Fork Arbitrary Amount of Children from a Parent in C?

Question

I have found examples of how to fork multiple children by having something like this:

if ( fork() = 0 ) {
    //In child
} else {
    if ( fork() = 0 ) {
       //in second child

But if I don't know how many children I am going to need, how might I do this?

For instance, if I have a linked list of commands, and I want to fork and exec for each of them... So I guess I need to know which child it is as well...

Answer 1

Taking you at your word that you need to do this for a linked list:

linked_list_of_commands_t *node = root;
while (node != NULL) {
   int pid = fork();
   if (pid == -1) {
       break; // handle error
   } else if (pid == 0) {
       // child
       execv(node->command, node->argv);
       exit(1); // execv should not return, but just in case the execv call fails
   } else {
       node = node->next;
   }
}

This will launch a separate process for every item in the list.

Answer 2

But the number of routines would have to be fixed, even if the execution over those branches is unbounded. So what about a while loop with some sort of switch statement logic to each routine?

Answer 3

How about

for (i=0; i< 1000; i++) {
    pid = fork();
    if (pid) {
        // Error handling for pid==-1 
        break;
    }
    // Some logic dependent on value of 'i'
}

Answer 4

for(i = 0; i < num_children_to_spawn(); ++i) {
    pid_t pid = fork();
    if (pid == -1) {
        exit(-1); /* error */
    } else if (pid == 0) {
        /* child */
        do_child_things();
        break;
    } else {
        /* parent */
    }
}

Note that I didn't use a switch() because it would make it more cumbersome to break out of the loop.

Answer 5

pid_t children_pids[MAX_CHILDREN];
int last_child_index = 0;
for (int i=0; i < num_wanted_children; i++) {
  pid_t pid = fork();
  if (pid == 0)
    // in child
  else
    children_pids[last_child_index++] = pid;
}