Stuff

/**********************************************************************
 *
 * ProcReaders.c
 *
 * Module for the Readers / Writers problem which favors readers using
 * processes.
 *
 * Author: Nick DiZazzo, April 2007
 * CS305 Programming Assignment 3 - Readers / Writers Problem
 *
 *********************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/sem.h>
#include <sys/wait.h>

#include "Constants.h"
#include "Processes.h"
#include "ProcReaders.h"
#include "ReadWrite.h"
#include "Sem.h"

/* Global Variables - absolutely nessecary, shared between processes. */
int readCount;

/**********************************************************************
 * pr_readersWriters()
 *
 * This function creates the specified number of reader processes, and
 * the specified number of writers processes, then has each new
 * process execute its associated reader/writer algorithm. This version
 * favors readers.
 *
 * It is nearly impossible to have a perfect outcome every time
 * demonstrating the solution clearly, but there are some techniques that
 * improve the demonstration. Interleaving the process creation helps this
 * slightly, but there are still too many factors to create a perfect
 * demonstration.
 *
 * By creating two child processes which in turn create readers and
 * writers, we achieve pseudo-process interleaving, naturally
 * generated by the OS process scheduler. This sharing of CPU time between
 * the two launcher children is what achieves this effect.
 *
 * Parameters: readers  - The number of reader processes to create.
 *             writers  - The number of writer processes to create.
 *
 * Output: None.
 * Pre-Conditions: None.
 * Post-Conditions: None.
 * Other: This function modifies globals:
 *      readCount
 *      C_W_XSEM
 *      C_W_RSEM
 *********************************************************************/
void pr_readersWriters(int readers, int writers)
{
        pid_t pPID;                             // parent manages semiphores
        pid_t c1PID;                            // c1 launches readers
        pid_t c2PID;                            // c2 launches writers
        key_t semkey;
        int semid;
        readCount = 0;

        // Create semaphores
        semkey = ftok(C_KEYFILE, C_KEYCHAR);
        semid = semget(semkey, C_R_NUMSEMS, 0666 | IPC_CREAT);

        // Initialize semaphores
        sem_init(semid, C_R_XSEM);                      // Initialize XSEM
        sem_init(semid, C_R_WSEM);                      // Initialize WSEM

        // Create child processes
        pPID = getpid();                                // Store parent PID
        c1PID = fork();                                 // Create child
if (pPID == getpid())
        {
                c2PID = fork();
                if (c2PID == (pid_t)C_CHILD)
                        c1PID = C_STOPFLG;              // C2 -> Mark C1 PID
        }
        else if (c1PID == (pid_t)C_CHILD)
        {
                c2PID = C_STOPFLG;                      // C1 -> Mark C2 PID
        }
        else
        {
                printf("%s\n", C_EM_FORKERR);
                exit(1);
        }

        // C1 Spawns readers, C2 Spawns writers
        if (c1PID == (pid_t)C_CHILD)
        {
                if (readers > 0)
                {
                        createProcesses(readers-1);     // Create # of readers
                        rreader(semid);                 // start read routene
                }
                exit(0);                                // Children do no more
        }
        else if (c2PID == (pid_t)C_CHILD)