如何让父进程等待所有子进程完成？

Question

I'm hoping someone could shed some light on how to make the parent wait for ALL child processes to finish before continuing after the fork. I have cleanup code which I want to run but the child processes need to have returned before this can happen.

for (int id=0; id<n; id++) {
  if (fork()==0) {
    // Child
    exit(0);      
  } else {
    // Parent
    ...
  }
  ...
}

Answer 1

pid_t child_pid, wpid;
int status = 0;

//Father code (before child processes start)

for (int id=0; id<n; id++) {
    if ((child_pid = fork()) == 0) {
        //child code
        exit(0);
    }
}

while ((wpid = wait(&status)) > 0); // this way, the father waits for all the child processes 

//Father code (After all child processes end)

wait waits for a child process to terminate, and returns that child process's pid . On error (eg when there are no child processes), -1 is returned. So, basically, the code keeps waiting for child processes to finish, until the wait ing errors out, and then you know they are all finished.

Answer 2

POSIX defines a function: wait(NULL)； . It's the shorthand for waitpid(-1, NULL, 0); , which will suspends the execution of the calling process until any one child process exits. Here, 1st argument of waitpid indicates wait for any child process to end.

In your case, have the parent call it from within your else branch.

Answer 3

Use waitpid() like this:

pid_t childPid;  // the child process that the execution will soon run inside of. 
childPid = fork();

if(childPid == 0)  // fork succeeded 
{   
   // Do something   
   exit(0); 
}

else if(childPid < 0)  // fork failed 
{    
   // log the error
}

else  // Main (parent) process after fork succeeds 
{    
    int returnStatus;    
    waitpid(childPid, &returnStatus, 0);  // Parent process waits here for child to terminate.

    if (returnStatus == 0)  // Verify child process terminated without error.  
    {
       printf("The child process terminated normally.");    
    }

    if (returnStatus == 1)      
    {
       printf("The child process terminated with an error!.");    
    }
}

Answer 4

Just use:

while(wait(NULL) > 0);

This ensures that you wait for ALL the child processes and only when all have returned, you move to the next instruction.