Python os.waitpid Function

Last modified April 11, 2025

This comprehensive guide explores Python’s os.waitpid function, which waits for a child process to complete and retrieves its exit status. We’ll cover process management, status interpretation, and practical examples.

Basic Definitions

The os.waitpid function waits for a child process specified by process ID (pid) to complete and returns its exit status information.

Key parameters: pid (process ID to wait for), options (flags like WNOHANG). Returns a tuple of (pid, status). Status contains exit code and signal info.

Basic Process Waiting

This example demonstrates the simplest use of os.waitpid to wait for a child process to complete. The parent process waits indefinitely.

basic_wait.py

import os import sys

Fork a child process

pid = os.fork()

if pid == 0: # Child process print(“Child process running”) sys.exit(42) else: # Parent process print(f"Parent waiting for child {pid}") child_pid, status = os.waitpid(pid, 0) print(f"Child {child_pid} exited with status {status}") if os.WIFEXITED(status): print(f"Exit code: {os.WEXITSTATUS(status)}")

The parent process forks a child that exits with code 42. The parent waits for the child to complete using waitpid. WIFEXITED checks for normal exit.

The status contains more information than just the exit code, including signal information if the process was terminated by a signal.

Non-blocking Wait with WNOHANG

The WNOHANG option makes os.waitpid non-blocking, returning immediately if the child hasn’t exited yet. This allows polling.

non_blocking.py

import os import time

pid = os.fork()

if pid == 0: # Child process runs for 3 seconds time.sleep(3) os._exit(0)

Parent polls child status

while True: child_pid, status = os.waitpid(pid, os.WNOHANG) if child_pid == 0: print(“Child still running…”) time.sleep(1) else: print(f"Child {child_pid} exited") break

The parent checks the child status every second without blocking. When the child exits after 3 seconds, waitpid returns the child’s PID and status.

This pattern is useful when you need to monitor a process while continuing to perform other tasks in the parent process.

Waiting for Any Child Process

Using pid of -1 waits for any child process. This is useful when managing multiple children and you don’t know which will complete first.

wait_any.py

import os import time import random

Create 3 child processes

children = [] for i in range(3): pid = os.fork() if pid == 0: # Child sleeps for random time and exits sleep_time = random.randint(1, 5) time.sleep(sleep_time) os._exit(sleep_time) else: children.append(pid)

Wait for any child to complete

while children: pid, status = os.waitpid(-1, 0) if os.WIFEXITED(status): print(f"Child {pid} exited after {os.WEXITSTATUS(status)} seconds") children.remove(pid)

print(“All children exited”)

The parent creates 3 children that sleep for random durations. Using pid=-1, it waits for whichever child exits first, removing it from the tracking list.

This approach is common in servers that handle multiple client processes concurrently.

Handling Terminated Children

This example shows how to handle children terminated by signals using the status information returned by os.waitpid.

signal_handling.py

import os import signal import time

pid = os.fork()

if pid == 0: # Child process waits to be killed print(“Child running (PID: {})".format(os.getpid())) while True: time.sleep(1) else: # Parent waits briefly then kills child time.sleep(2) os.kill(pid, signal.SIGTERM)

# Wait for child and check status
child_pid, status = os.waitpid(pid, 0)
if os.WIFSIGNALED(status):
    sig = os.WTERMSIG(status)
    print(f"Child {child_pid} killed by signal {sig} ({signal.Signals(sig).name})")
elif os.WIFEXITED(status):
    print(f"Child exited normally with status {os.WEXITSTATUS(status)}")

The parent sends SIGTERM to the child after 2 seconds. waitpid returns status information that we decode using WIFSIGNALED and WTERMSIG.

This demonstrates how to distinguish between normal exits and signal-induced terminations when managing child processes.

Waiting for Process Group

Using a pid less than -1 waits for any child in the specified process group. This is useful for managing groups of related processes.

process_group.py

import os import time

Create a new process group

os.setpgrp()

Create 3 child processes in the same group

for i in range(3): pid = os.fork() if pid == 0: # Children sleep and exit time.sleep(i + 1) os._exit(0)

Parent waits for any child in our process group

Negative PID means wait for any in that process group

group_pid = -os.getpgrp() exited = 0

while exited < 3: child_pid, status = os.waitpid(group_pid, 0) exited += 1 print(f"Child {child_pid} exited (total exited: {exited})”)

The parent creates a new process group and spawns 3 children. Using a negative pid equal to the negative of the group ID, it waits for any child in the group.

This technique is particularly useful when you want to manage all descendants of a process together.

Advanced Status Interpretation

This example demonstrates comprehensive status interpretation using all the available macros for examining the waitpid status value.

status_interpretation.py

import os import signal import random

def analyze_status(pid, status): print(f"\nAnalysis for process {pid}:") if os.WIFEXITED(status): print(f" Exited normally with status {os.WEXITSTATUS(status)}") if os.WIFSIGNALED(status): print(f" Killed by signal {os.WTERMSIG(status)}") print(f" Core dumped: {os.WCOREDUMP(status)}") if os.WIFSTOPPED(status): print(f" Stopped by signal {os.WSTOPSIG(status)}") if os.WIFCONTINUED(status): print(" Continued")

Create child that might exit normally, be killed, or stopped

pid = os.fork() if pid == 0: # Child process fate = random.choice([’exit’, ‘segfault’, ‘stop’]) if fate == ’exit’: os._exit(42) elif fate == ‘segfault’: os.kill(os.getpid(), signal.SIGSEGV) else: os.kill(os.getpid(), signal.SIGSTOP) os._exit(0) else: # Parent waits and analyzes child_pid, status = os.waitpid(pid, 0) analyze_status(child_pid, status)

The child randomly chooses to exit normally, segfault, or stop. The parent uses various macros to interpret the status returned by waitpid.

This demonstrates the full range of status interpretation possibilities when working with child processes in Python.

Waiting for Orphaned Processes

This example shows how os.waitpid handles orphaned processes that have been reparented to init (PID 1) when the original parent exits.

orphaned.py

import os import time import sys

pid = os.fork()

if pid == 0: # Child becomes orphan print(f"Child PID: {os.getpid()}") print(“Child sleeping for 10 seconds (will be orphaned)”) time.sleep(10) print(“Orphan child exiting”) sys.exit(0) else: # Parent exits immediately print(f"Parent exiting, child will be orphaned") sys.exit(0)

This code only runs in the original parent

The orphaned child continues running but can’t be waited for

by this process as it has been reparented to init

The parent exits immediately, orphaning the child which continues running. The original parent can no longer wait for this child as it’s been reparented.

This demonstrates an important limitation - you can only wait for direct child processes, not grandchildren or orphaned processes.

Security Considerations

• PID reuse: Be aware of potential PID reuse between wait calls

• Zombie processes: Always wait for children to avoid zombies

• Race conditions: Children may exit before waitpid is called

• Permission issues: Need permissions to wait for specific processes

• Signal interference: Signals can interrupt waitpid calls

Best Practices

• Always wait: Prevent zombie processes by always waiting

• Check status: Properly interpret the exit status

• Handle errors: Account for ECHILD and EINTR errors

• Use WNOHANG carefully: Avoid busy-waiting with WNOHANG

• Consider subprocess: For new code, consider subprocess module

Source References

• Python os.waitpid Documentation

• Linux waitpid(2) man page

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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