5 次分叉后只有 1 child (C)

Question

我正在尝试在 C 中创建一个处理器场。我从打开消息队列开始，然后尝试创建工作进程：（请注意 NROF_WORKERS 是 5）

static void
makechildren (void) {
    // Only the parent should fork. Any children created will become workers. 
    pid_t   processID; 
    pid_t   farmerPID = getpid(); // To identify who the farmer is

    // Loop creating processes, indexed by NROF_WORKERS
    int i = 0; 
    while (i < NROF_WORKERS){
        if (getpid() == farmerPID){
            i++; 
            printf ("Parent is creating a child!%d\n", getpid()); 
            processID = fork();
        }
    }

    if (processID < 0){
        perror("fork() failed");
        exit(1);
    }
    else {
    // If parent, start farming
        if (processID == farmerPID) {
            printf("Parent reporting in!%d\n");
        }
    // If child, become a worker
        if (processID == 0) {
            printf("Child reporting in!%d\n", getpid()); 
            join();
        }
    }
}

如您所见，我希望 parent 在创建 child 时进行报告，之后我希望 parent 和所有 children报告。然而，这就是我得到的全部：

Parent is creating a child!11909
Parent is creating a child!11909
Parent is creating a child!11909
Parent is creating a child!11909
Parent is creating a child!11909
Child reporting in!11914

现在，我确实注意到 11909 和 11914 的差异是 5。所以我的问题是：是否创建了其他进程？如果是这样，他们怎么不报告？如果不是，我做错了什么？另外， parent 根本没有报告，这是怎么造成的？

Answer 1

所有 children 都已创建，但将在 while 循环中永远循环，因为 i 仅针对 parent:

int i = 0; 
while (i < NROF_WORKERS){
    if (getpid() == farmerPID){    
        i++;             // <---- This is happening for the parent process only.
        printf ("Parent is creating a child!%d\n", getpid()); 
        processID = fork();
    }
}

唯一要终止的child是最后一个，i等于NROF_WORKERS。

另外 parent 是 "not reporting"，因为您要检查的 processID 等于 parent PID 永远不会等于它，因为它等于最新 fork 结果，即最新创建的 child PID:

.........
 processID = fork();
.........
.........
 if (processID == farmerPID) {
            printf("Parent reporting in!%d\n");
 }

Answer 2

你总是打印 farmerPid！但是由于消息被打印了 5 次，您实际上创建了 5 个进程：

while (i < NROF_WORKERS){
    if (getpid() == farmerPID){
        i++; 
        printf ("Parent is creating a child!%d\n", getpid()); 
        processID = fork();
    }
}

如果你想打印 children pids 那么你的代码必须在 parent 和 child 之间有所不同，如：

while (i < NROF_WORKERS){
    if (getpid() == farmerPID){
        i++; 
        printf ("Parent is creating a child!\n"); 
        processID = fork();
        if (processID==0) { // child
            printf("I am the child %d\n",getpid());
        } else { // parent
            printf("Parent just created child %d\n",processID);
        }
    }
}

5 次分叉后只有 1 child (C)

Only 1 child after 5 forks (C)

c

fork

pid

process