I wish to make a process run in the background, but only one instance of it, and not many,
so when the program is loaded, it has to check whether another instance of the same
program is running and if so to exit. How do I do this ?
If your system supports the semget() and semop() calls that are included in one of the optional features in the POSIX standards, you can use semget() to get access to a semaphore with a key that is known to your daemon. After getting the semaphore ID you can use semop() with an array of two operations at the start of your program. These operations are:
If the semop() call fails with EAGAIN, another daemon is already running and owns the semaphore. If the semop() call succeeds, this daemon owns the semaphore until it terminates. (It doesn't matter if this daemon terminates due to receipt of an uncaught signal or terminates normally by calling exit, the SEM_UNDO flag will release ownership of the semaphore when the daemon terminates.)
If you don't have access to a similar set of semaphore operations, a common approach is to atomically create (with open flags O_CREAT | O_EXCL) a lock file and write the PID of the daemon that created the lock into the lock file. The daemon should then remove the lock file before it exits. (If the daemon is killed before it can remove the lock file, another daemon won't be able to start until you manually remove the lock file.)
This User Gave Thanks to Don Cragun For This Post:
Hi,
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rish (1 Reply)
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Hello,
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Hi,
HI ,
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Discussion started by: nks342
3 Replies
LEARN ABOUT SUSE
ncftpbatch
ncftpbatch(1) General Commands Manual ncftpbatch(1)NAME
ncftpbatch - Individual batch FTP job processor
SYNOPSIS
ncftpbatch -d
ncftpbatch -l
ncftpbatch -D
OPTIONS
Command line flags:
-d Begin background processing of FTP jobs in the current user's $HOME/.ncftp/spool directory. This returns immediately, because a
daemon process is spawned and ran in the background.
-l Lists the contents of the user's job queue.
-D This is like -d, except that the process does not become a daemon.
DESCRIPTION
This program is responsible for processing background FTP requests. It is normally only run by ncftp and not manually by a human being,
however you can run it to manually process the FTP job queue.
The jobs are spool files written to a user's $HOME/.ncftp/spool directory and have a special format and file-naming convention (which con-
tains when the job is to be run). ncftp runs this program when it needs to, but if the ncftpbatch daemon dies unexpectedly the jobs that
are left in the queue will not be processed until another instance of ncftpbatch is run.
ncftpget and ncftpput can also be used to submit jobs for batch processing, using those utilities' -b command-line flag. If desired, you
can also manually create the spool files although this procedure is not documented here (see the manual page for ncftpspooler for more
information on how to do that).
DIAGNOSTICS
ncftpbatch writes to its own log file, the $HOME/.ncftp/spool/log file. This file should be examined to determine if any ncftpbatch pro-
cesses are actively working on jobs.
AUTHOR
Mike Gleason, NcFTP Software (http://www.ncftp.com).
SEE ALSO ncftp(1), ncftpput(1), ncftpget(1).
ncftpbatch NcFTP Software ncftpbatch(1)