5-Process-API

1. Write a program that calls fork(). Before calling fork(), have the main process access a variable (e.g., x) and set its value to some- thing (e.g., 100). What value is the variable in the child process? What happens to the variable when both the child and parent change the value of x?

Each process has its own separate copy of x:

$ ./variable-in-child
Parent process - x: 99
Child process - x: 101

2. Write a program that opens a file (with the open() system call) and then calls fork() to create a new process. Can both the child and parent access the file descriptor returned by open()? What happens when they are writing to the file concurrently, i.e., at the same time?

Yes, both the child and parent can access the file descriptor returned by open(). However, concurrent writes from the parent and child processes can lead to the interleaved data in the file:

$ ./access-file-descriptor
writing from the parent
writing from the child

$ cat /tmp/file
from parent
from child

To ensure that the writes are cooperated and not interleaved:

• synchronization mechanism (locks)
• IPC (inter-process communication)

3. Write another program using fork(). The child process should print “hello”; the parent process should print “goodbye”. You should try to ensure that the child process always prints first; can you do this without calling wait() in the parent?

To ensure that the child process always prints first, we can leverage the synchronization of pipe communication between the parent and child processes:

child process:

• print "hello"
• send a signal to the parent process

parent process:

• wait to receive the signal
• print "goodbye"

$ ./ensure-child-first
hello
goodbye

4. Write a program that calls fork() and then calls some form of exec() to run the program /bin/ls. See if you can try all of the variants of exec(), including (on Linux) execl(), execle(), execlp(), execv(), execvp(), and execvpe(). Why do you think there are so many variants of the same basic call?

• execv: takes the program name and command-line arguments as an array of pointers
• execvp: searches for the program name in the PATH env
• execl: takes the program name and command-line arguments as separate parameters

5. Now write a program that uses wait() to wait for the child process to finish in the parent. What does wait() return? What happens if you use wait() in the child?

$ ./wait-in-child
Child process: my pid is 43702
wait() failed in child process: No child processes
error code: 10
Child process terminated with status: 1

$ awk '$3 == 10 { print }' (xcrun --show-sdk-path)/usr/include/sys/errno.h
#define ECHILD          10              /* No child processes */

6. Write a slight modification of the previous program, this time using waitpid()instead of wait(). When would waitpid() be useful?

$ ./waitpid
child process is still running at Fri Jun 23 11:01:02 2023
child process is still running at Fri Jun 23 11:01:03 2023
child process is still running at Fri Jun 23 11:01:04 2023
child process is still running at Fri Jun 23 11:01:05 2023
child process is still running at Fri Jun 23 11:01:06 2023
child process finished.

7. Write a program that creates a child process, and then in the child closes standard output (STDOUT FILENO). What happens if the child calls printf() to print some output after closing the descriptor?

The output will not be visible in the terminal:

$ ./print-after-closing-stdout
Child process: before closing STDOUT_FILENO
Parent process: child process completed

8. Write a program that creates two children, and connects the standard output of one to the standard input of the other, using the pipe() system call.

first process:

• redirect stdout to the write end of the pipe
• run ls -l

second process:

• redirect the stdin to the read end of the pipe
• run wc -l