basic.lock

sets one of 64 'execution' locks, in the range 0 to 63. This prevents program 're-entrancy', allowing only one process to run the program at any given time.

If the specified lock is found 'unlocked', this statement 'locks' the lock and the 'then' clause, if present, is executed. If the lock is already locked by another process or by the current process at a different level, it remains locked and the 'else' clause, if present, is executed. If the lock is locked by another process or by the current process at a different level and no 'else' clause is used, the program waits until the lock is unlocked.

If the 'lock.number' expression evaluates to greater than 63, the result is divided by 64, and the 'lock.number' is equal to the remainder of the equation.

See the 'then/else construct' for an explanation on the use of 'then' and 'else' clauses in statements that allow or require them.