I have a VS840 multimeter, and i found some code for PC readouts, through rs232 port

the source: http://pastebin.com/483265
or you can find it at: http://www.lrr.in.tum.de/~acher/vc840/vc840.c (it's the same code)

the program reads out the measurings right, but the problem is that you can only stop it with C-c

after that if you run the prog. again, it doesnt read anything until a reboot.
setserial said that the port was in use

is there a way to manually close an open serial port? or another way to stop the program that would close the port (there is a close_serial function, but it is not used)?

it is boring if I have to make 20 reading to reboot my pc 20 times.

thank you for any help given

As root type this command to find out which application is using the serial port.

lsof /dev/ttyS0

Here's an example output, my ups daemon is on the first port.
There's another possibility, the application may be using some kind of lock file that isn't cleaned up when you ctrl-c it but gets removed when you reboot. Some programs create a file (maybe in /var/lock) and if the file exists it won't reopen the serial port even though no one is actually using the port.

lsof /dev/ttyS0 doesnt output anything
there is no lock file specified in the program, and none exists for the serial port on the pc

still.. after a reboot, i start the program and it outputs me data, when i stop it, and then rerun it again, it doesnt do anything. unplugging and replugging the cables doesnt help, so does not turning off/on the meter

Do you have the source code to the application?
Also, just try runing the lsof command from a different terminal while your application is also running and see if it prints out the name of your application.
Then when you ctrl-C the app does lsof stop listing the application?