bufcall(9F)

NAME

     bufcall - call a function when a buffer becomes available

SYNOPSIS

     #include <sys/types.h>
     #include <sys/stream.h>

     bufcall_id_t   bufcall(size_t   size,   uint_t   pri,   void
     (*func)(void *arg), void *arg);

INTERFACE LEVEL

     Architecture independent level 1 (DDI/DKI).

PARAMETERS

      size Number of bytes required for the buffer.

     pri   Priority of the  allocb(9F)  allocation  request  (not
           used).

     func  Function or driver routine to be called when a  buffer
           becomes available.

     arg   Argument to the function to be called  when  a  buffer
           becomes available.

DESCRIPTION

     bufcall() serves as  a  timeout(9F)  call  of  indeterminate
     length.  When  a  buffer allocation request fails, bufcall()
     can be used to schedule the routine func, to be called  with
     the  argument  arg when a buffer becomes available. func may
     call allocb() or it may do something else.

RETURN VALUES

     If successful, bufcall() returns a bufcall ID  that  can  be
     used  in a call to unbufcall() to cancel the request. If the
     bufcall() scheduling fails, func is never called  and  0  is
     returned.

CONTEXT

     bufcall() can be called from user or interrupt context.

EXAMPLES

     Example 1: Calling a function when a buffer  becomes  avail-
     able:

     The purpose of this srv(9E) service  routine  is  to  add  a
     header to all M_DATA messages. Service routines must process
     all messages on their queues before returning, or arrange to
     be rescheduled

     While there are messages to be processed (line 13), check to
     see  if  it  is a high priority message or a normal priority
     message that can be sent on (line 14). Normal priority  mes-
     sage  that  cannot be sent are put back on the message queue
     (line 34). If the message was a high priority one, or if  it
     was  normal priority and canputnext(9F) succeeded, then send
     all but M_DATA messages to the next module with  putnext(9F)
     (line 16).

     For M_DATA messages, try to allocate a buffer  large  enough
     to  hold  the  header (line 18). If no such buffer is avail-
     able, the service routine must be  rescheduled  for  a  time
     when a buffer is available. The original message is put back
     on the queue (line 20) and bufcall  (line  21)  is  used  to
     attempt  the rescheduling. It will succeed if the reschedul-
     ing succeeds, indicating that qenable will be called  subse-
     quently  with  the argument q once a buffer of the specified
     size (sizeof (struct hdr)) becomes available.  If  it  does,
     qenable(9F)  will  put q on the list of queues to have their
     service routines called.  If  bufcall()  fails,  timeout(9F)
     (line 22) is used to try again in about a half second.

     If the buffer  allocation  was  successful,  initialize  the
     header  (lines  25-28),  make the message type M_PROTO (line
     29), link the M_DATA message to it (line 30), and pass it on
     (line 31).

     Note that this example ignores  the  bookkeeping  needed  to
     handle  bufcall() and timeout(9F) cancellation for ones that
     are still outstanding at close time.

      1  struct hdr {
      2     unsigned int h_size;
      3     int          h_version;
      4  };
      5
      6  void xxxsrv(q)
      7     queue_t *q;
      8  {
      9     mblk_t *bp;
     10     mblk_t *mp;
     11     struct hdr *hp;
     12
     13     while ((mp = getq(q)) != NULL) { /* get next message */
     14         if (mp->b_datap->db_type >= QPCTL ||   /* if high priority */
                      canputnext(q)) {  /* normal & can be passed */
     15            if (mp->b_datap->db_type != M_DATA)
     16                 putnext(q, mp); /* send all but M_DATA */
     17            else {
     18                bp = allocb(sizeof(struct hdr), BPRI_LO);
     19                if (bp == NULL) {     /* if unsuccessful */
     20                     putbq(q, mp);    /* put it back */
     21                     if (!bufcall(sizeof(struct hdr), BPRI_LO,
                                qenable, q)) /* try to reschedule */

     22                         timeout(qenable, q, drv_usectohz(500000));
     23                        return (0);
     24                 }
     25                 hp = (struct hdr *)bp->b_wptr;
     26                 hp->h_size = msgdsize(mp);     /* initialize header */
     27                 hp->h_version = 1;
     28                 bp->b_wptr += sizeof(struct hdr);
     29                 bp->b_datap->db_type = M_PROTO;     /* make M_PROTO  */
     30                 bp->b_cont = mp;     /* link it */
     31                 putnext(q, bp); /* pass it on */
     32            }
     33         } else { /* normal priority, canputnext failed */
     34           putbq(q, mp);    /* put back on the message queue */
     35           return (0);
     36         }
     37        }
          return (0);
     38  }

SEE ALSO

     srv(9E),    allocb(9F),    canputnext(9F),     esballoc(9F),
     esbbcall(9F),     putnext(9F),    qenable(9F),    testb(9F),
     timeout(9F), unbufcall(9F)

     Writing Device Drivers

     STREAMS Programming Guide

WARNINGS

     Even when func is called by bufcall(), allocb(9F)  can  fail
     if  another module or driver had allocated the memory before
     func was able to call allocb(9F).