PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

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

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

   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         top

       None.

RATIONALE         top

       None.

FUTURE DIRECTIONS         top

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

SEE ALSO         top

       Section 2.6, STREAMS, poll(3p), putmsg(3p), read(3p), write(3p)

       The Base Definitions volume of POSIX.1‐2008, stropts.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                          GETMSG(3P)

Pages that refer to this page: stropts.h(0p)getpmsg(3p)ioctl(3p)poll(3p)putmsg(3p)