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Technical Reference: Communications, Volume 2

wantio Utility


Register direct I/O entry points with the stream head.


#include <sys/stream.h>

int wantio(queue_t *q, struct wantio *w)


q Pointer to the queue structure.
w Pointer to the wantio structure.


The wantio STREAMS routine can be used by a STREAMS module or driver to register input/output (read/write/select) entry points with the stream head. The stream head then calls these entry points directly, by-passing all normal STREAMS processing, when an I/O request is detected. This service may be useful to increase STREAMS performance in cases where normal module processing is not required or where STREAMS processing is to be performed outside of this operating system.

STREAMS modules and drivers should precede a wantio call by sending a high priority M_LETSPLAY message upstream. The M_LETSPLAY message format is a message block containing an integer followed by a pointer to the write queue of the module or driver originating the M_LETSPLAY message. The integer counts the number of modules that can permit direct I/O. Each module passes this message to its neighbor after incrementing the count if direct I/O is possible. When this message reaches the stream head, the stream head compares the count field with the number of modules and drivers in the stream. If the count is not equal to the number of modules, then a M_DONTPLAY message is sent downstream indicating direct I/O will not be permitted on the stream. If the count is equal, then queued messages are cleared by sending them downstream as M_BACKWASH messages. When all messages are cleared, then an M_BACKDONE message is sent downstream. This process starts at the stream head and is repeated in every module in the stream. Modules will wait to receive an M_BACKDONE message from upstream. Upon receipt of this message, the module will send all queued data downstream as M_BACKWASH messages. When all data is cleared, the module will send an M_BACKDONE message to its downstream neighbor indicating that all data has been cleared from the stream to this point. wantio registration is cleared from a stream by issuing a wantio call with a NULL pointer to the wantio structure.

Multiprocessor serialization is the responsibility of the driver or module requesting direct I/O. The stream head acquires no STREAMS locks before calling the wantio entry point.

Currently, the write entry point of the wantio structure is ignored.

Return Values

Returns 0 always.

Related Information

The wantmsg utility.

The queue and wantio structures in /usr/include/sys/stream.h.

The STREAMS Entry Points article in InfoExplorer.

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