CIS 350, Process Creation in Unix

1. My files on astro

% ls
a.out      execl2.c   fork2.c    fork4.c    fork6.c    hello.out
execl1.c   fork1.c    fork3.c    fork5.c    hello.c


2. Program hello.c

% cat hello.c
main()
{
    printf("Hello\n");
}

% cc hello.c -o hello.out
% hello.out
Hello
%

3. The execl system call overlays the calling process with the named file.
   Here, execl is used to execute my "hello.c" program.

% cat execl1.c
main()
{
        printf("I am execl1 and I am about to execl to hello.out \n");
        execl("/usr2/cis/stafford/cis120/code/hello.out", "hello.out", 0);
        printf("This statement will never be executed \n");
}

% cc execl1.c
% a.out
I am execl1 and I am about to execl to hello.out 
Hello
%

CIS 350, Process Creation in Unix, page 2

4. Execl is used here to execute /bin/ls. Note that the second argument to
   execl is the name of the program (ls), followed by any arguments to ls.

% cat execl2.c
main()
{
        printf("I am execl2 and I am about to execl the ls program \n");
        execl("/bin/ls", "ls", "-al", 0);
        printf("This statement will never be executed \n");
}

% cc execl2.c
% a.out
I am execl2 and I am about to execl the ls program 
total 195
drwxr-xr-x   2 stafford staff        512 Apr  8 08:38 .
drwxr-xr-x   4 stafford staff        512 Apr  6 12:53 ..
-rwxr-xr-x   1 stafford staff      46512 Apr  8 08:38 a.out
-rw-r--r--   1 stafford staff        217 Apr  7 17:20 execl1.c
-rw-r--r--   1 stafford staff        186 Apr  7 17:31 execl2.c
-rw-r--r--   1 stafford staff         46 Apr  6 14:28 fork1.c
-rw-r--r--   1 stafford staff        175 Apr  6 14:46 fork2.c
-rw-r--r--   1 stafford staff        262 Apr  6 15:02 fork3.c
-rw-r--r--   1 stafford staff        363 Apr  6 15:20 fork4.c
-rw-r--r--   1 stafford staff        546 Apr  7 12:58 fork5.c
-rw-r--r--   1 stafford staff        273 Apr  7 18:04 fork6.c
-rw-r--r--   1 stafford staff         34 Apr  6 15:13 hello.c
-rwxr-xr-x   1 stafford staff      45680 Apr  8 08:37 hello.out
%

5. The "fork" system call creates a child process that is a duplicate of the
   calling process (the parent process) except for different process ID's.

% cat fork1.c
main()
{
    fork();
    printf("Hello\n");
}

% cc fork1.c
% a.out
Hello
Hello
%

CIS 350, Process Creation in Unix, page 3

6. To the parent, fork returns the child's process ID (pid). To the child,
   fork returns zero. You can use an if or case statement to do one thing
   in the parent and another in the child. Note that the childs line could
   have been printed before the parents line. However, in this case, the
   parent exits (dies) first. When the child exits (dies), it tries to
   notify (signal) its parent. Since the parent has died, it is possible
   for the child to become a "zombie" process that will stay around forever
   trying to tell its parent about its own imminent demise.

% cat fork2.c
main()
{
    int pid;

    pid=fork();
    if (pid == 0) {
        printf("I am the child %d \n", pid);
    }
    else {
        printf("I am the parent %d \n", pid);
    }
}

% cc fork2.c
% a.out
I am the parent 5915 
I am the child 0 
%


7. The wait system call causes a parent to wait until one or more of its
   children exits. (If a process has no children, wait will return -1.)
   Note that no zombies are created by the following code.

cat fork3.c
main()
{
    int pid, qid;

    pid=fork();
    if (pid == 0) {
        printf("I am the child %d \n", pid);
    }
    else {
        printf("I am the parent %d \n", pid);
        qid=wait(0);
        printf("I am the parent after the wait %d \n", qid);
    }
}

% cc fork3.c
% a.out
I am the child 0 
I am the parent 5932 
I am the parent after the wait 5932 
%

CIS 350, Process Creation in Unix, page 4

8. The following program combines the execl, fork, and wait system calls

% cat fork4.c
main()
{
    int pid, qid;

    pid=fork();
    if (pid == 0) {
        printf("I am the child %d \n", pid);
        execl("/bin/ls", "ls", "-l", 0);
        printf("This statement will never be executed \n");
    }
    else {
        printf("I am the parent %d \n", pid);
        qid=wait(0);
        printf("I am the parent after the wait %d \n", qid);
    }
}

% cc fork4.c
% a.out
I am the parent 5968 
I am the child 0 
total 194
-rwxr-xr-x   1 stafford staff      47068 Apr  8 08:39 a.out
-rw-r--r--   1 stafford staff        217 Apr  7 17:20 execl1.c
-rw-r--r--   1 stafford staff        186 Apr  7 17:31 execl2.c
-rw-r--r--   1 stafford staff         46 Apr  6 14:28 fork1.c
-rw-r--r--   1 stafford staff        175 Apr  6 14:46 fork2.c
-rw-r--r--   1 stafford staff        262 Apr  6 15:02 fork3.c
-rw-r--r--   1 stafford staff        363 Apr  6 15:20 fork4.c
-rw-r--r--   1 stafford staff        546 Apr  7 12:58 fork5.c
-rw-r--r--   1 stafford staff        273 Apr  7 18:04 fork6.c
-rw-r--r--   1 stafford staff         34 Apr  6 15:13 hello.c
-rwxr-xr-x   1 stafford staff      45680 Apr  8 08:37 hello.out
I am the parent after the wait 5968 
%

CIS 350, Process Creation in Unix, page 5

9. The getpid and getppid system calls return the processes ID (pid) and
   the process ID of the processes parent (ppid for parent process ID.
   The "ps" command will give you a list of your processes. "ps -lax" will
   give you a long listing of all proesses running in the computer, beginning
   with process 0001, init.

% cat fork5.c
main()
{
    int pid, ppid, cpid, cpid2;

    cpid=fork();
    if (cpid == 0) {
        pid=getpid();
        ppid=getppid();
        printf("I am the child,  pid=%d  ppid=%d \n", pid, ppid);
        execl("/bin/ps", "ps", "-l", 0);
        printf("This statement will never be executed \n");
    }
    else {
        pid=getpid();
        ppid=getppid();
        printf("I am the parent, pid=%d ppid=%d cpid=%d\n", pid, ppid, cpid);
        cpid2=wait(0);
        printf("Parent after wait, pid=%d ppid=%d cpid=%d\n", pid, ppid, cpid2);
    }
}

% cc fork5.c
% a.out
I am the parent, pid=6006 ppid=5606 cpid=6007
I am the child,  pid=6007  ppid=6006 
 F S   UID   PID  PPID  C PRI NI     ADDR     SZ    WCHAN TTY      TIME COMD
10 S   124  6006  5606 10  30 20 801bd918      5 801bd808 ttyx3    0:00 a.out
18 O   124  6007  6006 12  61 20 801befd8     18          ttyx3    0:00 ps
 0 S     0  5604   286  0  30 20 801c3418     18 801c3308 ttyx3    0:00 telnetd
10 S   124  5606  5604  9  39 20 801c82d8     36 ffffc000 ttyx3    0:01 csh
Parent after wait, pid=6006 ppid=5606 cpid=6007
%

CIS 350, Process Creation in Unix, page 6

10. Here is a nice simple program. Can you predict the output?

% cat fork6.c
main()
{
    int pid1, pid2, pid3, pid4;

    pid1=fork();
    pid2=fork();
    pid3=fork();
    printf ("%d, %d, %d, %d, %d \n", getpid(), getppid(), pid1, pid2, pid3);
    while ((pid4=wait(0)) != -1)
        printf ("%d, %d, %d \n", getpid(), getppid(), pid4);       
}

% cc fork6.c
% a.out
6067, 6066, 6064, 0, 0 
6069, 6065, 0, 0, 0 
6070, 6063, 6064, 6066, 0 
6066, 6063, 6064, 0, 6067 
6066, 6063, 6067 
6064, 6063, 0, 6065, 6068 
6068, 6064, 0, 6065, 0 
6064, 6063, 6068 
6065, 6064, 0, 0, 6069 
6065, 6064, 6069 
6064, 6063, 6065 
6063, 5606, 6064, 6066, 6070 
6063, 5606, 6070 
6063, 5606, 6066 
6063, 5606, 6064 
%