qbufcall.9

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.TH QBUFCALL 9 "2004/01/12 23:33:10" "LiS-2_16_18-8" "Linux STREAMS DDI/DKI"
.SH NAME
.B qbufcall
\- install a \fISTREAMS\fR buffer callback
.SH SYNOPSIS
.PP
.B #include <sys/stream.h>
.HP 8
.BI "int " retval " = bufcall(unsigned " size ", int " priority ", void (*" function ")(long), long " arg );
.SH DESCRIPTION
.PP
When allocation of a buffer fails as a result of a failed call to
.BR allocb (9),
the service routine should
place the
.IR STREAMS (4)
message that invoked the
.BR allocb (9)
call back on the queue with
.BR putbq (9)
and call the
.BR bufcall ()
function with the queue's service routine as the
.I function
argument and a pointer to the queue as the
.I arg
argument to the call.
The
.I size
and
.I priority
arguments should be the same as the
.I size
and
.I priority
arguments that were issued to
.BR allocb (9).
.PP
In this way, the queue service routine will be recalled once there is
sufficient memory to allocate the requested buffer and message blocks.
.PP
The arguments to
.BR bufcall ()
are interpreted as follows:
.TP 12
.I size
The size of the requested data buffer.
.TP 12
.I priority
The priority of the requested message.  The priority can be one of the
following values:
.RS
.TP 12
.B BPRI_LO
The buffer needed is a high priority buffer.
.TP 12
.B BPRI_MED
The buffer needed is a medium priority buffer.
.TP 12
.B BPRI_HI
The buffer needed is a low priority buffer.
.PP
The
.I priority
argument is ignored by post-SVR 4.2 STREAMS
.[
svr42
.]
implementations.
.RE
.TP 12
.I function
The callback function to call when a buffer of the requested size and message of the
requested priority becomes available.
.TP 12
.I arg
The argument to pass to the callback function.
.SH "RETURN VALUES"
.PP
Upon success,
.BR bufcall ()
returns true (1) and the specified
.I function
will be called with the sepcified
.I arg
at a later time when buffers of the necessary size become available.
.PP
Upon failure,
.BR bufcall ()
returns false (0) and no call will be generated to the specified
.IR function .
.SH ERRORS
.PP
When
.BR bufcall ()
encounters an error, it returns false (0).
.PP
.BR bufcall ()
returns false (0) when the function pointer to the callback function,
.I function
is
.BR NULL ,
or cannot allocate the necessary internal structures.
.SH CAVEATS
.PP
The
.B LiS
.[
LiS
.]
implementation of
.BR bufcall ()
does not actually generate a callback function once a buffer of the
appropriate size becomes available.  Instead
.B LiS
.[
LiS
.]
starts a timer.  When the timer expires, the ability to allocate the necessary
buffers is checked and the callback function invoked if it is now possible to
allocate the necessary structures and buffers.
This is not entirely consistent with SVR 4.2
.[
svr42
.]
operation.
.PP
.B LiS
.[
LiS
.]
builds a linked list of buffer call structures and assigns an integer id to
each structure.  Searches for unique buffer ids when ids are assigned and
searches for buffer ids when
.BR unbufcall (9)
is called are linear searches that are slow and inefficient.
In fact, \*(lqThe Magic Garden\*(rq explains that in SVR 4.2
.[
svr42
.]
a
.BR bufcall ()
results in the creation of a
.B strevent
structure which is linked to the
.I strbcalls
system variable
and sets the
.I strbcwait
flag which is then causes
.BR kmem_free ()
to set the
.IR qrunflag .
At no point in this process is an integer id assigned to the
.B strevent
structure.  This means that
.B LiS
.[
LiS
.]
meaninglessly assigns an integer id to the buffer call structure and the
inefficiencies of linear searches are pointless.
.SH "SEE ALSO"
.PP
.BR putbq (9),
.BR allocb "(9) and "
.BR unbufcall (9).
.SH HISTORY
.PP
.BR bufcall ()
first appeared in SVR 3.
.[
svr3
.]
.SH VERSIONS
.PP
This manpage was written for
.B LiS\c
.
.[
LiS
.]
.\"
.\"
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.[
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.]
.[
svr42
.]
.[
svr4
.]
.\"
.\"
.[
$LIST$
.]
.TA