This can be implemented via an unnamed pipe or similar communication channel that is created before the firstĪnd hence available in both the original and the daemon process. In the daemon process, drop privileges, if possible and applicable.įrom the daemon process, notify the original process started that initialization is complete.
This must be implemented in race-free fashion so that the PID file is only updated when it is verified at the same time that the PID previously stored in the PID file no longer exists or belongs to a foreign process. (for a hypothetical daemon "foobar") to ensure that the daemon cannot be started more than once. In the daemon process, write the daemon PID (as returned by In the daemon process, change the current directory to the root directory (/), in order to avoid that the daemon involuntarily blocks mount points from being unmounted. In the daemon process, reset the umask to 0, so that the file modes passed toĪnd suchlike directly control the access mode of the created files and directories. This ensures that the daemon process is re-parented to init/PID 1, as all daemons should be. In the first child, so that only the second child (the actual daemon process) stays around. To detach from any terminal and create an independent session.Īgain, to ensure that the daemon can never re-acquire a terminal again. Sanitize the environment block, removing or resetting environment variables that might negatively impact daemon runtime. This is best done by iterating through the available signals up to the limit of _NSIG and resetting them to Reset all signal handlers to their default. proc/self/fd, with a fallback of iterating from file descriptor 3 to the value returned by On Linux, this is best implemented by iterating through This ensures that no accidentally passed file descriptor stays around in the daemon process. the first three file descriptors 0, 1, 2). Note that these steps are unnecessary for new-style daemons (see below), and should only be implemented if compatibility with SysV is essential.Ĭlose all open file descriptors except standard input, output, and error (i.e. When a traditional SysV daemon starts, it should execute the following steps as part of the initialization.
LINUX PROCESS MONITOR DAEMON MANUAL
This manual page covers both schemes, and in particular includes recommendations for daemons that shall be included in the systemd init system. Modern daemons should follow a simpler yet more powerful scheme (here called "new-style" daemons), as implemented by Traditionally, daemons are implemented following a scheme originating in SysV Unix. Daemon - Writing and packaging system daemonsĪ daemon is a service process that runs in the background and supervises the system or provides functionality to other processes.
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