'\" te
.\" Copyright 1989 AT&T
.\" Copyright (c) 2006, Sun Microsystems, Inc.
.TH testb 9F "16 Jan 2006" "SunOS 5.11" "Kernel Functions for Drivers"
.SH NAME
testb \- check for an available buffer
.SH SYNOPSIS
.LP
.nf
#include <sys/stream.h>

\fBint\fR \fBtestb\fR(\fBsize_t\fR \fIsize\fR, \fBuint_t\fR \fIpri\fR);
.fi

.SH INTERFACE LEVEL
.sp
.LP
Architecture independent level 1 (DDI/DKI).
.SH PARAMETERS
.sp
.ne 2
.mk
.na
\fB\fIsize\fR\fR
.ad
.RS 8n
.rt  
Size of the requested buffer.
.RE

.sp
.ne 2
.mk
.na
\fB\fIpri\fR\fR
.ad
.RS 8n
.rt  
Priority of the allocb request.
.RE

.SH DESCRIPTION
.sp
.LP
The \fBtestb()\fR function checks to see if an \fBallocb\fR(9F) call is likely to succeed if a buffer of \fBsize\fR bytes at priority \fIpri\fR is requested. Even if \fBtestb()\fR returns successfully, the call to \fBallocb\fR(9F) can fail. The \fIpri\fR argument is no longer used, but is retained for compatibility.
.SH RETURN VALUES
.sp
.LP
Returns \fB1\fR if a buffer of the requested size is available, and \fB0\fR if one is not.
.SH CONTEXT
.sp
.LP
The \fBtestb()\fR function can be called user, interrupt, or kernel context.
.SH EXAMPLES
.LP
\fBExample 1 \fR\fBtestb()\fR example
.sp
.LP
In a service routine, if \fBcopymsg\fR(9F) fails (line 6), the message is put back on the queue (line 7) and a routine, \fBtryagain\fR, is scheduled to be run in one tenth of a second. Then the service routine returns.

.sp
.LP
When the \fBtimeout\fR(9F) function runs, if there is no message on the front of the queue, it just returns. Otherwise, for each message block in the first message, check to see if an allocation would succeed.  If the number of message blocks equals the number we can allocate, then enable the service procedure. Otherwise, reschedule \fBtryagain\fR to run again in another tenth of a second. Note that \fBtryagain\fR is merely an approximation. Its accounting may be faulty. Consider the case of a message comprised of two 1024-byte message blocks. If there is only one free 1024-byte message block and no free 2048-byte message blocks, then \fBtestb()\fR will still succeed twice. If no message blocks are freed of these sizes before the service procedure runs again, then the \fBcopymsg\fR(9F) will still fail. The reason \fBtestb()\fR is used here is because it is significantly faster than calling \fBcopymsg\fR. We must minimize the amount of time spent in a \fBtimeout()\fR routine.

.sp
.in +2
.nf
1  xxxsrv(q)
 2      queue_t *q;
 3  {
 4	mblk_t *mp;
 5	mblk_t *nmp;
        . . .
 6	if ((nmp = copymsg(mp)) == NULL) {
 7		putbq(q, mp);
 8		timeout(tryagain, (intptr_t)q, drv_usectohz(100000));
 9		return;
10	}
	. . .
11  }
12
13  tryagain(q)
14      queue_t *q;
15  {
16	register int can_alloc = 0;
17	register int num_blks = 0;
18	register mblk_t *mp;
19
20	if (!q->q_first)
21		return;
22	for (mp = q->q_first; mp; mp = mp->b_cont) {
23		num_blks++;
24		can_alloc += testb((mp->b_datap->db_lim -
25		    mp->b_datap->db_base), BPRI_MED);
26	}
27	if (num_blks == can_alloc)
28		qenable(q);
29	else
30		timeout(tryagain, (intptr_t)q, drv_usectohz(100000));
31  }
.fi
.in -2

.SH SEE ALSO
.sp
.LP
\fBallocb\fR(9F), \fBbufcall\fR(9F), \fBcopymsg\fR(9F), \fBtimeout\fR(9F)
.sp
.LP
\fIWriting Device Drivers for Oracle Solaris 11.2\fR
.sp
.LP
\fISTREAMS Programming Guide\fR
.SH NOTES
.sp
.LP
The \fIpri\fR argument is provided for compatibility only. Its value is ignored.