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IOCTL(3P) POSIX Programmer's Manual IOCTL(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult 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 func-
tions 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 asso-
ciated 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 sig-
naled 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 informa-
tion 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 down-
stream. 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 inter-
nal 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 speci-
fied 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.
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, respec-
tively, 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
The process has the maximum number of file descriptors currently open that it is allowed.
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 con-
tinuing downstream until either the end of the STREAM is reached, or the number of requested modules ( sl_nmods) is sat-
isfied.
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 down-
stream. 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 mes-
sage 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 dis-
connect 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 mul-
tiplexed 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 connec-
tion 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 mul-
tiplexed 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 func-
tion. 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 fildes argument is not associated with 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
None.
SEE ALSO
STREAMS, close(), fcntl(), getmsg(), open(), pipe(), poll(), putmsg(), read(), sigaction(), write(), the Base Definitions
volume of IEEE Std 1003.1-2001, <stropts.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
right (C) 2001-2003 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.open-
group.org/unix/online.html .
IEEE/The Open Group 2003 IOCTL(3P)
