how to create multiple databases running in different dirs

Hi, I'm new to postgresql and sql in general. I desired to write a program in C that used an sql data base for IPC and because multiple copies of my program might run on the same machine I wanted a way to ensure that only one copy of each multithreaded program got one database but I'm uncertain how to go about this. As the program might be run several different times and each time it should only get the data base from it's predisesor I can't use the pid as a unique data base name. I thought that I might start multiple pgsql instances but somehow that seems wrong. Any ideas?

 

Thanks, David