Homework B - Processes

The following answers should be handwritten, either on paper or tablet. Submit by uploading photos/images to Canvas.

Q1 - State precisely in one sentence the effect and return value of the fork() system call in Unix.

Q2 - State precisely in one sentence the effect and return value of the execl() system call in Unix.

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Now, the most common and expected way of using fork and execl is this:

pid = fork();
if(pid==0) {
    execl("program",...);
    _exit(1);
} else if(pid>0) {
    wait(&status);
}

Which results in a single child process running “program”. And, the parent process waits for the completion of the child. This can be sketched as a process tree, like this:

But there are many other ways to put the pieces together, some of them useful, and some of them just plain wrong. as shown below. Suppose that a program named “shell” executes the following code snippets. (You may assume that fork and execl always succeed in these examples.)

Q3 - Sketch a diagram showing the outcome of this program:

fork();
execl("program",...);

Q4 - Sketch a diagram showing the outcome of this program:

execl("program",...);
fork();

Q5 - Sketch the outcome of this program:

fork();
fork();
execl("program",...");

Q6 - Sketch the outcome of this program:

for(i=0;i<8;i++) {
    pid = fork();
    if(pid==0) {
        execl("program",...):
    }
}

Q7 - Sketch the outcome of this program: (notice the !=)

for(i=0;i<8;i++) {
    pid = fork();
    if(pid!=0) {
        execl("program",...):
    }
}

Q8 - Sketch the outcome of this program:

for(i=0;i<3;i++) {
    fork();
}
execl("program",...):

Q9 - Sketch the outcome of this program:

for(i=0;i<8;i++) {
    execl("program",...):
}
fork();

Q10 - Sketch the outcome of this programm, and suggest why this might be a useful technique:

pid = fork();
if(pid==0) {
    execl("program",...);
} else {
    _exit(0);
}