1 files changed, 46 insertions, 0 deletions
diff --git a/demo4/parent.py b/demo4/parent.py
new file mode 100644
index 0000000..79f988c
--- /dev/null
+++ b/demo4/parent.py
@@ -0,0 +1,46 @@
+import subprocess
+import sysv_ipc
+import time
+import os
+
+sem = sysv_ipc.Semaphore(None, sysv_ipc.IPC_CREX, initial_value = 1)
+print("Parent: created semaphore {}.".format(sem.key))
+
+sem.acquire()
+
+# Spawn a child that will wait on this semaphore.
+path, _ = os.path.split(__file__)
+print("Parent: spawning child process...")
+subprocess.Popen(["python", os.path.join(path, 'child.py'), str(sem.key)])
+
+for i in range(3, 0, -1):
+    print("Parent: child process will acquire the semaphore in {} seconds...".format(i))
+    time.sleep(1)
+
+sem.release()
+
+# Sleep for a second to give the child a chance to acquire the semaphore. 
+# This technique is a little sloppy because technically the child could still 
+# starve, but it's certainly sufficient for this demo.
+time.sleep(1)
+
+# Wait for the child to release the semaphore.
+print("Parent: waiting for the child to release the semaphore.")
+sem.acquire()
+
+# Clean up.
+print("Parent: destroying the semaphore.")
+sem.release()
+sem.remove()
+
+msg = """ 
+By the time you're done reading this, the parent will have exited and so the
+operating system will have destroyed the semaphore. You can prove that  the
+semaphore is gone by running this command and observing that it raises
+sysv_ipc.ExistentialError --
+
+   python -c "import sysv_ipc; sysv_ipc.Semaphore({})"
+
+""".format(sem.key)
+
+print(msg)