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Acquires a multi-thread lock.

void __cdecl _lock
   int locknum

[in] locknum

The identifier of the lock to acquire.

If the lock has already been acquired, this method acquires the lock anyway and causes an internal C run-time (CRT) error. If the method cannot acquire a lock, it exits with a fatal error and sets the error code to _RT_LOCK.

Source: mlock.c