putmsg(2) putmsg(2)
NAME
putmsg, putpmsg - send a message on a stream
SYNOPSIS
#include <stropts.h>
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);
DESCRIPTION
putmsg() 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. The semantics of each
part is defined by the STREAMS module that receives the message.
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 per-
tains 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 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 to be included in the message. flags indicates what type of mes-
sage should be sent.
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
RSHIPRI, a high priority message is sent. If no control part is
specified, and flags is set to RSHIPRI, 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.
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For putpmsg(), the flags are different. flags is a bitmask with the
mutually-exclusive flags MSGHIPRI and MSGBAND. If flags is set to 0,
putpmsg() fails and sets errno to EINVAL. If a control part is speci-
fied and flags is set to MSGHIPRI and band is set to 0, a high-
priority message is sent. If flags is set to MSGHIPRI 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 MSGBAND,
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
MSGBAND, 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 ONDELAY or ONONBLOCK is
set. Instead, it fails and sets errno to EAGAIN.
putmsg() or putpmsg() also blocks, unless prevented by lack of inter-
nal resources, waiting for the availability of message blocks in the
stream, regardless of priority or whether ONDELAY or ONONBLOCK has
been specified. No partial message is sent.
ERRORS
The following error code descriptions are function-specific. You will
find a general description in introprm2(2) or in errno(5).
putmsg() fails if one or more of the following apply:
EAGAIN A non-priority message was specified, the ONDELAY or
ONONBLOCK flag is set and the stream write queue is full due
to internal flow control conditions.
EBADF fd is not a valid file descriptor open for writing.
EFAULT ctlptr or dataptr points outside the allocated address space.
EINTR A signal was caught during the putmsg() system call.
EINVAL An undefined value was specified in flags, or flags is set to
RSHIPRI and no control part was supplied.
EINVAL The stream referenced by fd is linked below a multiplexor.
EINVAL In the case of putpmsg(), if flags is set to MSGHIPRI and
band is non-zero.
ENOSR Buffers could not be allocated for the message that was to be
created due to insufficient STREAMS memory resources.
ENOSTR A stream is not associated with fd.
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ENXIO A hangup condition was generated downstream for the specified
stream, or the other end of the pipe is closed.
ERANGE The size of the data part of the message does not fall within
the range specified by the maximum and minimum packet sizes
of the topmost stream module. This value is also returned if
the control part of the message is larger than the maximum
configured size of the control part of a message, or if the
data part of a message is larger than the maximum configured
size of the data part of a message.
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.
RESULT
Upon successful completion, a value of 0 is returned. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
NOTES
If two processes open a FIFO file, in which one of them uses putmsg()
to write a high priority message, and the other reads a high priority
message using getmsg(), it is possible for messages to be lost. Such a
loss can be avoided if the sending process is slowed down between suc-
cessive invocations of putmsg(), by means of sleep().
SEE ALSO
getmsg(2), poll(2), putmsg(2), read(2), write(2), stropts(5).
"Programmer's Guide: STREAMS".
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