int putmsg(int fd, const struct strbuf *ctlptr, const struct strbuf *dataptr, int flags);
int putpmsg(int fd, const struct strbuf *ctlptr, const struct strbuf *dataptr, int band, int flags);
The function putpmsg 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.
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; /* not used */ int len; /* length of data */ void *buf; /* ptr to buffer */
ctlptr points to the structure describing the control part, if any, to be included in the message. The buf field in the strbuf structure points to the buffer where the control information resides, and the len field indicates the number of bytes to be sent. The maxlen field is not used in putmsg [see getmsg(2)]. In a similar manner, dataptr specifies the data, if any, to be included in the message. flags 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 field 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 field of dataptr (ctlptr) is set to -1.
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. flags 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_NONBLOCK is set. Instead, it fails and sets errno to EAGAIN.
putmsg or putpmsg 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_NONBLOCK has been specified. No partial message is sent.
putmsg also fails if a STREAMS error message had been processed by the stream head before the call to putmsg. The error returned is the value contained in the STREAMS error message.
While one thread is blocked, siblings might still be executing.