Shared Memory Fork Example

#include <sys/shm.h>
#include <sys/wait.h>
#include <malloc.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <error.h>
#include <math.h>

/*
 * This code is not complete.
 * It is only a starting point, or for reference.
 * It is missing several parts
 * E.g. time-stamping, multiple matrices
 */

#define N 4

typedef struct matrixstruct {
    int C[N][N];
} matstr;

matstr *Cms; /* pointer to output matrix C */
int Cid;     /* segment id for output matrix C */
int base_pid, status, pid;

int main()
{
    int i,j;
    int rank, chunk, u, t;
    // Get the size of the work that each process will have to do.
    chunk = N / sqrt(4);

    /*
    *
    * STEP 1: create the shared segment
    */
    if ((Cid = shmget(IPC_PRIVATE, sizeof(matstr), IPC_CREAT | 0666)) < 0)
    {
        perror("smget returned -1\n");
        error(-1, errno, " ");
        exit(-1);
    }
    printf("Allocated %d, at id %d\n", (int) sizeof(matstr), Cid);

 
    /*
    *
    * STEP 2: fork the children processes
    */
    base_pid = getpid();
    for(i=1; i<N; i++) {
        if (getpid()==base_pid)
            pid = fork();
        else
            break;
    }

    /*
    *
    * STEP 3: each child process attaches the segment
    */
    if ((Cms = (matstr *) shmat(Cid, NULL, 0)) == (matstr *) -1){
        perror("Process shmat returned NULL\n");
        error(-1, errno, " ");
    }
    else
        printf("Process %d attached the segment %d\n", getpid(), Cid);


    /*
    *
    *
    *
    * STEP 4: use the shared segment 
    *
    *
    *
    */

    // Get the process id
    rank = getpid()-base_pid;

    // Calculate the offsets for the result matrix
    u = (rank % (N/chunk)) * chunk;
    t = floor((rank * chunk) / N) * chunk;

    // Loop over the matrix chunk for this process
    for (i=u;i<(u+chunk);i++) {
        for (j=t;j<(t+chunk);j++) {
            Cms->C[i][j] = rank;
        }
    }

    /*
    *
    * STEP 5: each child process detaches the segment
    */
    if (shmdt(Cms) == -1){
        perror("shmdt returned -1\n");
        error(-1, errno, " ");
    }else
        printf("Process %d detached the segment %d\n", getpid(), Cid);

    /*
    *
    * STEP 6: each child process exits and the parent joins
    */
    if (getpid()==base_pid)
        for(i=1; i<N; i++) {
            wait(NULL);
        }
    else
        exit(0);

    // Print out the matrix
    Cms = (matstr *) shmat(Cid, NULL, 0);
    for (i=0;i<N;i++) {
            printf("[");
        for (j=0;j<N;j++) {
            printf("%d, ", Cms->C[i][j]);
        }
        printf("]\n");
    }
    printf("\n\n");

    /*
    *
    * STEP 7: base process deletes the segment
    */
    if (shmctl(Cid,IPC_RMID,NULL) == -1){
        perror("shmctl returned -1\n");
        error(-1, errno, " ");
    }


    return 0;
}

Perfect ! I wish you the best

U r a lifesaver man

Thanks. It was used within student's lection.

I'm curious what will happen if fork AFTER shmat. Will the same virtual address acquired by shmat remain vaild shared memory between the processes?