Kernel Functions for Drivers                            testb(9F)



NAME
     testb - check for an available buffer

SYNOPSIS
     #include 



     int testb(sizet size, uintt pri);


INTERFACE LEVEL
     Architecture independent level 1 (DI/DKI).

PARAMETERS
     size    Size of the requested buffer.


     pri     Priority of the allocb request.


DESCRIPTION
     The testb() function checks to see if an allocb(9F) call  is
     likely  to succeed if a buffer of size bytes at priority pri
     is requested. Even if testb() returns successfully, the call
     to  allocb(9F) can fail. The pri argument is no longer used,
     but is retained for compatibility.

RETURN VALUES
     Returns 1 if a buffer of the requested  size  is  available,
     and 0 if one is not.

CONTEXT
     The testb() function can be called user, interrupt, or  ker-
     nel context.

EXAMPLES
     Example 1 testb() example


     In a service routine, if copymsg(9F)  fails  (line  6),  the
     message  is  put  back  on the queue (line 7) and a routine,
     tryagain, is scheduled to be run in one tenth of  a  second.
     Then the service routine returns.



     When the timeout(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



SunOS 5.11          Last change: 16 Jan 2006                    1






Kernel Functions for Drivers                            testb(9F)



     procedure.  Otherwise,  reschedule  tryagain to run again in
     another tenth of a second. Note that tryagain 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  testb()  will  still
     succeed twice. If no message blocks are freed of these sizes
     before  the  service  procedure   runs   again,   then   the
     copymsg(9F) will still fail. The reason testb() is used here
     is because it is significantly faster than calling  copymsg.
     We  must  minimize  the  amount of time spent in a timeout()
     routine.


       1  xxxsrv(q)
        2      queuet *q;
        3  {
        4   mblkt *mp;
        5   mblkt *nmp;
               . . .
        6   if ((nmp = copymsg(mp)) == NUL) {
        7        putbq(q, mp);
        8        timeout(tryagain, (intptrt)q, drvusectohz(100000));
        9        return;
       10   }
            . . .
       11  }
       12
       13  tryagain(q)
       14      queuet *q;
       15  {
       16   register int canalloc = 0;
       17   register int numblks = 0;
       18   register mblkt *mp;
       19
       20   if (!q->qfirst)
       21        return;
       22   for (mp = q->qfirst; mp; mp = mp->bcont) {
       23        numblks];
       24        canalloc ]= testb((mp->bdatap->dblim -
       25            mp->bdatap->dbbase), BPRIMED);
       26   }
       27   if (numblks == canalloc)
       28        qenable(q);
       29   else
       30        timeout(tryagain, (intptrt)q, drvusectohz(100000));
       31  }


SEE ALSO
     allocb(9F), bufcall(9F), copymsg(9F), timeout(9F)




SunOS 5.11          Last change: 16 Jan 2006                    2






Kernel Functions for Drivers                            testb(9F)



     Writing Device Drivers


     STREAMS Programming Guide

NOTES
     The pri argument is provided  for  compatibility  only.  Its
     value is ignored.















































SunOS 5.11          Last change: 16 Jan 2006                    3