12.9.3 Interacting with foreign streams
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Availability:C-language interface function
int Sgcclose(IOSTREAM *s, int flags)
As Sclose(), but intended to be used from the atom garbage collector if a stream is closed because it is garbage. The SWI-Prolog atom garbage collector normally runs in a separate thread and thus may be unable to obtain a lock on s if some thread lost access to the stream while it is locked. For this situation flags may be SIO_CLOSE_TRYLOCK which causes Sgcclose() to return -1 with errno set to EDEADLK if the stream is locked. Alternatively, using SIO_CLOSE_FORCE the stream is closed and released without gaining a lock. This should be safe because the stream is garbage and thus no thread can use the lock.

In addition, Sgcclose() never raises a Prolog exception because Prolog interaction is not allowed from the blob release hook and there is no meaningful way to raise a Prolog exception from this context.