Trouble understanding Parent and Child Processes

[cardinals03]

I need help understanding how to get the following answers. Please note that these are review questions, not something I am trying to have done. I want to understand the concepts behind how the answers were achieved. The instructor has told us that questions similiar to these will be on our next exam.

1) Including the initial parent process, how many processes are created in the below program?

#include <stdio.h>
#include <unistd.h>

int main()
{

//fork a child process
fork();

//fork another child process
fork();

//and fork another
fork();

return 0;
}

2) Using the below program, identify the values A, B, C, and D. (assume the actual pids of the parent and child are 2600 and 2603, respectively)

#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>

int main()
{
pid_t pid, pid1;

//fork a child process
pid = fork();

if (pid< 0) //error occurred
{
fprintf(stderr, "Fork Failed");
return 1;
}
else if (pid == 0) //child process
{
pid1 = getpid();
printf("child: pid = %d", pid); //line A
printf("child: pid1 = %d", pid1); //line B
}
else //parent process
{
pid1 = getpid();
printf("child: pid = %d", pid); //line C
printf("child: pid1 = %d", pid1); //line D
wait(NULL);
}

return 0;
}

3) Using the below program, explain what the output will be at line A

#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>

int value = 5;

int main()
{
pid_t pid;
pid = fork()

if (pid == 0) //child process
{
value += 15;
return 0;
}
else (id pid > 0) //parent process
{
wait(NULL);
printf("Parent: value = %d", value); //Line A
return 0;
}
}

Please help if you can...I may be way off base here, but here is what I think:

One the first question, I am thinking either 7 or 8. Doesn't the fork() copy both a child and a parent process, thus giving 7?

On the second I believe it is:
A 0
B 2603
C 2600
D 2600

On the third, would in not simply be 5?

Thanks for any advice...I am trying hard, but I am at my wits end..

[chrisname]

On the first, I'm not sure. Forking copies the parent and child process exactly, (although I think after the point of forking, or you'd get unlimited forks). I think it is seven -- the parent creates one, (1) the child and parent both create another (3) and then the parent and all 3 children create a child (7).

The second you are correct, it is 0, 2603, 2600, 2600.

Third you are also correct -- if value was a static variable then I think it would become 20 in both processes, but as it is a plain integer you are correct, it is 5.

Read this for clarity:
http://en.wikipedia.org/wiki/Fork_(operating_system)

Particularly http://en.wikipedia.org/wiki/Fork_(operating_system)#Example_in_C