feedback.md

Feedback — Exercise Sheet 3

General

Output Parameters

Some C functions take non-const pointers as parameters to return additional information. Take for example pid_t wait(int* wstatus): it returns the PID of the child process (on success), but also takes an int* as argument. The man page states:

If wstatus is not NULL, wait() and waitpid() store status information in the int to which it points.

This technique is used because functions in C can only return one value — here we are already returning the PID. We commonly refer to such parameters as output parameters as their sole purpose is to output additional values by the function.

Furthermore, the if is not NULL part tells us that it is an optional parameter. If we are not interested in this particular value, we can just provide NULL instead of a valid pointer.

Note that output parameters can sometimes be non-optional, in which case you have to provide a valid pointer. Always read the documentation carefully.

Task 1

Parallel Execution

One of the main advantages of using child processes is the parallel execution of computations. The function calls wait(NULL) and waitpid(-1, NULL, 0) are blocking, which means that execution stops until the the function returns (i.e. is finished). Using only one loop, like in the following code snippet, results in a sequential execution of the child processes. This means that for every loop iteration: the (parent) process spawns a child process and waits until the child process is finished. The next child process will be spawned in the following loop iteration.

for (int i = 0; i < processCount; ++i) {
    pid_t pid = fork();
    if (pid < 0) {
        // error handling
    } else if (pid == 0) {
        // child
        exit(0);
    } else {
        // parent
        wait(NULL);
    }
}

The correct solution requires you to use two loops, one for starting child process, the other to wait for them to finish.

for (int i = 0; i < processCount; ++i) {
    pid_t pid = fork();
    if (pid < 0) {
        // error handling
    } else if (pid == 0) {
        // child
        exit(0);
    }
}

while (wait(NULL) > 0) {
}