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@ -3,17 +3,17 @@ |
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Configurable daemon behaviors: |
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Configurable daemon behaviors: |
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1.) The current working directory set to the "/" directory. |
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1.) The current working directory set to the "/" directory. |
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2.) The current file creation mode mask set to 0. |
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2.) The current file creation mode mask set to 0. |
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3.) Close all open files (1024). |
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3.) Close all open files (1024). |
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4.) Redirect standard I/O streams to "/dev/null". |
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4.) Redirect standard I/O streams to "/dev/null". |
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A failed call to fork() now raises an exception. |
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A failed call to fork() now raises an exception. |
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References: |
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References: |
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1) Advanced Programming in the Unix Environment: W. Richard Stevens |
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1) Advanced Programming in the Unix Environment: W. Richard Stevens |
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2) Unix Programming Frequently Asked Questions: |
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2) Unix Programming Frequently Asked Questions: |
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http://www.erlenstar.demon.co.uk/unix/faq_toc.html |
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http://www.erlenstar.demon.co.uk/unix/faq_toc.html |
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""" |
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""" |
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__author__ = "Chad J. Schroeder" |
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__author__ = "Chad J. Schroeder" |
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@ -23,8 +23,8 @@ __revision__ = "$Id$" |
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__version__ = "0.2" |
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__version__ = "0.2" |
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# Standard Python modules. |
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# Standard Python modules. |
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import os # Miscellaneous OS interfaces. |
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import os # Miscellaneous OS interfaces. |
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import sys # System-specific parameters and functions. |
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#import sys # System-specific parameters and functions. |
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# Default daemon parameters. |
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# Default daemon parameters. |
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# File mode creation mask of the daemon. |
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# File mode creation mask of the daemon. |
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@ -38,144 +38,144 @@ MAXFD = 1024 |
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# The standard I/O file descriptors are redirected to /dev/null by default. |
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# The standard I/O file descriptors are redirected to /dev/null by default. |
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if (hasattr(os, "devnull")): |
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if (hasattr(os, "devnull")): |
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REDIRECT_TO = os.devnull |
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REDIRECT_TO = os.devnull |
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else: |
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else: |
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REDIRECT_TO = "/dev/null" |
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REDIRECT_TO = "/dev/null" |
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def createDaemon(): |
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def createDaemon(): |
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"""Detach a process from the controlling terminal and run it in the |
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"""Detach a process from the controlling terminal and run it in the |
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background as a daemon. |
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background as a daemon. |
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""" |
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""" |
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try: |
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try: |
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# Fork a child process so the parent can exit. This returns control to |
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# Fork a child process so the parent can exit. This returns control to |
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# the command-line or shell. It also guarantees that the child will not |
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# the command-line or shell. It also guarantees that the child will not |
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# be a process group leader, since the child receives a new process ID |
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# be a process group leader, since the child receives a new process ID |
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# and inherits the parent's process group ID. This step is required |
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# and inherits the parent's process group ID. This step is required |
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# to insure that the next call to os.setsid is successful. |
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# to insure that the next call to os.setsid is successful. |
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pid = os.fork() |
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pid = os.fork() |
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except OSError, e: |
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except OSError, e: |
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raise Exception, "%s [%d]" % (e.strerror, e.errno) |
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raise Exception, "%s [%d]" % (e.strerror, e.errno) |
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if (pid == 0): # The first child. |
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if (pid == 0): # The first child. |
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# To become the session leader of this new session and the process group |
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# To become the session leader of this new session and the process group |
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# leader of the new process group, we call os.setsid(). The process is |
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# leader of the new process group, we call os.setsid(). The process is |
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# also guaranteed not to have a controlling terminal. |
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# also guaranteed not to have a controlling terminal. |
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os.setsid() |
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os.setsid() |
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# Is ignoring SIGHUP necessary? |
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# Is ignoring SIGHUP necessary? |
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# |
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# |
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# It's often suggested that the SIGHUP signal should be ignored before |
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# It's often suggested that the SIGHUP signal should be ignored before |
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# the second fork to avoid premature termination of the process. The |
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# the second fork to avoid premature termination of the process. The |
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# reason is that when the first child terminates, all processes, e.g. |
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# reason is that when the first child terminates, all processes, e.g. |
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# the second child, in the orphaned group will be sent a SIGHUP. |
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# the second child, in the orphaned group will be sent a SIGHUP. |
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# |
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# |
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# "However, as part of the session management system, there are exactly |
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# "However, as part of the session management system, there are exactly |
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# two cases where SIGHUP is sent on the death of a process: |
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# two cases where SIGHUP is sent on the death of a process: |
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# |
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# |
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# 1) When the process that dies is the session leader of a session that |
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# 1) When the process that dies is the session leader of a session that |
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# is attached to a terminal device, SIGHUP is sent to all processes |
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# is attached to a terminal device, SIGHUP is sent to all processes |
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# in the foreground process group of that terminal device. |
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# in the foreground process group of that terminal device. |
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# 2) When the death of a process causes a process group to become |
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# 2) When the death of a process causes a process group to become |
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# orphaned, and one or more processes in the orphaned group are |
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# orphaned, and one or more processes in the orphaned group are |
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# stopped, then SIGHUP and SIGCONT are sent to all members of the |
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# stopped, then SIGHUP and SIGCONT are sent to all members of the |
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# orphaned group." [2] |
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# orphaned group." [2] |
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# |
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# |
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# The first case can be ignored since the child is guaranteed not to have |
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# The first case can be ignored since the child is guaranteed not to have |
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# a controlling terminal. The second case isn't so easy to dismiss. |
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# a controlling terminal. The second case isn't so easy to dismiss. |
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# The process group is orphaned when the first child terminates and |
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# The process group is orphaned when the first child terminates and |
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# POSIX.1 requires that every STOPPED process in an orphaned process |
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# POSIX.1 requires that every STOPPED process in an orphaned process |
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# group be sent a SIGHUP signal followed by a SIGCONT signal. Since the |
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# group be sent a SIGHUP signal followed by a SIGCONT signal. Since the |
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# second child is not STOPPED though, we can safely forego ignoring the |
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# second child is not STOPPED though, we can safely forego ignoring the |
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# SIGHUP signal. In any case, there are no ill-effects if it is ignored. |
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# SIGHUP signal. In any case, there are no ill-effects if it is ignored. |
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# |
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# |
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# import signal # Set handlers for asynchronous events. |
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# import signal # Set handlers for asynchronous events. |
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# signal.signal(signal.SIGHUP, signal.SIG_IGN) |
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# signal.signal(signal.SIGHUP, signal.SIG_IGN) |
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try: |
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try: |
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# Fork a second child and exit immediately to prevent zombies. This |
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# Fork a second child and exit immediately to prevent zombies. This |
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# causes the second child process to be orphaned, making the init |
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# causes the second child process to be orphaned, making the init |
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# process responsible for its cleanup. And, since the first child is |
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# process responsible for its cleanup. And, since the first child is |
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# a session leader without a controlling terminal, it's possible for |
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# a session leader without a controlling terminal, it's possible for |
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# it to acquire one by opening a terminal in the future (System V- |
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# it to acquire one by opening a terminal in the future (System V- |
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# based systems). This second fork guarantees that the child is no |
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# based systems). This second fork guarantees that the child is no |
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# longer a session leader, preventing the daemon from ever acquiring |
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# longer a session leader, preventing the daemon from ever acquiring |
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# a controlling terminal. |
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# a controlling terminal. |
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pid = os.fork() # Fork a second child. |
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pid = os.fork() # Fork a second child. |
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except OSError, e: |
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except OSError, e: |
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raise Exception, "%s [%d]" % (e.strerror, e.errno) |
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raise Exception, "%s [%d]" % (e.strerror, e.errno) |
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if (pid == 0): # The second child. |
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if (pid == 0): # The second child. |
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# Since the current working directory may be a mounted filesystem, we |
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# Since the current working directory may be a mounted filesystem, we |
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# avoid the issue of not being able to unmount the filesystem at |
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# avoid the issue of not being able to unmount the filesystem at |
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# shutdown time by changing it to the root directory. |
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# shutdown time by changing it to the root directory. |
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os.chdir(WORKDIR) |
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os.chdir(WORKDIR) |
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# We probably don't want the file mode creation mask inherited from |
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# We probably don't want the file mode creation mask inherited from |
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# the parent, so we give the child complete control over permissions. |
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# the parent, so we give the child complete control over permissions. |
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os.umask(UMASK) |
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os.umask(UMASK) |
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else: |
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else: |
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# exit() or _exit()? See below. |
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# exit() or _exit()? See below. |
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os._exit(0) # Exit parent (the first child) of the second child. |
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os._exit(0) # Exit parent (the first child) of the second child. |
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else: |
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else: |
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# exit() or _exit()? |
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# exit() or _exit()? |
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# _exit is like exit(), but it doesn't call any functions registered |
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# _exit is like exit(), but it doesn't call any functions registered |
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# with atexit (and on_exit) or any registered signal handlers. It also |
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# with atexit (and on_exit) or any registered signal handlers. It also |
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# closes any open file descriptors. Using exit() may cause all stdio |
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# closes any open file descriptors. Using exit() may cause all stdio |
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# streams to be flushed twice and any temporary files may be unexpectedly |
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# streams to be flushed twice and any temporary files may be unexpectedly |
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# removed. It's therefore recommended that child branches of a fork() |
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# removed. It's therefore recommended that child branches of a fork() |
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# and the parent branch(es) of a daemon use _exit(). |
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# and the parent branch(es) of a daemon use _exit(). |
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os._exit(0) # Exit parent of the first child. |
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os._exit(0) # Exit parent of the first child. |
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# Close all open file descriptors. This prevents the child from keeping |
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# Close all open file descriptors. This prevents the child from keeping |
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# open any file descriptors inherited from the parent. There is a variety |
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# open any file descriptors inherited from the parent. There is a variety |
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# of methods to accomplish this task. Three are listed below. |
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# of methods to accomplish this task. Three are listed below. |
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# |
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# |
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# Try the system configuration variable, SC_OPEN_MAX, to obtain the maximum |
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# Try the system configuration variable, SC_OPEN_MAX, to obtain the maximum |
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# number of open file descriptors to close. If it doesn't exists, use |
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# number of open file descriptors to close. If it doesn't exists, use |
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# the default value (configurable). |
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# the default value (configurable). |
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# |
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# |
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# try: |
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# try: |
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# maxfd = os.sysconf("SC_OPEN_MAX") |
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# maxfd = os.sysconf("SC_OPEN_MAX") |
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# except (AttributeError, ValueError): |
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# except (AttributeError, ValueError): |
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# maxfd = MAXFD |
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# maxfd = MAXFD |
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# |
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# |
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# OR |
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# OR |
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# |
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# |
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# if (os.sysconf_names.has_key("SC_OPEN_MAX")): |
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# if (os.sysconf_names.has_key("SC_OPEN_MAX")): |
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# maxfd = os.sysconf("SC_OPEN_MAX") |
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# maxfd = os.sysconf("SC_OPEN_MAX") |
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# else: |
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# else: |
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# maxfd = MAXFD |
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# maxfd = MAXFD |
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# |
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# |
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# OR |
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# OR |
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# |
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# |
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# Use the getrlimit method to retrieve the maximum file descriptor number |
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# Use the getrlimit method to retrieve the maximum file descriptor number |
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# that can be opened by this process. If there is not limit on the |
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# that can be opened by this process. If there is not limit on the |
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# resource, use the default value. |
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# resource, use the default value. |
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# |
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# |
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import resource # Resource usage information. |
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import resource # Resource usage information. |
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maxfd = resource.getrlimit(resource.RLIMIT_NOFILE)[1] |
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maxfd = resource.getrlimit(resource.RLIMIT_NOFILE)[1] |
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if (maxfd == resource.RLIM_INFINITY): |
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if (maxfd == resource.RLIM_INFINITY): |
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maxfd = MAXFD |
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maxfd = MAXFD |
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# Iterate through and close all file descriptors. |
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# Iterate through and close all file descriptors. |
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for fd in range(0, maxfd): |
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for fd in range(0, maxfd): |
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try: |
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try: |
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os.close(fd) |
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os.close(fd) |
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except OSError: # ERROR, fd wasn't open to begin with (ignored) |
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except OSError: # ERROR, fd wasn't open to begin with (ignored) |
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pass |
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pass |
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# Redirect the standard I/O file descriptors to the specified file. Since |
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# Redirect the standard I/O file descriptors to the specified file. Since |
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# the daemon has no controlling terminal, most daemons redirect stdin, |
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# the daemon has no controlling terminal, most daemons redirect stdin, |
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# stdout, and stderr to /dev/null. This is done to prevent side-effects |
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# stdout, and stderr to /dev/null. This is done to prevent side-effects |
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# from reads and writes to the standard I/O file descriptors. |
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# from reads and writes to the standard I/O file descriptors. |
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# This call to open is guaranteed to return the lowest file descriptor, |
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# This call to open is guaranteed to return the lowest file descriptor, |
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# which will be 0 (stdin), since it was closed above. |
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# which will be 0 (stdin), since it was closed above. |
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os.open(REDIRECT_TO, os.O_RDWR) # standard input (0) |
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os.open(REDIRECT_TO, os.O_RDWR) # standard input (0) |
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# Duplicate standard input to standard output and standard error. |
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# Duplicate standard input to standard output and standard error. |
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os.dup2(0, 1) # standard output (1) |
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os.dup2(0, 1) # standard output (1) |
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os.dup2(0, 2) # standard error (2) |
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os.dup2(0, 2) # standard error (2) |
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return(0) |
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return(0) |
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Ruud
14 years ago