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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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PUTMSG(3P) POSIX Programmer's Manual PUTMSG(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
putmsg, putpmsg — send a message on a STREAM (STREAMS)
#include <stropts.h>
int putmsg(int fildes, const struct strbuf *ctlptr,
const struct strbuf *dataptr, int flags);
int putpmsg(int fildes, const struct strbuf *ctlptr,
const struct strbuf *dataptr, int band, int flags);
The putmsg() function shall create a message from a process
buffer(s) and send the message to a STREAMS file. The message may
contain either a data part, a control part, or both. The data and
control parts are distinguished by placement in separate buffers,
as described below. The semantics of each part are defined by the
STREAMS module that receives the message.
The putpmsg() function is equivalent to putmsg(), except that the
process can send messages in different priority bands. Except
where noted, all requirements on putmsg() also pertain to
putpmsg().
The fildes argument specifies a file descriptor referencing an
open STREAM. The ctlptr and dataptr arguments each point to a
strbuf structure.
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 by putmsg(). In a
similar manner, the argument 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 further below.
To send the data part of a message, the application shall ensure
that dataptr is not a null pointer and the len member of dataptr
is 0 or greater. To send the control part of a message, the
application shall ensure that the corresponding values are set for
ctlptr. No data (control) part shall be sent if either
dataptr(ctlptr) is a null pointer or the len member 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 shall be sent. If no control
part is specified, and flags is set to RS_HIPRI, putmsg() shall
fail and set errno to [EINVAL]. If flags is set to 0, a normal
message (priority band equal to 0) shall be sent. If a control
part and data part are not specified and flags is set to 0, no
message shall be sent and 0 shall be returned.
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() shall fail
and set 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 shall be 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() shall fail and set errno to [EINVAL]. If flags is set
to MSG_BAND, then a message shall be 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 shall be sent
and 0 shall be returned.
The putmsg() function shall block if the STREAM write queue is
full due to internal flow control conditions, with the following
exceptions:
* For high-priority messages, putmsg() shall not block on this
condition and continues processing the message.
* For other messages, putmsg() shall not block but shall fail
when the write queue is full and O_NONBLOCK is set.
The putmsg() function shall also block, unless prevented by lack
of internal resources, while waiting for the availability of
message blocks in the STREAM, regardless of priority or whether
O_NONBLOCK has been specified. No partial message shall be sent.
Upon successful completion, putmsg() and putpmsg() shall return 0;
otherwise, they shall return -1 and set errno to indicate the
error.
The putmsg() and putpmsg() functions shall fail if:
EAGAIN A non-priority message was specified, the O_NONBLOCK flag
is set, and the STREAM write queue is full due to internal
flow control conditions; or buffers could not be allocated
for the message that was to be created.
EBADF fildes is not a valid file descriptor open for writing.
EINTR A signal was caught during putmsg().
EINVAL An undefined value is specified in flags, or flags is set
to RS_HIPRI or MSG_HIPRI and no control part is supplied,
or the STREAM or multiplexer referenced by fildes is linked
(directly or indirectly) downstream from a multiplexer, or
flags is set to MSG_HIPRI and band is non-zero (for
putpmsg() only).
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 fildes.
ENXIO A hangup condition was generated downstream for the
specified STREAM.
EPIPE or EIO
The fildes argument refers to a STREAMS-based pipe and the
other end of the pipe is closed. A SIGPIPE signal is
generated for the calling thread.
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.
In addition, putmsg() and putpmsg() shall fail if the STREAM head
had processed an asynchronous error before the call. In this case,
the value of errno does not reflect the result of putmsg() or
putpmsg(), but reflects the prior error.
The following sections are informative.
Sending a High-Priority Message
The value of fd is assumed to refer to an open STREAMS file. This
call to putmsg() does the following:
1. Creates a high-priority message with a control part and a data
part, using the buffers pointed to by ctrlbuf and databuf,
respectively.
2. Sends the message to the STREAMS file identified by fd.
#include <stropts.h>
#include <string.h>
...
int fd;
char *ctrlbuf = "This is the control part";
char *databuf = "This is the data part";
struct strbuf ctrl;
struct strbuf data;
int ret;
ctrl.buf = ctrlbuf;
ctrl.len = strlen(ctrlbuf);
data.buf = databuf;
data.len = strlen(databuf);
ret = putmsg(fd, &ctrl, &data, MSG_HIPRI);
Using putpmsg()
This example has the same effect as the previous example. In this
example, however, the putpmsg() function creates and sends the
message to the STREAMS file.
#include <stropts.h>
#include <string.h>
...
int fd;
char *ctrlbuf = "This is the control part";
char *databuf = "This is the data part";
struct strbuf ctrl;
struct strbuf data;
int ret;
ctrl.buf = ctrlbuf;
ctrl.len = strlen(ctrlbuf);
data.buf = databuf;
data.len = strlen(databuf);
ret = putpmsg(fd, &ctrl, &data, 0, MSG_HIPRI);
None.
None.
The putmsg() and putpmsg() functions may be removed in a future
version.
Section 2.6, STREAMS, getmsg(3p), poll(3p), read(3p), write(3p)
The Base Definitions volume of POSIX.1‐2017, stropts.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 PUTMSG(3P)
Pages that refer to this page: stropts.h(0p), getmsg(3p), ioctl(3p), poll(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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