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DAEMON(7)

DESCRIPTION

A daemon is a service process that runs in the background and supervises the system or provides functionality to other processes. Traditionally, daemons are implemented following a scheme originating in SysV Unix. Modern daemons should follow a simpler yet more powerful scheme (here called "new-style" daemons), as implemented by systemd(1). This manual page covers both schemes, and in particular includes recommendations for daemons that shall be included in the systemd init system.

SysV Daemons

When a traditional SysV daemon starts, it should execute the following steps as part of the initialization. Note that these steps are unnecessary for new-style daemons (see below), and should only be implemented if compatibility with SysV is essential.

  1. Close all open file descriptors except standard input, output, and error (i.e. the first three file descriptors 0, 1, 2). This ensures that no accidentally passed file descriptor stays around in the daemon process. On Linux, this is best implemented by iterating through /proc/self/fd, with a fallback of iterating from file descriptor 3 to the value returned by getrlimit() for RLIMIT_NOFILE.

SUMMARY : 文件描述符标志close-on-exec

  1. Reset all signal handlers to their default. This is best done by iterating through the available signals up to the limit of _NSIG and resetting them to SIG_DFL.

  2. Reset the signal mask using sigprocmask().

  3. Sanitize the environment block, removing or resetting environment variables that might negatively impact daemon runtime.

  4. Call fork(), to create a background process.

  5. In the child, call setsid() to detach from any terminal and create an independent session.

  6. In the child, call fork() again, to ensure that the daemon can never re-acquire a terminal again.

  7. Call exit() in the first child, so that only the second child (the actual daemon process) stays around. This ensures that the daemon process is re-parented to init/PID 1, as all daemons should be.

  8. In the daemon process, connect /dev/null to standard input, output, and error.

  9. In the daemon process, reset the umask to 0, so that the file modes passed to open(), mkdir() and suchlike directly control the access mode of the created files and directories.

  10. In the daemon process, change the current directory to the root directory (/), in order to avoid that the daemon involuntarily（无意的） blocks mount points from being unmounted.

  11. In the daemon process, write the daemon PID (as returned by getpid()) to a PID file, for example /run/foobar.pid (for a hypothetical daemon "foobar") to ensure that the daemon cannot be started more than once. 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.

  12. In the daemon process, drop privileges, if possible and applicable.

  13. From the daemon process, notify the original process started that initialization is complete. This can be implemented via an unnamed pipe or similar communication channel that is created before the first fork() and hence available in both the original and the daemon process.

  14. Call exit() in the original process. The process that invoked the daemon must be able to rely on that this exit() happens after initialization is complete and all external communication channels are established and accessible.

The BSD daemon() function should not be used, as it implements only a subset of these steps.

A daemon that needs to provide compatibility with SysV systems should implement the scheme pointed out above. However, it is recommended to make this behavior optional and configurable via a command line argument to ease debugging as well as to simplify integration into systems using systemd.