NAME

getmsg - get next message off a stream

SYNOPSIS

#include <stropts.h>

int getmsg(int fd, struct strbuf *ctlptr,
               struct strbuf *dataptr, int *flagsp);

int getpmsg(int fd, struct strbuf *ctlptr,
                struct strbuf *dataptr, int *bandp, int *flagsp);

DESCRIPTION

getmsg retrieves the contents of a  message  [see  intro(2)]
located  at  the stream head read queue from a STREAMS file,
and places the contents into user specified buffer(s).   The
message  must contain either a data part, a control part, or
both.  The data and control parts of the message are  placed
into separate buffers, as described below.  The semantics of
each part is defined by the STREAMS  module  that  generated
the  message.   The  function getpmsg does the same thing as
getmsg, but provides finer control over the priority of  the
messages received.  Except where noted, all information per-
taining to getmsg also pertains to getpmsg.  fd specifies  a
file  descriptor  referencing  an  open  stream.  ctlptr and
dataptr each point to a strbuf structure, which contains the
following members:
     int maxlen;     /* maximum buffer length */
     int len;        /* length of data */
     char *buf;      /* ptr to buffer */
buf points to a buffer in which the data or control informa-
tion  is  to  be  placed,  and  maxlen indicates the maximum
number of bytes this buffer can hold.  On return,  len  con-
tains  the  number  of  bytes of data or control information
actually received, or 0 if there is a zero-length control or
data  part,  or  -1  if  no  data  or control information is
present in the message.  flagsp should point to  an  integer
that  indicates  the  type  of  message  the user is able to
receive.  This is described later.

ctlptr is used to hold the control part from the message and dataptr is used to hold the data part from the message. If ctlptr (or dataptr) is NULL or the maxlen field is -1, the control (or data) part of the message is not processed and is left on the stream head read queue. If ctlptr (or dataptr) is not NULL and there is no corresponding control (or data) part of the messages on the stream head read queue, len is set to -1. If the maxlen field is set to 0 and there is a zero-length control (or data) part, that zero-length part is removed from the read queue and len is set to 0. If the maxlen field is set to 0 and there are more than zero bytes of control (or data) information, that information is left on the read queue and len is set to 0. If the maxlen field in ctlptr or dataptr is less than, respectively, the control or data part of the message, maxlen bytes are retrieved. In this case, the remainder of the message is left on the stream head read queue and a non-zero return value is provided, as described below under DIAGNOSTICS.

By default, getmsg processes the first available message on the stream head read queue. However, a user may choose to retrieve only high priority messages by setting the integer pointed by flagsp to RS_HIPRI. In this case, getmsg processes the next message only if it is a high priority message. If the integer pointed by flagsp is 0, getmsg retrieves any message available on the stream head read queue. In this case, on return, the integer pointed to by flagsp will be set to RS_HIPRI if a high priority message was retrieved, or 0 otherwise.

For getpmsg, the flags are different. flagsp points to a bitmask with the following mutually-exclusive flags defined: MSG_HIPRI, MSG_BAND, and MSG_ANY. Like getmsg, getpmsg processes the first available message on the stream head read queue. A user may choose to retrieve only high-priority messages by setting the integer pointed to by flagsp to MSG_HIPRI and the integer pointed to by bandp to 0. In this case, getpmsg will only process the next message if it is a high-priority message. In a similar manner, a user may choose to retrieve a message from a particular priority band by setting the integer pointed to by flagsp to MSG_BAND and the integer pointed to by bandp to the priority band of interest. In this case, getpmsg will only process the next message if it is in a priority band equal to, or greater than, the integer pointed to by bandp, or if it is a high-priority message. If a user just wants to get the first message off the queue, the integer pointed to by flagsp should be set to MSG_ANY and the integer pointed to by bandp should be set to 0. On return, if the message retrieved was a high-priority message, the integer pointed to by flagsp will be set to MSG_HIPRI and the integer pointed to by bandp will be set to 0. Otherwise, the integer pointed to by flagsp will be set to MSG_BAND and the integer pointed to by bandp will be set to the priority band of the message.

If O_NDELAY and O_NONBLOCK are clear, getmsg blocks until a message of the type specified by flagsp is available on the stream head read queue. If O_NDELAY or O_NONBLOCK has been set and a message of the specified type is not present on the read queue, getmsg fails and sets errno to EAGAIN. If a hangup occurs on the stream from which messages are to be retrieved, getmsg continues to operate normally, as described above, until the stream head read queue is empty. Thereafter, it returns 0 in the len fields of ctlptr and dataptr. getmsg or getpmsg will fail if one or more of the following are true:

EAGAIN The O_NDELAY or O_NONBLOCK flag is set, and no messages are available.

EBADF fd is not a valid file descriptor open for reading.

EBADMSG Queued message to be read is not valid for getmsg.

EFAULT ctlptr, dataptr, bandp, or flagsp points to a location outside the allocated address space.

EINTR A signal was caught during the getmsg system call.

EINVAL An illegal value was specified in flagsp, or the stream referenced by fd is linked under a multiplexor.

ENOSTR A stream is not associated with fd.

getmsg can also fail if a STREAMS error message had been received at the stream head before the call to getmsg. The error returned is the value contained in the STREAMS error message.

SEE ALSO

intro(2), poll(2), putmsg(2), read(2), write(2).
Programmer's Guide: STREAMS.

DIAGNOSTICS

Upon  successful  completion,  a   non-negative   value   is
returned.   A  value  of 0 indicates that a full message was
read successfully.  A return value of MORECTL indicates that
more control information is waiting for retrieval.  A return
value of MOREDATA indicates that more data are  waiting  for
retrieval.   A  return value of MORECTL | MOREDATA indicates
that both types of information  remain.   Subsequent  getmsg
calls  retrieve the remainder of the message.  However, if a
message of higher priority has come in on  the  stream  head
read  queue,  the  next  call  to  getmsg will retrieve that
higher priority message before retrieving the  remainder  of
the previously received partial message.