bcanput.9

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.TH BCANPUT 9 "2004/01/12 23:32:58" "LiS-2_16_18-8" "Linux STREAMS DDI/DKI"
.SH "NAME"
.B bcanput
\- test flow control on a \fISTREAMS\fR message queue
.SH "SYNOPSIS"
.PP
.B #include <sys/stream.h>
.HP 8
.BI "int " retval " = bcanput(queue_t *" q ", unsigned char " band );
.SH "DESCRIPTION"
.PP
It is the responsibility of a
.IR STREAMS (4)
module puting a banded non-priority
message to a subsequent message queue to check for flow control using
.BR bcanput ()
for the band associated with the message before placing a message on the queue
with
.BR putq (9).
.PP
.BR bcanput ()
checks whether the stream is flow controlled starting at the specified queue,
.IR q ,
and for the specified 
.IR band .
Starting at the
.I q
specified and for the
.I band
specified,
.BR bcanput ()
looks for the next forward queue which has a service routine or which has no
further forward queue.
.PP
If this forward queue has the
.B QB_FULL
flag clear indicating that it is not full for the specified
.IR band ,
.BR bcanput ()
will return true (1).
.PP
If this forward queue has the
.B QB_FULL
flag set indicating that it is full for the specified
.IR band ,
.BR bcanput ()
will return false (0).
.PP
When
.BR bcanput ()
returns false (0), it also sets the
.B QB_WANTW
flag for the specified
.IR band ,
indicating that the
.IR STREAMS (4)
executive should backenable service
procedures backwards up the queues when the flow controlled
.I band
is no longer full.
.SH "RETURN VALUES"
.PP
.BR bcanput ()
will return false (0) when the specified
.IR STREAMS (4)
message queue is flow
controlled for the specified band, and return true (1) when the specified
.IR STREAMS (4)
message queue is not flow controlled for the specified band.
.SH "ERRORS"
.PP
.BR bcanput ()
always succeed.
.SH "CAVEATS"
.PP
In the
.B LiS
.[
LiS
.]
implementation, if
.BR bcanput ()
suffers an internal error, it will return false (0).
This is problematic because the calling queue will never be backenabled.
.PP
.B LiS
.[
LiS
.]
implements
\fB\)canput(\fI\)q\fB\))\fR\),
\fB\)canputnext(\fI\)q\fB\))\fR\) and
\fB\)bcanputnext(\fI\)q\fB\), \fI\)band\fB\))\fR\)
with macros that call
\fB\)bcanput(\fI\)q\fB\), \fI\)band\fB\))\fR\).
.SH "NOTES"
.PP
.B LfS
.[
LfS
.]
implements
.BR bcanput ()
and
.BR bcanputnext (9)
functions directly and ensures integrity of dereferencing of
the
.I q
pointer.  This means that, while equivalent,
.BI bcanput( q->q_next ", " band )
and
.BI bcanputnext( q ", " band )
are different in that the caller is responsible for protecting the integrity of
the
.I q->q_next
dereferencing for
.BI bcanput( q->q_next ", " band )\fR.
Driver and module writers should always use
.BI bcanputnext( q ", " band )
instead of
.BI bcanput( q->q_next ", " band )\fR.
In fact, driver and module writers should avoid the dereference
.I q->q_next
altogether.
.SH "SEE ALSO"
.PP
.BR putq (9),
.BR canput (9),
.BR canputnext (9),
.BR bcanputnext (9),
.BR noenable "(9), and "
.BR enableok (9).
.SH HISTORY
.PP
.BR bcanput ()
first appeared in SVR 4.
.[
svr4
.]
.SH "VERSIONS"
.PP
This manpage was written for
.B LiS\c
.
.[
LiS
.]
.\"
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.XX
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.[
svr42
.]
.[
svr4
.]
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$LIST$
.]
.TA