There can be a data race any time a processor loads a value, modifies it, and writes it back. Without an atomic update operation like compare_exchange() generally you need to lock the database file against other processes and threads. The typical solution is to only have one process update the file, only have one thread perform the writes, and combine it with a TCP server.

Suppose you have a big data file and want to mark which pages are occupied and which pages are free. Suppose a writer wants to read a bit from an index page to the stack to check whether a data page is occupied, modify the page bit in the stack to claim the data page if another process hasn't claimed it, and write the updated value back to memory to claim the data page to store the data value if another process hasn't claimed it.

If each process read()s the index bits, they can both see that page 2 bit in the index is unset and try to claim it, then write() back the updated index value. The updates to the index will collide, both writers will think the claimed page 2 when only one should have, and one of the data values written to that page will get lost.