I create a linked list dynamically and initialize the first node in main(), and I add to the list every time I spawn a worker process. Before the worker process exits, I print the list. Also, I print the list inside my sigchld signal handler.

in main():

head = NULL;
tail = NULL;
// linked list to keep track of worker process
dll_node_t *node;
node = (dll_node_t *) malloc(sizeof(dll_node_t)); // initialize list, allocate memory
append_node(node);
node->pid = mainPID; // the first node is the MAIN process
node->type = MAIN;

in a fork()'d process:

    // add to list
    dll_node_t *node;
    node = (dll_node_t *) malloc(sizeof(dll_node_t));
    append_node(node);
    node->pid = mmapFileWorkerStats->childPID;
    node->workerFileName = mmapFileWorkerStats->workerFileName;
    node->type = WORK;

functions:

void append_node(dll_node_t *nodeToAppend) {
    /*
     * append param node to end of list
     */

    // if the list is empty
    if (head == NULL) {
        // create the first/head node
        head = nodeToAppend;
        nodeToAppend->prev = NULL;
    } else {
        tail->next = nodeToAppend;
        nodeToAppend->prev = tail;
    }

    // fix the tail to point to the new node
    tail = nodeToAppend;
    nodeToAppend->next = NULL;
}

finally... the signal handler:

void chld_signalHandler() {
    dll_node_t *temp1 = head;
    while (temp1 != NULL) {
        printf("2. node's pid: %d\n", temp1->pid);
        temp1 = temp1->next;
    }

    int termChildPID = waitpid(-1, NULL, WNOHANG);
    dll_node_t *temp = head;
    while (temp != NULL) {
        if (temp->pid == termChildPID) {
            printf("found process: %d\n", temp->pid);
        }
        temp = temp->next;
    }
    return;
}

Is it true that upon the worker process exiting, the SIGCHLD signal handler is triggered? If so, that would mean that after I print the tree before exiting, the next thing I do is in the signal handler which is print the tree... which would mean i would print the tree twice?

But the tree isn't the same. The node I add in the worker process doesn't exist when I print in the signal handler or at the very end of main(). Any idea why?

Thanks, Hristo