esbbcall.9

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.TH ESBBCALL 9 "2004/01/12 23:33:00" "LiS-2_16_18-8" "Linux STREAMS DDI/DKI"
.SH NAME
.B esbbcall
\- installs a buffer callback for an extended \fISTREAMS\fR message block
.SH SYNOPSIS
.PP
.B #include <sys/stream.h>
.HP 8
.BI "int " retval " = esbbcall(int " priority ", void (*" function ")(long), long " arg );
.SH DESCRIPTION
.PP
When allocation of a message block fails as a result of a failed call to
.BR esballoc (9),
the service routine should place a
.IR STREAMS (4)
message that invoked
.BR esballoc (9)
call back on the queue with
.BR putbq (9)
and call the
.BR esbbcall ()
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 priority
argument should be the same as the
.I priority
argument issued to the
.BR esballoc (9).
.PP
In this way, the queue service routine will be recalled once there is
sufficient memory to allocate the requested message block.
.PP
The arguments to
.BR esbbcall ()
are interpreted as follows:
.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 message block needed is for a low priority buffer.
.TP 12
.B BPRI_MED
The message block needed is for a medium priority buffer.
.TP 12
.B BPRI_HI
The message block needed is for a high priority buffer.
.PP
The
.I priority
argument is ignored by SVR 4.2
.[
svr42
.]
and later
.IR STREAMS (4)
implementations.  This
argument is only used by SVR 3
.[
svr3
.]
implementations.
This should be the priority which was provided to the failed
.BR esballoc (9)
call.
.RE
.TP
.I function
The callback function to invoke when a buffer of the required size and message
of the requested priority becomes available.  This is normally the service
routine which failed a call to
.BR esballoc (9).
.TP
.I arg
The argument to pass to the callback function.  This is normally a pointer to
the queue whose service function failed the
.BR esballoc (9).
.SH "RETURN VALUES"
.PP
Upon success,
.BR esbbcall ()
returnes true (1) and the sepcified
.I function
will be called with the specified
.I arg
at a later time when buffers of the necessary size become available.
.PP
Upon failure,
.BR esbbcall ()
returns false (0) and no call will be generated to the specified
.IR function .
.SH ERRORS
.PP
When
.BR esbbcall ()
encounters an error, it returns false (0).
.PP
.BR esbbcall ()
returns false (0) when the function pointer to the callback function,
.I function
is
.BR NULL ,
or
.BR esbbcall ()
cannot allocate the necessary internal structures.
.SH CAVEATS
.PP
The
.B LiS
.[
LiS
.]
implementation of
.BR esbbcall ()
does not actually generate a callback function once a message block of the
appropriate priority 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 able to
allocate the necessary structures.  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 ides 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
.[
magic
.]
explains that in SVR 4.2
.[
svr42
.]
a
.BR esbbcall ()
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 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 esballoc "(9) and "
.BR unbufcall (9).
.SH HISTORY
.PP
.BR esbbcall ()
first appeared in SVR 3.
.[
svr3
.]
.SH VERSIONS
.PP
This manpage was written for
.B LiS\c
.
.[
LiS
.]
.\"
.\"
.XX
.[
magic
.]
.[
svr42
.]
.[
svr4
.]
.\"
.\"
.[
$LIST$
.]
.TA