Appendix-04.Fork_in_Unix
Appendix-04.Fork_in_Unix
What is os.fork
import os
pid = os.fork()
if pid == 0:
print("I am the child process!")
else:
print(f"I am the parent process. My child has PID {pid}")Use case: Pipes for Parent–Child Communication
Python’s os.pipe() creates two file descriptors:
-
r— read end -
w— write end
After fork(), the parent and child each inherit both ends, so you must close the unused ends in each process.
import os
# Create a pipe: r for reading, w for writing
r, w = os.pipe()
pid = os.fork()
if pid == 0:
# --- Child process ---
os.close(r) # child doesn't read
message = b"Hello from child!"
os.write(w, message)
os.close(w) # important: tell parent we are done writing
else:
# --- Parent process ---
os.close(w) # parent doesn't write
# Read data sent by child
data = os.read(r, 1024)
print("Parent received:", data.decode())
os.close(r)
Use case: Pipes for Bidirectional communication
To communicate both ways, you use two pipes:
-
Pipe 1: parent → child
-
Pipe 2: child → parent
Here’s a minimal round-trip example:
import os
import time
# Create two pipes
pr, pw = os.pipe() # parent writes, child reads
cr, cw = os.pipe() # child writes, parent reads
pid = os.fork()
if pid == 0:
# --- Child ---
os.close(pw)
os.close(cr)
msg = os.read(pr, 1024) # Will block to wait compuate
print("Child got:", msg.decode())
reply = b"Message received!"
os.write(cw, reply)
os.close(pr)
os.close(cw)
else:
# --- Parent ---
os.close(pr)
os.close(cw)
print("[Parent] Started, waiting 2 seconds before sending message...")
time.sleep(2)
os.write(pw, b"Hello child!")
reply = os.read(cr, 1024)
print("Parent got:", reply.decode())
os.close(pw)
os.close(cr)
Use case: call a function from child process
import os
def my_func(x, y):
print(f"Child running my_func with {x=} and {y=}")
pid = os.fork()
if pid == 0:
# This is the child process
my_func(10, 20)
os._exit(0) # terminate child cleanly
else:
# This is the parent process
print(f"Parent: child PID is {pid}")
Child process will be copied global variables from parent
