/*
    sharedmem.c

    Started by Jeff Ondich on 4/11/96
    Last modified on 4/7/10

    This program demonstrates how to use shared memory
    in Linux.
*/

#include    <sys/types.h>
#include    <sys/wait.h>
#include    <sys/ipc.h>
#include    <sys/shm.h>
#include    <unistd.h>
#include    <stdlib.h>
#include    <string.h>
#include    <stdio.h>


#define        kSharedMemKey        ((key_t)1113)
#define        kSharedBufferSize    100


int main( void )
{
    pid_t pid;
    int status;
    char *sharedBuffer;
    int sharedBufferID;

    /* Create the shared memory. */
    sharedBufferID = shmget( kSharedMemKey,
                             kSharedBufferSize,
                             0664 | IPC_CREAT | IPC_EXCL  );
    if( sharedBufferID == -1 )
    {
        perror( "Shmget trouble" );
        exit( 1 );
    }

    /* Attach the shared memory to the current process' address space. */
    sharedBuffer = (char *)shmat( sharedBufferID, (char *)0, 0 );
    if( sharedBuffer == (char *)-1 )
    {
        perror( "Shmat trouble" );
        shmctl( sharedBufferID, IPC_RMID, NULL );
        exit( 1 );
    }

    /* Initialize the shared memory. */
    strcpy( sharedBuffer, "Mom, there's a singing moose outside!" );
    fprintf( stdout, "[Parent:%d before fork] Buffer: '%s'\n", getpid(), sharedBuffer );

    /* Fork a child process */
    pid = fork();
    if( pid == -1 )
    {
        perror( "fork() failure" );
    }

    else if( pid != 0 )
    {
        fprintf( stdout, "[Parent:%d] Waiting...\n", getpid() );
        wait( &status );
        fprintf( stdout, "[Parent:%d] Done waiting. Buffer: '%s'\n", getpid(), sharedBuffer );
        fprintf( stdout, "[Parent:%d] Quitting\n", getpid() );
    }

    else
    {
        fprintf( stdout, "[Child:%d] Changing the buffer.\n", getpid() );
        strcpy( sharedBuffer, "The moose is gone now." );
        fprintf( stdout, "[Child:%d] Buffer: '%s'\n", getpid(), sharedBuffer );
        fprintf( stdout, "[Child:%d] Quitting\n", getpid() );
    }

    /* Clean up the shared memory. */
    shmdt( sharedBuffer );
    shmctl( sharedBufferID, IPC_RMID, NULL );

    return 0;
}