Hi.

I have a rather advanced question in Linux C programming.

I'm writing some code for a library. This library loads a data structure from disk, and I need to keep it in memory as long as there are processes using it. The library has init and uninit functions, so that could serve for reference counting, and when the last process stops using it, the library can unload the data structure from memory.
My main problem is that the library is used from various processes, and all of them must share the same copy of the data structure.

I've looked at shared memory, but I would like to re-use some implementation of the data structure (hash table), like for example the GLib one, but those allocate on the heap, and AFAIK they can't be changed to allocate in the shared memory segment.

If your data structures are on the heap, they are private, and cannot be shared. I do not think you will be able to use your present approach. Perhaps your objective can be accomplished using threads instead of multiple processes. Using shared memory also requires the use of mutex semaphores to prevent clashes when more than one process tries to write to the shared memory segment. Simply coercing an existing library to use a shared memory segment would also require it to use this or some other type of synchronization method.
--- rod.