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PROLOG
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.
NAME
ioctl — control a STREAMS device (STREAMS)
SYNOPSIS
#include <stropts.h>
int ioctl(int fildes, int request, ... /* arg */);
DESCRIPTION
The ioctl() function shall perform a variety of control functions on STREAMS devices. For non-STREAMS
devices, the functions performed by this call are unspecified. The request argument and an optional third
argument (with varying type) shall be passed to and interpreted by the appropriate part of the STREAM
associated with fildes.
The fildes argument is an open file descriptor that refers to a device.
The request argument selects the control function to be performed and shall depend on the STREAMS device
being addressed.
The arg argument represents additional information that is needed by this specific STREAMS device to
perform the requested function. The type of arg depends upon the particular control request, but it shall
be either an integer or a pointer to a device-specific data structure.
The ioctl() commands applicable to STREAMS, their arguments, and error conditions that apply to each
individual command are described below.
The following ioctl() commands, with error values indicated, are applicable to all STREAMS files:
I_PUSH Pushes the module whose name is pointed to by arg onto the top of the current STREAM, just
below the STREAM head. It then calls the open() function of the newly-pushed module.
The ioctl() function with the I_PUSH command shall fail if:
EINVAL Invalid module name.
ENXIO Open function of new module failed.
ENXIO Hangup received on fildes.
I_POP Removes the module just below the STREAM head of the STREAM pointed to by fildes. The arg
argument should be 0 in an I_POP request.
The ioctl() function with the I_POP command shall fail if:
EINVAL No module present in the STREAM.
ENXIO Hangup received on fildes.
I_LOOK Retrieves the name of the module just below the STREAM head of the STREAM pointed to by
fildes, and places it in a character string pointed to by arg. The buffer pointed to by arg
should be at least FMNAMESZ+1 bytes long, where FMNAMESZ is defined in <stropts.h>.
The ioctl() function with the I_LOOK command shall fail if:
EINVAL No module present in the STREAM.
I_FLUSH Flushes read and/or write queues, depending on the value of arg. Valid arg values are:
FLUSHR Flush all read queues.
FLUSHW Flush all write queues.
FLUSHRW Flush all read and all write queues.
The ioctl() function with the I_FLUSH command shall fail if:
EINVAL Invalid arg value.
EAGAIN or ENOSR
Unable to allocate buffers for flush message.
ENXIO Hangup received on fildes.
I_FLUSHBAND Flushes a particular band of messages. The arg argument points to a bandinfo structure. The
bi_flag member may be one of FLUSHR, FLUSHW, or FLUSHRW as described above. The bi_pri member
determines the priority band to be flushed.
I_SETSIG Requests that the STREAMS implementation send the SIGPOLL signal to the calling process when
a particular event has occurred on the STREAM associated with fildes. I_SETSIG supports an
asynchronous processing capability in STREAMS. The value of arg is a bitmask that specifies
the events for which the process should be signaled. It is the bitwise-inclusive OR of any
combination of the following constants:
S_RDNORM A normal (priority band set to 0) message has arrived at the head of a STREAM
head read queue. A signal shall be generated even if the message is of zero
length.
S_RDBAND A message with a non-zero priority band has arrived at the head of a STREAM head
read queue. A signal shall be generated even if the message is of zero length.
S_INPUT A message, other than a high-priority message, has arrived at the head of a
STREAM head read queue. A signal shall be generated even if the message is of
zero length.
S_HIPRI A high-priority message is present on a STREAM head read queue. A signal shall be
generated even if the message is of zero length.
S_OUTPUT The write queue for normal data (priority band 0) just below the STREAM head is
no longer full. This notifies the process that there is room on the queue for
sending (or writing) normal data downstream.
S_WRNORM Equivalent to S_OUTPUT.
S_WRBAND The write queue for a non-zero priority band just below the STREAM head is no
longer full. This notifies the process that there is room on the queue for
sending (or writing) priority data downstream.
S_MSG A STREAMS signal message that contains the SIGPOLL signal has reached the front
of the STREAM head read queue.
S_ERROR Notification of an error condition has reached the STREAM head.
S_HANGUP Notification of a hangup has reached the STREAM head.
S_BANDURG When used in conjunction with S_RDBAND, SIGURG is generated instead of SIGPOLL
when a priority message reaches the front of the STREAM head read queue.
If arg is 0, the calling process shall be unregistered and shall not receive further SIGPOLL
signals for the stream associated with fildes.
Processes that wish to receive SIGPOLL signals shall ensure that they explicitly register to
receive them using I_SETSIG. If several processes register to receive this signal for the
same event on the same STREAM, each process shall be signaled when the event occurs.
The ioctl() function with the I_SETSIG command shall fail if:
EINVAL The value of arg is invalid.
EINVAL The value of arg is 0 and the calling process is not registered to receive the SIGPOLL
signal.
EAGAIN There were insufficient resources to store the signal request.
I_GETSIG Returns the events for which the calling process is currently registered to be sent a SIGPOLL
signal. The events are returned as a bitmask in an int pointed to by arg, where the events
are those specified in the description of I_SETSIG above.
The ioctl() function with the I_GETSIG command shall fail if:
EINVAL Process is not registered to receive the SIGPOLL signal.
I_FIND Compares the names of all modules currently present in the STREAM to the name pointed to by
arg, and returns 1 if the named module is present in the STREAM, or returns 0 if the named
module is not present.
The ioctl() function with the I_FIND command shall fail if:
EINVAL arg does not contain a valid module name.
I_PEEK Retrieves the information in the first message on the STREAM head read queue without taking
the message off the queue. It is analogous to getmsg() except that this command does not
remove the message from the queue. The arg argument points to a strpeek structure.
The application shall ensure that the maxlen member in the ctlbuf and databuf strbuf
structures is set to the number of bytes of control information and/or data information,
respectively, to retrieve. The flags member may be marked RS_HIPRI or 0, as described by
getmsg(). If the process sets flags to RS_HIPRI, for example, I_PEEK shall only look for a
high-priority message on the STREAM head read queue.
I_PEEK returns 1 if a message was retrieved, and returns 0 if no message was found on the
STREAM head read queue, or if the RS_HIPRI flag was set in flags and a high-priority message
was not present on the STREAM head read queue. It does not wait for a message to arrive. On
return, ctlbuf specifies information in the control buffer, databuf specifies information in
the data buffer, and flags contains the value RS_HIPRI or 0.
I_SRDOPT Sets the read mode using the value of the argument arg. Read modes are described in read().
Valid arg flags are:
RNORM Byte-stream mode, the default.
RMSGD Message-discard mode.
RMSGN Message-nondiscard mode.
The bitwise-inclusive OR of RMSGD and RMSGN shall return [EINVAL]. The bitwise-inclusive OR
of RNORM and either RMSGD or RMSGN shall result in the other flag overriding RNORM which is
the default.
In addition, treatment of control messages by the STREAM head may be changed by setting any
of the following flags in arg:
RPROTNORM Fail read() with [EBADMSG] if a message containing a control part is at the front
of the STREAM head read queue.
RPROTDAT Deliver the control part of a message as data when a process issues a read().
RPROTDIS Discard the control part of a message, delivering any data portion, when a
process issues a read().
The ioctl() function with the I_SRDOPT command shall fail if:
EINVAL The arg argument is not valid.
I_GRDOPT Returns the current read mode setting, as described above, in an int pointed to by the
argument arg. Read modes are described in read().
I_NREAD Counts the number of data bytes in the data part of the first message on the STREAM head read
queue and places this value in the int pointed to by arg. The return value for the command
shall be the number of messages on the STREAM head read queue. For example, if 0 is returned
in arg, but the ioctl() return value is greater than 0, this indicates that a zero-length
message is next on the queue.
I_FDINSERT Creates a message from specified buffer(s), adds information about another STREAM, and sends
the message downstream. The message contains a control part and an optional data part. The
data and control parts to be sent are distinguished by placement in separate buffers, as
described below. The arg argument points to a strfdinsert structure.
The application shall ensure that the len member in the ctlbuf strbuf structure is set to the
size of a t_uscalar_t plus the number of bytes of control information to be sent with the
message. The fildes member specifies the file descriptor of the other STREAM, and the offset
member, which must be suitably aligned for use as a t_uscalar_t, specifies the offset from
the start of the control buffer where I_FDINSERT shall store a t_uscalar_t whose
interpretation is specific to the STREAM end. The application shall ensure that the len
member in the databuf strbuf structure is set to the number of bytes of data information to
be sent with the message, or to 0 if no data part is to be sent.
The flags member specifies the type of message to be created. A normal message is created if
flags is set to 0, and a high-priority message is created if flags is set to RS_HIPRI. For
non-priority messages, I_FDINSERT shall block if the STREAM write queue is full due to
internal flow control conditions. For priority messages, I_FDINSERT does not block on this
condition. For non-priority messages, I_FDINSERT does not block when the write queue is full
and O_NONBLOCK is set. Instead, it fails and sets errno to [EAGAIN].
I_FDINSERT 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.
The ioctl() function with the I_FDINSERT command shall fail if:
EAGAIN A non-priority message is specified, the O_NONBLOCK flag is set, and the STREAM write
queue is full due to internal flow control conditions.
EAGAIN or ENOSR
Buffers cannot be allocated for the message that is to be created.
EINVAL One of the following:
-- The fildes member of the strfdinsert structure is not a valid, open STREAM
file descriptor.
-- The size of a t_uscalar_t plus offset is greater than the len member for the
buffer specified through ctlbuf.
-- The offset member does not specify a properly-aligned location in the data
buffer.
-- An undefined value is stored in flags.
ENXIO Hangup received on the STREAM identified by either the fildes argument or the fildes
member of the strfdinsert structure.
ERANGE The len member for the buffer specified through databuf does not fall within the range
specified by the maximum and minimum packet sizes of the topmost STREAM module; or the
len member for the buffer specified through databuf is larger than the maximum
configured size of the data part of a message; or the len member for the buffer
specified through ctlbuf is larger than the maximum configured size of the control
part of a message.
I_STR Constructs an internal STREAMS ioctl() message from the data pointed to by arg, and sends
that message downstream.
This mechanism is provided to send ioctl() requests to downstream modules and drivers. It
allows information to be sent with ioctl(), and returns to the process any information sent
upstream by the downstream recipient. I_STR shall block until the system responds with either
a positive or negative acknowledgement message, or until the request times out after some
period of time. If the request times out, it shall fail with errno set to [ETIME].
At most, one I_STR can be active on a STREAM. Further I_STR calls shall block until the
active I_STR completes at the STREAM head. The default timeout interval for these requests is
15 seconds. The O_NONBLOCK flag has no effect on this call.
To send requests downstream, the application shall ensure that arg points to a strioctl
structure.
The ic_cmd member is the internal ioctl() command intended for a downstream module or driver
and ic_timout is the number of seconds (−1=infinite, 0=use implementation-defined timeout
interval, >0=as specified) an I_STR request shall wait for acknowledgement before timing out.
ic_len is the number of bytes in the data argument, and ic_dp is a pointer to the data
argument. The ic_len member has two uses: on input, it contains the length of the data
argument passed in, and on return from the command, it contains the number of bytes being
returned to the process (the buffer pointed to by ic_dp should be large enough to contain the
maximum amount of data that any module or the driver in the STREAM can return).
The STREAM head shall convert the information pointed to by the strioctl structure to an
internal ioctl() command message and send it downstream.
The ioctl() function with the I_STR command shall fail if:
EAGAIN or ENOSR
Unable to allocate buffers for the ioctl() message.
EINVAL The ic_len member is less than 0 or larger than the maximum configured size of the
data part of a message, or ic_timout is less than −1.
ENXIO Hangup received on fildes.
ETIME A downstream ioctl() timed out before acknowledgement was received.
An I_STR can also fail while waiting for an acknowledgement if a message indicating an error
or a hangup is received at the STREAM head. In addition, an error code can be returned in
the positive or negative acknowledgement message, in the event the ioctl() command sent
downstream fails. For these cases, I_STR shall fail with errno set to the value in the
message.
I_SWROPT Sets the write mode using the value of the argument arg. Valid bit settings for arg are:
SNDZERO Send a zero-length message downstream when a write() of 0 bytes occurs. To not
send a zero-length message when a write() of 0 bytes occurs, the application
shall ensure that this bit is not set in arg (for example, arg would be set to
0).
The ioctl() function with the I_SWROPT command shall fail if:
EINVAL arg is not the above value.
I_GWROPT Returns the current write mode setting, as described above, in the int that is pointed to by
the argument arg.
I_SENDFD Creates a new reference to the open file description associated with the file descriptor arg,
and writes a message on the STREAMS-based pipe fildes containing this reference, together
with the user ID and group ID of the calling process.
The ioctl() function with the I_SENDFD command shall fail if:
EAGAIN The sending STREAM is unable to allocate a message block to contain the file pointer;
or the read queue of the receiving STREAM head is full and cannot accept the message
sent by I_SENDFD.
EBADF The arg argument is not a valid, open file descriptor.
EINVAL The fildes argument is not connected to a STREAM pipe.
ENXIO Hangup received on fildes.
The ioctl() function with the I_SENDFD command may fail if:
EINVAL The arg argument is equal to the fildes argument.
I_RECVFD Retrieves the reference to an open file description from a message written to a STREAMS-based
pipe using the I_SENDFD command, and allocates a new file descriptor in the calling process
that refers to this open file description. The arg argument is a pointer to a strrecvfd data
structure as defined in <stropts.h>.
The fd member is a file descriptor. The uid and gid members are the effective user ID and
effective group ID, respectively, of the sending process.
If O_NONBLOCK is not set, I_RECVFD shall block until a message is present at the STREAM head.
If O_NONBLOCK is set, I_RECVFD shall fail with errno set to [EAGAIN] if no message is present
at the STREAM head.
If the message at the STREAM head is a message sent by an I_SENDFD, a new file descriptor
shall be allocated for the open file descriptor referenced in the message. The new file
descriptor is placed in the fd member of the strrecvfd structure pointed to by arg.
The ioctl() function with the I_RECVFD command shall fail if:
EAGAIN A message is not present at the STREAM head read queue and the O_NONBLOCK flag is set.
EBADMSG
The message at the STREAM head read queue is not a message containing a passed file
descriptor.
EMFILE All file descriptors available to the process are currently open.
ENXIO Hangup received on fildes.
I_LIST Allows the process to list all the module names on the STREAM, up to and including the
topmost driver name. If arg is a null pointer, the return value shall be the number of
modules, including the driver, that are on the STREAM pointed to by fildes. This lets the
process allocate enough space for the module names. Otherwise, it should point to a str_list
structure.
The sl_nmods member indicates the number of entries the process has allocated in the array.
Upon return, the sl_modlist member of the str_list structure shall contain the list of module
names, and the number of entries that have been filled into the sl_modlist array is found in
the sl_nmods member (the number includes the number of modules including the driver). The
return value from ioctl() shall be 0. The entries are filled in starting at the top of the
STREAM and continuing downstream until either the end of the STREAM is reached, or the number
of requested modules (sl_nmods) is satisfied.
The ioctl() function with the I_LIST command shall fail if:
EINVAL The sl_nmods member is less than 1.
EAGAIN or ENOSR
Unable to allocate buffers.
I_ATMARK Allows the process to see if the message at the head of the STREAM head read queue is marked
by some module downstream. The arg argument determines how the checking is done when there
may be multiple marked messages on the STREAM head read queue. It may take on the following
values:
ANYMARK Check if the message is marked.
LASTMARK Check if the message is the last one marked on the queue.
The bitwise-inclusive OR of the flags ANYMARK and LASTMARK is permitted.
The return value shall be 1 if the mark condition is satisfied; otherwise, the value shall be
0.
The ioctl() function with the I_ATMARK command shall fail if:
EINVAL Invalid arg value.
I_CKBAND Checks if the message of a given priority band exists on the STREAM head read queue. This
shall return 1 if a message of the given priority exists, 0 if no such message exists, or −1
on error. arg should be of type int.
The ioctl() function with the I_CKBAND command shall fail if:
EINVAL Invalid arg value.
I_GETBAND Returns the priority band of the first message on the STREAM head read queue in the integer
referenced by arg.
The ioctl() function with the I_GETBAND command shall fail if:
ENODATA
No message on the STREAM head read queue.
I_CANPUT Checks if a certain band is writable. arg is set to the priority band in question. The
return value shall be 0 if the band is flow-controlled, 1 if the band is writable, or −1 on
error.
The ioctl() function with the I_CANPUT command shall fail if:
EINVAL Invalid arg value.
I_SETCLTIME This request allows the process to set the time the STREAM head shall delay when a STREAM is
closing and there is data on the write queues. Before closing each module or driver, if
there is data on its write queue, the STREAM head shall delay for the specified amount of
time to allow the data to drain. If, after the delay, data is still present, it shall be
flushed. The arg argument is a pointer to an integer specifying the number of milliseconds to
delay, rounded up to the nearest valid value. If I_SETCLTIME is not performed on a STREAM, an
implementation-defined default timeout interval is used.
The ioctl() function with the I_SETCLTIME command shall fail if:
EINVAL Invalid arg value.
I_GETCLTIME Returns the close time delay in the integer pointed to by arg.
Multiplexed STREAMS Configurations
The following commands are used for connecting and disconnecting multiplexed STREAMS configurations.
These commands use an implementation-defined default timeout interval.
I_LINK Connects two STREAMs, where fildes is the file descriptor of the STREAM connected to the
multiplexing driver, and arg is the file descriptor of the STREAM connected to another
driver. The STREAM designated by arg is connected below the multiplexing driver. I_LINK
requires the multiplexing driver to send an acknowledgement message to the STREAM head
regarding the connection. This call shall return a multiplexer ID number (an identifier used
to disconnect the multiplexer; see I_UNLINK) on success, and −1 on failure.
The ioctl() function with the I_LINK command shall fail if:
ENXIO Hangup received on fildes.
ETIME Timeout before acknowledgement message was received at STREAM head.
EAGAIN or ENOSR
Unable to allocate STREAMS storage to perform the I_LINK.
EBADF The arg argument is not a valid, open file descriptor.
EINVAL The fildes argument does not support multiplexing; or arg is not a STREAM or is
already connected downstream from a multiplexer; or the specified I_LINK operation
would connect the STREAM head in more than one place in the multiplexed STREAM.
An I_LINK can also fail while waiting for the multiplexing driver to acknowledge the request,
if a message indicating an error or a hangup is received at the STREAM head of fildes. In
addition, an error code can be returned in the positive or negative acknowledgement message.
For these cases, I_LINK fails with errno set to the value in the message.
I_UNLINK Disconnects the two STREAMs specified by fildes and arg. fildes is the file descriptor of
the STREAM connected to the multiplexing driver. The arg argument is the multiplexer ID
number that was returned by the I_LINK ioctl() command when a STREAM was connected downstream
from the multiplexing driver. If arg is MUXID_ALL, then all STREAMs that were connected to
fildes shall be disconnected. As in I_LINK, this command requires acknowledgement.
The ioctl() function with the I_UNLINK command shall fail if:
ENXIO Hangup received on fildes.
ETIME Timeout before acknowledgement message was received at STREAM head.
EAGAIN or ENOSR
Unable to allocate buffers for the acknowledgement message.
EINVAL Invalid multiplexer ID number.
An I_UNLINK can also fail while waiting for the multiplexing driver to acknowledge the
request if a message indicating an error or a hangup is received at the STREAM head of
fildes. In addition, an error code can be returned in the positive or negative
acknowledgement message. For these cases, I_UNLINK shall fail with errno set to the value in
the message.
I_PLINK Creates a persistent connection between two STREAMs, where fildes is the file descriptor of
the STREAM connected to the multiplexing driver, and arg is the file descriptor of the STREAM
connected to another driver. This call shall create a persistent connection which can exist
even if the file descriptor fildes associated with the upper STREAM to the multiplexing
driver is closed. The STREAM designated by arg gets connected via a persistent connection
below the multiplexing driver. I_PLINK requires the multiplexing driver to send an
acknowledgement message to the STREAM head. This call shall return a multiplexer ID number
(an identifier that may be used to disconnect the multiplexer; see I_PUNLINK) on success, and
−1 on failure.
The ioctl() function with the I_PLINK command shall fail if:
ENXIO Hangup received on fildes.
ETIME Timeout before acknowledgement message was received at STREAM head.
EAGAIN or ENOSR
Unable to allocate STREAMS storage to perform the I_PLINK.
EBADF The arg argument is not a valid, open file descriptor.
EINVAL The fildes argument does not support multiplexing; or arg is not a STREAM or is
already connected downstream from a multiplexer; or the specified I_PLINK operation
would connect the STREAM head in more than one place in the multiplexed STREAM.
An I_PLINK can also fail while waiting for the multiplexing driver to acknowledge the
request, if a message indicating an error or a hangup is received at the STREAM head of
fildes. In addition, an error code can be returned in the positive or negative
acknowledgement message. For these cases, I_PLINK shall fail with errno set to the value in
the message.
I_PUNLINK Disconnects the two STREAMs specified by fildes and arg from a persistent connection. The
fildes argument is the file descriptor of the STREAM connected to the multiplexing driver.
The arg argument is the multiplexer ID number that was returned by the I_PLINK ioctl()
command when a STREAM was connected downstream from the multiplexing driver. If arg is
MUXID_ALL, then all STREAMs which are persistent connections to fildes shall be disconnected.
As in I_PLINK, this command requires the multiplexing driver to acknowledge the request.
The ioctl() function with the I_PUNLINK command shall fail if:
ENXIO Hangup received on fildes.
ETIME Timeout before acknowledgement message was received at STREAM head.
EAGAIN or ENOSR
Unable to allocate buffers for the acknowledgement message.
EINVAL Invalid multiplexer ID number.
An I_PUNLINK can also fail while waiting for the multiplexing driver to acknowledge the
request if a message indicating an error or a hangup is received at the STREAM head of
fildes. In addition, an error code can be returned in the positive or negative
acknowledgement message. For these cases, I_PUNLINK shall fail with errno set to the value in
the message.
RETURN VALUE
Upon successful completion, ioctl() shall return a value other than −1 that depends upon the STREAMS
device control function. Otherwise, it shall return −1 and set errno to indicate the error.
ERRORS
Under the following general conditions, ioctl() shall fail if:
EBADF The fildes argument is not a valid open file descriptor.
EINTR A signal was caught during the ioctl() operation.
EINVAL The STREAM or multiplexer referenced by fildes is linked (directly or indirectly) downstream from
a multiplexer.
If an underlying device driver detects an error, then ioctl() shall fail if:
EINVAL The request or arg argument is not valid for this device.
EIO Some physical I/O error has occurred.
ENOTTY The file associated with the fildes argument is not a STREAMS device that accepts control
functions.
ENXIO The request and arg arguments are valid for this device driver, but the service requested cannot
be performed on this particular sub-device.
ENODEV The fildes argument refers to a valid STREAMS device, but the corresponding device driver does not
support the ioctl() function.
If a STREAM is connected downstream from a multiplexer, any ioctl() command except I_UNLINK and I_PUNLINK
shall set errno to [EINVAL].
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
The implementation-defined timeout interval for STREAMS has historically been 15 seconds.
RATIONALE
None.
FUTURE DIRECTIONS
The ioctl() function may be removed in a future version.
SEE ALSO
Section 2.6, STREAMS, close(), fcntl(), getmsg(), open(), pipe(), poll(), putmsg(), read(), sigaction(),
write()
The Base Definitions volume of POSIX.1‐2008, <stropts.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .