CECS 326 Assignment 4

CECS 326 Assignment 4 (5 points)


Cooperating processes need to communicate between them.  Linux supports shared memory for interprocess communication in two different implementations: POSIX and System V.  A System V shared memory segment can be created and controlled using system calls shmget, shmctl, shmat, shmdt, and a C library function memcpy.  Please consult the man pages of these system calls for details.  This assignment is designed to illustrate a critical problem with processes executing concurrently that try to access shared data.
For this assignment you need to copy the following two C++ programs (named shmp1.cpp and shmc1.cpp) and a header file registration.h into your Linux directory, compile the two programs into shmp1, and shmc1, respectively.  Then run shmp1 and observe what happens.  Run shmp1 at least 5 times and observe, report the results and explain what may have caused the peculiarities in the outcome.  Your observations and explanation should be reported on the cover page, along with the program description. 

The program must run successfully on Linux.

Do the following for this assignment:
1.    Compile shmp1.cpp and shmc1.cpp into shmp1 and shmc1, respectively, and run shmp1.  Document what the programs are designed to do on the cover page.
2.    Annotate the programs with adequate amount of comments throughout the program to explain what the program does. System calls require especially detailed comments and must be in your own words.  Simply copying the text from the man pages are not acceptable.
3.    Run shmp1 at least 5 times and observe the results.  Explain the problems you have observed in these runs and explain what may be causing the peculiar outcomes on the cover page.
4.    Submit on BeachBoard the annotated shmp1.cpp and shmc1.cpp, a screenshot that shows successful compile of the two programs as well as results of 5 runs, and a cover page that provides your name, your student ID, course # and section, assignment #, due date, submission date, a clear program description, and your observations & explanation of the run results.  Format of the cover page should follow the cover page template on BeachBoard.  
5.    The programs must be properly formatted and adequately commented to enhance readability and understanding.

 
/* registration.h */
/* Header file to be used with
 * shmp1.cpp and shmc1.cpp
 */

struct CLASS {
    char class_number[6];
    char date[7];
    char title[50];
    int seats_left;
};
 
/* shmp1.cpp */

#include "registration.h"
#include <sys/types.h
#include <sys/ipc.h
#include <sys/shm.h
#include <sys/wait.h
#include <unistd.h
#include <stdlib.h
#include <iostream
#include <stdio.h
#include <memory.h

using namespace std;

CLASS myclass = { "4321", "082620", "Operating Systems", 20 };

#define NCHILD    3

int    shm_init( void * );
void    wait_and_wrap_up( int [], void *, int );
void    rpterror( char *, char * );

main(int argc, char *argv[])
{
    int     child[NCHILD], i, shmid;
    void    *shm_ptr;
    char    ascshmid[10], pname[14];

    shmid = shm_init(shm_ptr);
    sprintf (ascshmid, "%d", shmid);

    for (i = 0; i < NCHILD; i++) {
        child[i] = fork();
        switch (child[i]) {
        case -1:
            sprintf (pname, "child%d", i+1);
rpterror ((char *)"fork failed", pname);
            exit(1);
        case 0:
            sprintf (pname, "shmc%d", i+1);
            execl("shmc1", pname, ascshmid, (char *)0);
            rpterror ((char *)"execl failed", pname);
            exit (2);
        }
    }
    wait_and_wrap_up (child, shm_ptr, shmid);
}

int shm_init(void *shm_ptr)
{
    int    shmid;

    shmid = shmget(ftok(".",'u'), sizeof(CLASS), 0600 | IPC_CREAT);
    if (shmid == -1) {
        perror ("shmget failed");
        exit(3);
    }
    shm_ptr = shmat(shmid, (void * ) 0, 0);
    if (shm_ptr == (void *) -1) {
        perror ("shmat failed");
        exit(4);
    }
    memcpy (shm_ptr, (void *) &myclass, sizeof(CLASS) );
    return (shmid);
}

void wait_and_wrap_up(int child[], void *shm_ptr, int shmid)
{
    int wait_rtn, w, ch_active = NCHILD;

    while (ch_active 0) {
        wait_rtn = wait( (int *)0 );
        for (w = 0; w < NCHILD; w++)
            if (child[w] == wait_rtn) {
                ch_active--;
                break;
            }
    }
    cout << "Parent removing shm" << endl;
    shmdt (shm_ptr);
    shmctl (shmid, IPC_RMID, (struct shmid_ds *) 0);
    exit (0);
}

void rpterror(char *string, char *pname)
{
    char errline[50];

    sprintf (errline, "%s %s", string, pname);
    perror (errline);
}
 
/* shmc1.cpp */

#include "registration.h"
#include <sys/types.h
#include <sys/ipc.h
#include <sys/shm.h
#include <sys/wait.h
#include <unistd.h
#include <stdlib.h
#include <iostream
#include <stdio.h
#include <memory.h

using namespace std;

CLASS *class_ptr;
void     *memptr;
char    *pname;
int    shmid, ret;
void rpterror(char *), srand(), perror(), sleep();
void sell_seats();

main(int argc, char* argv[])
{
    if (argc < 2) {
        fprintf (stderr, "Usage:, %s shmid\n", argv[0]);
        exit(1);
    }

    pname = argv[0];
    sscanf (argv[1], "%d", &shmid);
    memptr = shmat (shmid, (void *)0, 0);
    if (memptr == (char *)-1 ) {
        rpterror ((char *)"shmat failed");
        exit(2);
    }

    class_ptr = (struct CLASS *)memptr;

    sell_seats();

    ret = shmdt(memptr);
    exit(0);
}

void sell_seats() 
{
    int all_out = 0;

    srand ( (unsigned) getpid() );
    while ( !all_out) {   /* loop to sell all seats */
        if (class_ptr-seats_left 0) {
            sleep ( (unsigned)rand()%5 + 1);
            class_ptr-seats_left--;
            sleep ( (unsigned)rand()%5 + 1);
            cout << pname << " SOLD SEAT -- " 
                 << class_ptr-seats_left << " left" << endl;
        }
        else {
            all_out++;
            cout << pname << " sees no seats left" << endl;
        }
        sleep ( (unsigned)rand()%10 + 1);
    }
}

void rpterror(char* string)
{
    char errline[50];

    sprintf (errline, "%s %s", string, pname);
    perror (errline);
}