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PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

NAME

       getmsg, getpmsg — receive next message from a STREAMS file (STREAMS)

SYNOPSIS

       #include <stropts.h>

       int getmsg(int fildes, struct strbuf *restrict ctlptr,
           struct strbuf *restrict dataptr, int *restrict flagsp);
       int getpmsg(int fildes, struct strbuf *restrict ctlptr,
           struct strbuf *restrict dataptr, int *restrict bandp,
           int *restrict flagsp);

DESCRIPTION

       The  getmsg() function shall retrieve the contents of a message located at the head of the
       STREAM head read queue associated with a STREAMS file and place the contents into  one  or
       more  buffers.  The message contains either a data part, a control part, or both. The data
       and control parts of the message shall be  placed  into  separate  buffers,  as  described
       below. The semantics of each part are defined by the originator of the message.

       The  getpmsg()  function  shall  be  equivalent to getmsg(), except that it provides finer
       control over the priority of the messages received. Except where noted,  all  requirements
       on getmsg() also pertain to getpmsg().

       The fildes argument specifies a file descriptor referencing a STREAMS-based file.

       The ctlptr and dataptr arguments each point to a strbuf structure, in which the buf member
       points to a buffer in which the data or control information  is  to  be  placed,  and  the
       maxlen  member  indicates the maximum number of bytes this buffer can hold. On return, the
       len member shall contain the number of bytes  of  data  or  control  information  actually
       received.  The len member shall be set to 0 if there is a zero-length control or data part
       and len shall be set to -1 if no data or control information is present in the message.

       When getmsg() is called, flagsp should point to an integer  that  indicates  the  type  of
       message the process is able to receive. This is described further below.

       The  ctlptr  argument is used to hold the control part of the message, and dataptr is used
       to hold the data part of the message. If ctlptr (or dataptr) is  a  null  pointer  or  the
       maxlen  member is -1, the control (or data) part of the message shall not be processed and
       shall be left on the STREAM head read queue, and if the ctlptr (or dataptr) is not a  null
       pointer,  len  shall  be  set to -1. If the maxlen member is set to 0 and there is a zero-
       length control (or data) part, that zero-length part shall be removed from the read  queue
       and  len  shall  be  set  to 0. If the maxlen member is set to 0 and there are more than 0
       bytes of control (or data) information, that information shall be left on the  read  queue
       and  len  shall  be set to 0. If the maxlen member in ctlptr (or dataptr) is less than the
       control (or data) part of the message, maxlen bytes shall be retrieved. In this case,  the
       remainder of the message shall be left on the STREAM head read queue and a non-zero return
       value shall be provided.

       By default, getmsg() shall process the first available message on  the  STREAM  head  read
       queue.  However,  a  process may choose to retrieve only high-priority messages by setting
       the integer pointed to by flagsp to RS_HIPRI. In this case, getmsg()  shall  only  process
       the  next message if it is a high-priority message.  When the integer pointed to by flagsp
       is 0, any available message shall be retrieved. In  this  case,  on  return,  the  integer
       pointed to by flagsp shall be set to RS_HIPRI if a high-priority message was retrieved, or
       0 otherwise.

       For getpmsg(), the flags are different. The flagsp argument points to a bitmask  with  the
       following  mutually-exclusive  flags  defined:  MSG_HIPRI,  MSG_BAND,  and  MSG_ANY.  Like
       getmsg(), getpmsg() shall process the first available message  on  the  STREAM  head  read
       queue. A process 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()  shall  only  process  the  next message if it is a high-priority message.  In a
       similar manner, a process 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()  shall  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 process 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 shall be set to MSG_HIPRI and the
       integer pointed to by bandp shall be set to 0. Otherwise, the integer pointed to by flagsp
       shall  be set to MSG_BAND and the integer pointed to by bandp shall be set to the priority
       band of the message.

       If O_NONBLOCK is not set, getmsg() and getpmsg() shall block until a message of  the  type
       specified by flagsp is available at the front of the STREAM head read queue. If O_NONBLOCK
       is set and a message of the specified type is not present at the front of the read  queue,
       getmsg() and getpmsg() shall fail and set errno to [EAGAIN].

       If a hangup occurs on the STREAM from which messages are retrieved, getmsg() and getpmsg()
       shall continue to operate normally, as described above, until the STREAM head  read  queue
       is empty. Thereafter, they shall return 0 in the len members of ctlptr and dataptr.

RETURN VALUE

       Upon  successful  completion,  getmsg() and getpmsg() shall return a non-negative value. 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 is waiting for retrieval. A return value of the bitwise-
       logical  OR  of  MORECTL  and  MOREDATA  indicates  that both types of information remain.
       Subsequent getmsg() and getpmsg() calls shall  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() or getpmsg() shall  retrieve  that  higher-priority  message  before
       retrieving the remainder of the previous message.

       If  the high priority control part of the message is consumed, the message shall be placed
       back on the queue as a normal message of band 0. Subsequent getmsg() and  getpmsg()  calls
       shall  retrieve  the  remainder of the message. If, however, a priority message arrives or
       already exists on the STREAM head, the subsequent call  to  getmsg()  or  getpmsg()  shall
       retrieve  the  higher-priority message before retrieving the remainder of the message that
       was put back.

       Upon failure, getmsg() and getpmsg() shall return -1 and set errno to indicate the error.

ERRORS

       The getmsg() and getpmsg() functions shall fail if:

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

       EBADF  The fildes argument is not a valid file descriptor open for reading.

       EBADMSG
              The queued message to be read is not valid for getmsg() or getpmsg() or  a  pending
              file descriptor is at the STREAM head.

       EINTR  A signal was caught during getmsg() or getpmsg().

       EINVAL An  illegal  value was specified by flagsp, or the STREAM or multiplexer referenced
              by fildes is linked (directly or indirectly) downstream from a multiplexer.

       ENOSTR A STREAM is not associated with fildes.

       In addition, getmsg() and getpmsg() shall  fail  if  the  STREAM  head  had  processed  an
       asynchronous  error before the call. In this case, the value of errno does not reflect the
       result of getmsg() or getpmsg() but reflects the prior error.

       The following sections are informative.

EXAMPLES

   Getting Any Message
       In the following example, the value of fd is assumed to refer to an open STREAMS file. The
       call to getmsg() retrieves any available message on the associated STREAM-head read queue,
       returning control and data information to the buffers pointed to by ctrlbuf  and  databuf,
       respectively.

           #include <stropts.h>
           ...
           int fd;
           char ctrlbuf[128];
           char databuf[512];
           struct strbuf ctrl;
           struct strbuf data;
           int flags = 0;
           int ret;

           ctrl.buf = ctrlbuf;
           ctrl.maxlen = sizeof(ctrlbuf);

           data.buf = databuf;
           data.maxlen = sizeof(databuf);

           ret = getmsg (fd, &ctrl, &data, &flags);

   Getting the First Message off the Queue
       In  the  following example, the call to getpmsg() retrieves the first available message on
       the associated STREAM-head read queue.

           #include <stropts.h>
           ...

           int fd;
           char ctrlbuf[128];
           char databuf[512];
           struct strbuf ctrl;
           struct strbuf data;
           int band = 0;
           int flags = MSG_ANY;
           int ret;

           ctrl.buf = ctrlbuf;
           ctrl.maxlen = sizeof(ctrlbuf);

           data.buf = databuf;
           data.maxlen = sizeof(databuf);

           ret = getpmsg (fd, &ctrl, &data, &band, &flags);

APPLICATION USAGE

       None.

RATIONALE

       None.

FUTURE DIRECTIONS

       The getmsg() and getpmsg() functions may be removed in a future version.

SEE ALSO

       Section 2.6, STREAMS, poll(), putmsg(), read(), write()

       The Base Definitions volume of POSIX.1‐2017, <stropts.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1-2017,  Standard  for  Information Technology -- Portable Operating System Interface
       (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C)  2018  by
       the  Institute  of  Electrical  and Electronics Engineers, Inc and The Open Group.  In the
       event of any discrepancy between this version and the original IEEE  and  The  Open  Group
       Standard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most  likely  to  have
       been  introduced  during  the conversion of the source files to man page format. To report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .