If I can't use flock or a module, I usually go to create a "Semaphore" file, meaning, I define a filename and location and create a 0 byte file there. When the program starts, it tries to rename the file to some temporary name. If this succeeds, the program may run. If not, I retry or bail out.

the file has to be re-renamed when the program is done.

this algorithm assumes that file renaming is an atomic process, meaning that it can't be interrupted by the system and that a program, which renamed the file, really did so.