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9.5. Daemon

http://www.myelin.co.nz/post/2003/3/13/#200303135

	
#!/usr/bin/env python

import os, sys

print "I'm going to fork now - the child will write something to a pipe, and the parent will read it back"

r, w = os.pipe()           # r,w是檔案描述符, 不是檔案對象

pid = os.fork()
if pid:
    # 父進程
    os.close(w)           # 關閉一個檔案描述符
    r = os.fdopen(r)      # 將r轉化為檔案對象
    print "parent: reading"
    txt = r.read()
    os.waitpid(pid, 0)   # 確保子進程被撤銷
else:
    # 子進程
    os.close(r)
    w = os.fdopen(w, 'w')
    print "child: writing"
    w.write("here's some text from the child")
    w.close()
    print "child: closing"
    sys.exit(0)

print "parent: got it; text =", txt
	
	
	
import sys, os

if __name__ == "__main__":
    # do the UNIX double-fork magic, see Stevens' "Advanced
    # Programming in the UNIX Environment" for details (ISBN 0201563177)
    try:
        pid = os.fork()
        if pid > 0:
            # exit first parent
            sys.exit(0)
    except OSError, e:
        print >>sys.stderr, "fork #1 failed: %d (%s)" % (e.errno, e.strerror)
        sys.exit(1)

    # decouple from parent environment
    os.chdir("/")
    os.setsid()
    os.umask(0)

    # do second fork
    try:
        pid = os.fork()
        if pid > 0:
            # exit from second parent, print eventual PID before
            print "Daemon PID %d" % pid
            sys.exit(0)
    except OSError, e:
        print >>sys.stderr, "fork #2 failed: %d (%s)" % (e.errno, e.strerror)
        sys.exit(1)

    # start the daemon main loop
	
	

# Redirect standard file descriptors
sys.stdin = open('/dev/null', 'r')
sys.stdout = open('/dev/null', 'w')
sys.stderr = open('/dev/null', 'w')
	
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