Single Program Instance

Posted on 2004-09-30
Last Modified: 2010-04-15
How can I make some sort of lock to ensure that only one instance of a program is running at a time?
Question by:joele24

Expert Comment

ID: 12195241
In the code, at the beginning you can open a pipe to "ps -elf | grep <progname>" and check whether something with that name is running or not. But his has one drawback, if somebody changes the name of the executable/binary, the process name will be different.

The other way can be writing some information (e.g. process ID) into some file and everytime before starting just check whether that file exists. Similarly you need to remove that file just before the program is exiting.

Author Comment

ID: 12195454
I have the 2nd way you described but the problem is I appear to be hitting a race condition where 2 will start up and one both will read and write to the file in such a way as to miss that another process is running

Expert Comment

ID: 12196206
if you running on windows you can use named mutex object
LVL 45

Expert Comment

ID: 12196265

On linux you don't need to write to the file.  Simply open it for exclusive write.  The second program will be unable to open the file because it's "locked" and know that another instance is running.

When the program terminates the lock file will automatically be closed so there's no chance of a deadlock or improper status in the file.

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Expert Comment

by:Amritpal Singh
ID: 12197428
plz have a look at the following links

i hope they may be of any help

Author Comment

ID: 12201615
This is for the linux platform, but thx for all the additonal  info.


so what is wrong with this?

mutex= open("/path/to/file",O_RDWR|O_EXCL);

basically as I run multiple instances of the program they all are able to get this file descriptor and  do not return -1

LVL 45

Accepted Solution

Kdo earned 125 total points
ID: 12202286

My apologies.  I still tend to think in terms of other environments / systems and it's not quite this easy in *nix.

The O_EXCL flag doesn't work this way.  It's a modifier to to O_CREAT flag.

Basically, you're going to have to create the file, not just open it.

  mutex = open ("/path/to/file", O_RDWR|O_CREAT|O_EXCL, <permissions>);

  if (mutex < 0)
    open failed, presumably because the file already exists.

Then when the program finishes, delete the file.


Author Comment

ID: 12202338
ah one quick question do you know if i can call a system("touch path_to_file") or will the lock prevent this
LVL 45

Expert Comment

ID: 12202367

You should be able to do that.  If the touch(1) fails (which it shouldn't) the worst that will happen is that system() will return a non-zero value.


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