Question

This will probably require some looking into, but my question is very simple:

Why is numPassenger always 0 in the parentHandler2() function?

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

int getRand()
{
  return (rand() % 5001);
}

//////////GLOBAL//////////

const int CAPACITY = 100;
const int MEMSIZE = 1024;

char* sharedmem;

pid_t pid;

int numPassenger;
int numTram;

//////////GLOBAL//////////

//handles SIGALRM, generates passengers, sends SIGUSR1
void parentHandler1()
{
    numPassenger = getRand();
    sprintf(sharedmem, "%d", numPassenger);
    kill(getpid(), SIGUSR1);
}

//handles SIGUSR1, calculates number of trams needed, sends SIGUSR2
void childHandler()
{
    double n = atoi(sharedmem);
    numTram = (ceil(n/100));
    sprintf(sharedmem, "%d", numTram);
    kill(pid, SIGUSR2);
}

//outputs
void parentHandler2()
{
    int n = atoi(sharedmem);
    printf("Passengers: %d, Trams: %d\n", numPassenger, n);
}

int main (int argc, char* argv[])
{
    srand(time(0));

    key_t key;
    int shmemaddr;

    //shared memory
    key=ftok(argv[0],1);
    shmemaddr=shmget(key,MEMSIZE,IPC_CREAT|S_IRUSR|S_IWUSR);
    sharedmem = shmat(shmemaddr,NULL,0);

    pid = fork();
    if ( pid > 0 )
    {
        //timer
        struct itimerval timer;
        timer.it_value.tv_sec = 3; 
        timer.it_value.tv_usec = 0; 
        timer.it_interval.tv_sec = 3;
        timer.it_interval.tv_usec = 0;  
        setitimer (ITIMER_REAL, &timer, NULL);

        signal(SIGALRM, parentHandler1);
        signal(SIGUSR1, childHandler);
    }
    else if ( pid == 0 )
    {
        signal(SIGUSR2, parentHandler2);
    }

    //not so busy waiting
    while(1) sleep(1);

   return 0;
}

https://gist.github.com/4299915

Answer 1

Fork creates a new copy of the current process. Global variables aren’t shared between processes. The only memory that is shared between your two processes is the memory returned by shmget. The value of numPassenger is never set in the child process.