C++ Error: free(): invalid next size (fast):

It means that you have a memory error. You may be trying to free a pointer that wasn't allocated bymalloc (or delete an object that wasn't created by new) or you may be trying to free/deletesuch an object more than once. You may be overflowing a buffer or otherwise writing to memory to which you shouldn't be writing, causing heap corruption.

Any number of programming errors can cause this problem. You need to use a debugger, get a backtrace, and see what your program is doing when the error occurs. If that fails and you determine you have corrupted the heap at some previous point in time, you may be in for some painful debugging (it may not be too painful if the project is small enough that you can tackle it piece by piece).