Clicker Questions - 17-ForkExec
Question 1: How many lines will the following program print when standard output goes to the screen?
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#include <stdio.h>
#include <unistd.h>
int main()
{
if (fork() == 0) printf("Fred\n");
if (fork() == 0) printf("Fred\n");
return 0;
}
Answer
The answer is three. After the first fork() call, there are two processes, but only one of them (the child) returned 0 from fork. So it prints a line. Now, there are two processes calling the second fork(), so after the second fork() call, there are four processes and two of them (the child of the child, and the second child of the parent) returned 0 from fork(). They each print a line, for a total of three lines.Question 2: Suppose that fred.txt is two lines. How many lines will the following program print when standard output goes to the screen?
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#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int main()
{
int status;
printf("Hi\n");
if (fork() == 0) {
execlp("cat", "cat", "fred.txt", NULL);
execlp("cat", "cat", "fred.txt", NULL);
execlp("cat", "cat", "fred.txt", NULL);
exit(1);
} else {
(void) wait(&status);
printf("Done\n");
}
return 0;
}
Answer
UNIX> bin/click2 Hi Fred: Line 1 Fred: Line 2 Done UNIX>Since output is to the terminal, the stout buffer is flushed at the end of every line. So the "Hi" line is indeed printed after the printf() statement. The execlp() statement loads cat into the address space and runs it. It will print the two lines of fred.txt and exit. Since the execlp() call is successful, its address space is gone, and it doesn't return. Those other execlp() calls are never executed.
When the child exits, the parent prints "Done". The output is four lines.
Question 3: Suppose I compile the following program to bin/click3 and I then do:
UNIX> bin/click3 > tmp.txt
How many lines will tmp.txt be?
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#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int main()
{
int status;
printf("Hi\n");
if (fork() == 0) {
printf("I am a child.\n");
} else {
printf("I am no child.\n");
wait(&status);
}
return 0;
}
Answer
Suppose I compile the following program to bin/click3 and I then do:UNIX> bin/click3 > tmp.txt UNIX> cat tmp.txt Hi I am a child. Hi I am no child. UNIX>As you can see, it prints four lines. That's because we're redirecting stdout to a file, and the printf() call is buffered. The fork() call copies the buffer, and that is why "Hi" is printed twice.
Question 4: Suppose I compile the following program to bin/click4 and I then do:
UNIX> cat fred.txt Fred: Line 1 Fred: Line 2 UNIX> bin/click4 > tmp.txt
How many lines will tmp.txt be?
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#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int main()
{
int status;
printf("Hi\n");
if (fork() == 0) {
execlp("cat", "cat", "fred.txt", NULL);
exit(1);
} else {
wait(&status);
printf("Done\n");
}
return 0;
}
Answer
UNIX> cat fred.txt Fred: Line 1 Fred: Line 2 UNIX> bin/click4 > tmp.txt UNIX> cat tmp.txt Fred: Line 1 Fred: Line 2 Hi Done UNIX>As in the previous question, the "Hi" line goes into a buffer. That buffer is indeed copied to the child process in the fork() call. However, the execlp() call overwrites the address space, so the buffer is gone. As a result, the child never prints "Hi".
So there are four lines -- two from the parent and two from the child.
Question 5: What will the first line of tmp.txt be?
- A. “Hi”
- B. “Done”
- C. “Fred: Line 1”
- D. “Fred: Line 2”
Answer
UNIX> cat fred.txt Fred: Line 1 Fred: Line 2 UNIX> bin/click4 > tmp.txt UNIX> cat tmp.txt Fred: Line 1 Fred: Line 2 Hi Done UNIX>The first line is C. "Fred: Line 1". Remember, the "Hi" is in the parent's buffer, and it doesn't get printed until the parent exits. The parent has called wait(), so it doesn't print its buffer until after the child has printed its output.