Interprocess Communication fork() - Timing wait()

问题:

I've been asked to develop the consumer (client) side to a producer (server), where the producer creates processes, waits until the consumer has read shared memory and deleted processes, then passes control back to the producer for the killing of processes and the shutting down of the shared memory block.

I've researched the difference between sleep and wait, and realise that as soon as fork() is called, the child process begins running.

The below code is after the creation of processes and checks if they're parent processes. If they are, they wait(0). *Now for my question, how do I know where the code in the consumer starts to be executed, and how do I pass it back? *

else if(pid > 0)
                {
                    wait(0);
                }

Below can be seen the main loop the producer uses.

int noToCreate = atoi(argv[2]); // (user inputs on cmd line "./prod 20 10 5" - 20 size of shared mem, 10 process to be created, 5 processes to be deleted)

while(*memSig != 2)
    {
        while(*memSig == 1)   // set memsignature to sleep while..
        {
            sleep(1);
        }

        for(B = 0; B < noToCreate; B++)     
        {
            pid = fork();

            if(pid == -1)
            {
                perror("Error forking");
                exit(1);
            }
            else if(pid > 0)
            {
                wait(0);
            }
            else
            {
                srand(getpid());

                while(x == 0)
                {
                    if(*randNum == 101)
                    {
                        *randNum = rand() % (100 - 

1) + 1;
                        *pidNum = getpid();

                        printf("priority: %d 

Process ID: %d \n", *randNum, *pidNum);

                        x = 1;
                    }
                    else
                    {
                        *randNum++;
                        *pidNum++;
                    }
                }
                exit(0);
            }
        } /* Closes main for loop */

        if(*memSig == 0)
        {
            *memSig = 1;
        }
    } /* Closes main while loop */

Thanks a bunch guys :)

回答1:

wait make parent blocked until any child end .You can use waitpid let parent wait specific child.

When a child process end, it will set a signal SIG_CHILD.

回答2:

The pid is zero for the child process after the fork, so you are in the child process at your call to the srand function.

The other pid is that for the child process which allows he original thread to wait for the child to finish. If you wish to pass data between the processes consider using a pipe. A popen call returns two file descriptors, one to write end and the other to the read end. Set this up before the fork and the two processes can communicate.

回答3:

wait makes the parent wait for any child to terminate before going on (preferably use waitpid to wait for a certain child), whereas sleep puts the process to sleep and resumes it, as soon as the time passed as argument is over.
Both calls will make the process block.
And it is NOT said that the child will run immediately, this is indeterminate behavior!

If you want to pass data between producer and consumer, use pipes or *NIX sockets, or use the return-value of exit from the child if a single integer is sufficient.

See man wait, you can get the return value of the child with the macro WEXITSTATUS.

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

int
main(int argc, char *argv[])
{
    pid_t cpid, w;
    int status;

   cpid = fork();
    if (cpid == -1) {
        perror("fork");
        exit(EXIT_FAILURE);
    }

   if (cpid == 0) {            /* Code executed by child */
        printf("Child PID is %ld\n", (long) getpid());
        if (argc == 1)
            pause();                    /* Wait for signals */
        _exit(atoi(argv[1]));

   } else {                    /* Code executed by parent */
        do {
            w = waitpid(cpid, &status, WUNTRACED | WCONTINUED);
            if (w == -1) {
                perror("waitpid");
                exit(EXIT_FAILURE);
            }

           if (WIFEXITED(status)) {
                printf("exited, status=%d\n", WEXITSTATUS(status));
            } else if (WIFSIGNALED(status)) {
                printf("killed by signal %d\n", WTERMSIG(status));
            } else if (WIFSTOPPED(status)) {
                printf("stopped by signal %d\n", WSTOPSIG(status));
            } else if (WIFCONTINUED(status)) {
                printf("continued\n");
            }
        } while (!WIFEXITED(status) && !WIFSIGNALED(status));
        exit(EXIT_SUCCESS);
    }
}