requesting for semaphore resource
// in the Server. System calls provide means for
// synchronization of concurrent processes. Each system call
// will peform 4 specific task:
//
// 1) Prepare outgoing message m2
// 2) Send message to Server via pipe 2
// 3) Wait for reply message m3
// 4) Return information to caller
// ===========================================================

#include <stdlib.h>
#include <time.h>
#include <time.h>
#include <unistd.h>
#include "Message.h"

// ===========================================================
// Global variables
// ===========================================================

int inPd;       // read  in  pipe descriptor
int outPd;      // write out pipe descriptor

// ===========================================================
// Identifier:  vsinit(int*, char*[])
//
// Comments:    Pipe descriptors are recorded onto global
//              variables, inPd, outPd. The last two
//              entries of argv are removed and the argc
//              count is updated.
// ===========================================================

void vsinit(int *argcPt, char *argv[])
{
    outPd   = atoi(argv[*argcPt-1]);
    inPd    = atoi(argv[*argcPt-2]);

    argv[*argcPt-1] = NULL;
    argv[*argcPt-2] = NULL;

    *argcPt -= 2;
}

// ===========================================================
// Identifier:  int vsexit(void)
//
// Comments:    PID is sent as reply message m3.
//
// Returns:     -1 if pipe fail, 0 otherwise
// ===========================================================

int vsexit(void)
{
    int retValue = 0;
    RequestMsg m2;

    m2.timeStamp = time(NULL);
    m2.PID = getpid();
    m2.purpose = VS_EXIT;

    retValue = write(outPd, &m2, sizeof(RequestMsg));

    return (retValue == -1 ? -1 :0);
}

// ===========================================================
// Identifier:  int getSem(int, int, int*)
//
// Comments:    To obtain a semaphore by providing a semKey
//              and initial semaphore value. Server will
//              return a return value and semId.
//
// Returns:     return value of m3.
// ===========================================================

int getSem(int semKey, int semInitialVal, int *semIdPtr)
{
    RequestMsg m2;
    ReplyMsg m3;

    m2.timeStamp = time(NULL);
    m2.PID = getpid();
    m2.semKey = semKey;
    m2.semInitValue = semInitialVal;
    m2.purpose = GET_SEM;

    write(outPd, &m2, sizeof(RequestMsg));
    read(inPd, &m3, sizeof(ReplyMsg));

    *semIdPtr = m3.semId;

    return m3.returnValue;
}

// ===========================================================
// Identifier:  int signalSem(int)
//
// Comments:    Increments the semValue of a semaphore. Server
//              will return a return value. This operation
//              may wake up a waiting process.
//
// Returns:     return value of m3.
// ===========================================================

int signalSem(int semId)
{
    RequestMsg m2;
    ReplyMsg m3;

    m2.timeStamp = time(NULL);
    m2.PID = getpid();
    m2.semId = semId;
    m2.purpose = SIGNAL_SEM;

    write(outPd, &m2, sizeof(RequestMsg));
    read(inPd, &m3, sizeof(ReplyMsg));

    return m3.returnValue;
}

// ===========================================================
// Identifier:  waitSem(int)
//
// Comments:    Decrements the semValue of a semaphore. Server
//              will return a return value. A process must
//              wait if resulted semValue is < 0.
//
// Returns:     return value of m3.
// ===========================================================

int waitSem(int semId)
{
    RequestMsg m2;
    ReplyMsg m3;

    m2.timeStamp = time(NULL);
    m2.PID = getpid();
    m2.semId = semId;
    m2.purpose = WAIT_SEM;

    write(outPd, &m2, sizeof(RequestMsg));
    read(inPd, &m3, sizeof(ReplyMsg));

    return m3.returnValue;
}

// ===========================================================
// Identifier:  releaseSem(int)
//
// Comments:    Cancels the right of using a semaphore.
//              Server will return a return value.
//
// Returns:     return value of m3.
// ===========================================================

int releaseSem(int semId)
{
    RequestMsg m2;
    ReplyMsg m3;

    m2.timeStamp = time(NULL);
    m2.PID = getpid();
    m2.semId = semId;
    m2.purpose = RELEASE_SEM;

    write(outPd, &m2, sizeof(RequestMsg));
    read(inPd, &m3, sizeof(ReplyMsg));

    return m3.returnValue;
}