canputnext.9

7号信令功能代码,为开源代码

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.TH CANPUTNEXT 9 "2004/01/12 23:32:59" "LiS-2_16_18-8" "Linux STREAMS DDI/DKI"
.SH NAME
.B canputnext
\- test flow control on a \fISTREAMS\fR message queue
.SH SYNOPSIS
.PP
.B #include <sys/stream.h>
.HP 8
.BI "int " retval " = canputnext(queue_t *" q );
.SH DESCRIPTION"
.PP
It is the responsibility of a
.IR STREAMS (4)
module putting a band zero (0)
non-priority message to a subsequent message queue to check for flow control
using
.BR canputnext ()
before placing a message on the queue with
.BR putnext (9).
.PP
.BR canputnext ()
checks whether the stream is flow controlled starting at the
queue next from the specified queue,
.IR q .
Starting at the queue next from the
.I q
specified
.BR canputnext ()
looks for the next foward queue which has a service routine or which has no
further forward queue.
.PP
If this forward queue has the
.B QFULL
flag clear indicating that the queue is not full, then
.BR canputnext ()
will return true (1).
.PP
If this foward queue has the
.B QFULL
flag is set indicating that the queue if full, then
.BR canputnext ()
will return false (0).
.PP
When
.BR canputnext ()
returns false (0), it also sets the
.B QWANTW
flag for the queue indicating that the
.IR STREAMS (4)
executive should backenable the
service procedures feeding the flow controlled queue once the flow controlled
queue is no longer full (i.e. message have been read from the queue to reduce
the queue size below the low water mark for the queue).
.SH "RETURN VALUES"
.PP
.BR canputnext ()
will return false (0) when the specified
.IR STREAMS (4)
message queue is flow
controlled and return true (1) when the specified
.IR STREAMS (4)
message queue is not
flow controlled.
.SH ERRORS
.PP
.BR canputnext ()
always succeed.
.SH CAVEATS
.PP
In the
.B LiS
.[
LiS
.]
implementation, if
.BR canputnext ()
suffers an internal error, it will return false (0).
.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 a macros that call
\fB\)bcanput(\fI\)q\fB\), \fI\)band\fB\))\fR\).
This does not protect the
.I q->q_next
dereference and can cause problems when modules are being pushed onto or
popped from running streams.
.SH "NOTES"
.PP
.B LfS
.[
LfS
.]
implements
.BR bcanputnext ()
directly and protects the
.I q->q_next
dereference.
.SH "SEE ALSO"
.PP
.BR putnext (9),
.BR canput (9),
.BR bcanput (9),
.BR bcanputnext (9),
.BR noenable "(9), and "
.BR enableok (9).
.SH VERSIONS
.PP
.BR canputnext ()
first appeared in SVR 3 STREAMS
.[
svr3
.]
and continued in SVR 4.2 STREAMS.
.[
svr42
.]
.\"
.\"
.XX
.[
magic
.]
.[
svr42
.]
.[
svr4
.]
.\"
.\"
.[
$LIST$
.]
.TA