GETMSG(3)                  POSIX Programmer's Manual                 GETMSG(3)



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

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 .



IEEE/The Open Group                  2017                            GETMSG(3)
