SQLite's lock management in multithreaded environments required abandoning LinuxThreads due to its non-POSIX-compliant behavior, where locks were thread-scoped rather than process-wide. Starting with SQLite 3.7.0, the project shifted to the NPTL (Native POSIX Thread Library) to ensure correct lock semantics across threads within a single process.

Even with NPTL, Linux's kernel behavior poses a risk: closing a file descriptor releases all locks on that inode, regardless of which thread holds them. To prevent catastrophic lock loss, SQLite implements lazy file closing, deferring descriptor closure until all locks on the inode are released, trading temporary resource usage for transactional correctness.

The sqlite3OsLock API manages lock escalation internally, enforcing a strict order from NOLOCK to EXCLUSIVE. It avoids unnecessary system calls by handling compatible lock requests within the process and only invoking native fcntl locks for true escalation. This careful, non-blocking design guarantees serializable isolation without deadlocks.