getmsg(2)

getmsg, getpmsg -- 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 pertaining 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 information is to be placed, and maxlen indicates the maximum number of bytes this buffer can hold. On return, len contains 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 in Errors.

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.

Return values

On success, getmsg and getpmsg return a non-negative value:

0 indicates that a full message was read successfully.
MORECTL indicates that more control information is waiting for retrieval.
MOREDATA indicates that more data is waiting for retrieval.
(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.

On failure, getmsg and getpmsg return -1 and set errno to identify the error.

Errors

In the following conditions, getmsg and getpmsg fail and set errno to:

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.
EIO: fildes points to an open device that is in the process of closing.
EACCES: fildes points to a dynamic device and read permission on the device is denied.
ENODEV: fildes points to a device that does not support the getmsg operation.

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.

References

intro(2), poll(2), putmsg(2), read(2), write(2)

Notices

Considerations for threads programming

Open file descriptors are a process resource and available to any sibling thread; if used concurrently, actions by one thread can interfere with those of a sibling. In this case, data input by one thread will not be available to others.

While one thread is blocked, siblings might still be executing.