getmsg(2) (man pages section 2: System Calls)

man pages section 2: System Calls

getmsg(2)

NAME

getmsg, getpmsg- get next message off a stream

SYNOPSIS

#include <stropts.h>

fildes

ctlptr

dataptr

flagsp

fildes

ctlptr

dataptr

bandp

flagsp

DESCRIPTION

The getmsg() function 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 getpmsg() function behaved like getmsg(), but provides finer control over the priority of the messages received. Except where noted, all information pertaining to getmsg() also pertains to getpmsg().

The fildes argument specifies a file descriptor referencing an open stream. The ctlptr and dataptr arguments 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 */

The buf member points to a buffer into 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 contains the number of bytes of data or control information actually received; 0 if there is a zero-length control or data part; or -1 if no data or control information is present in the message. The flagsp argument should point to an integer that indicates the type of message the user is able to receive, as described below.

The ctlptr argument holds the control part from the message and the dataptr argument holds the data part from the message. If ctlptr (or dataptr) is NULL or the maxlen member 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 member 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 member 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 member 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 RETURN VALUES.

By default, getmsg() processes the first available message on the stream head read queue. A user may, however, choose to retrieve only high priority messages by setting the integer pointed to 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 to 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 to 0 otherwise.

For getpmsg(), the flagsp argument 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 member of ctlptr and dataptr.

RETURN VALUES

Upon successful completion, a non-negative value is returned. A return 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 been received by 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.

ERRORS

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

EAGAIN: The O_NDELAY or O_NONBLOCK flag is set and no messages are available.
EBADF: The fildes argument is not a valid file descriptor open for reading.
EBADMSG: Queued message to be read is not valid for getmsg.
EFAULT: The ctlptr, dataptr, bandp, or flagsp argument points to an illegal address.
EINTR: A signal was caught during the execution of the getmsg function.
EINVAL: An illegal value was specified in flagsp, or the stream referenced by fildes is linked under a multiplexor.
ENOSTR: A stream is not associated with fildes.

The getmsg() function 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.