The putmsg() function creates a message from user-specified buffer(s) and sends the message to a STREAMS file. The message may contain either a data part, a control part, or both.
The data and control parts to be sent are distinguished by placement in separate buffers, as described below. The semantics of each part is defined by the STREAMS module that receives the message.
The putpmsg() function does the same thing as putmsg(), but provides the user the ability to send messages in different priority bands. Except where noted, all
information pertaining to putmsg() also pertains to putpmsg().
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; /* not used here */
int len; /* length of data */
void *buf; /* ptr to buffer */
The ctlptr argument points to the structure describing the control part, if any, to be included in the message. The buf member in the strbuf
structure points to the buffer where the control information resides, and the len member indicates the number of bytes to be sent. The maxlen member is not used in putmsg() (see getmsg(2)). In a similar manner, dataptr specifies
the data, if any, to be included in the message. The flags argument indicates what type of message should be sent and is described later.
To send the data part of a message, dataptr must not be NULL, and the len member of dataptr must have
a value of 0 or greater. To send the control part of a message, the corresponding values must be set for ctlptr. No data (control) part is sent if either dataptr
(ctlptr) is NULL or the len member of dataptr (ctlptr) is negative.
For putmsg(), if a control part is specified, and flags is set to RS_HIPRI, a high priority message is sent. If no control part is specified,
and flags is set to RS_HIPRI, putmsg() fails and sets errno to EINVAL. If flags
is set to 0, a normal (non-priority) message is sent. If no control part and no data part are specified, and flags is set to 0, no message is sent, and 0 is returned.
The stream head guarantees that the control part of a message generated by putmsg() is at least 64 bytes in length.
For putpmsg(), the flags are different. The flags argument is a bitmask with the following mutually-exclusive flags defined: MSG_HIPRI
and MSG_BAND. If flags is set to 0, putpmsg() fails and sets errno to EINVAL. If a control part is
specified and flags is set to MSG_HIPRI and band is set to 0, a high-priority message is sent. If flags is set to MSG_HIPRI and either no control part is specified or band is set to a non-zero value, putpmsg() fails and sets errno to EINVAL. If flags is set to MSG_BAND, then a message is sent in the priority band specified by band. If a control part and data part are not specified
and flags is set to MSG_BAND, no message is sent and 0 is returned.
Normally, putmsg() will block if the stream write queue is full due to internal flow control conditions. For high-priority messages, putmsg() does not block
on this condition. For other messages, putmsg() does not block when the write queue is full and O_NDELAY or O_NONBLOCK is set. Instead, it fails
and sets errno to EAGAIN.
The putmsg() or putpmsg() function also blocks, unless prevented by lack of internal resources, waiting for the availability of message blocks in the stream,
regardless of priority or whether O_NDELAY or O_NONBLOCK has been specified. No partial message is sent.