IOCTL(3POSIX) POSIX Programmer's Manual IOCTL(3POSIX)

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.

ioctl — control a STREAMS device (STREAMS)

#include <stropts.h>
int ioctl(int fildes, int request, ... /* arg */);

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:

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:

Invalid module name.
Open function of new module failed.
Hangup received on fildes.
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:

No module present in the STREAM.
Hangup received on fildes.
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:

No module present in the STREAM.
Flushes read and/or write queues, depending on the value of arg. Valid arg values are:
Flush all read queues.
Flush all write queues.
Flush all read and all write queues.

The ioctl() function with the I_FLUSH command shall fail if:

Invalid arg value.

Unable to allocate buffers for flush message.
Hangup received on fildes.
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.
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:
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.
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.
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.
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.
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.
Equivalent to S_OUTPUT.
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.
A STREAMS signal message that contains the SIGPOLL signal has reached the front of the STREAM head read queue.
Notification of an error condition has reached the STREAM head.
Notification of a hangup has reached the STREAM head.
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:

The value of arg is invalid.
The value of arg is 0 and the calling process is not registered to receive the SIGPOLL signal.
There were insufficient resources to store the signal request.
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:

Process is not registered to receive the SIGPOLL signal.
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:

arg does not contain a valid module name.
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.

Sets the read mode using the value of the argument arg. Read modes are described in read(). Valid arg flags are:
Byte-stream mode, the default.
Message-discard mode.
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:

Fail read() with [EBADMSG] if a message containing a control part is at the front of the STREAM head read queue.
Deliver the control part of a message as data when a process issues a read().
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:

The arg argument is not valid.
Returns the current read mode setting, as described above, in an int pointed to by the argument arg. Read modes are described in read().
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.
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:

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.

Buffers cannot be allocated for the message that is to be created.
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.
Hangup received on the STREAM identified by either the fildes argument or the fildes member of the strfdinsert structure.
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.
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:


Unable to allocate buffers for the ioctl() message.
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.
Hangup received on fildes.
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.

Sets the write mode using the value of the argument arg. Valid bit settings for arg are:
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:

arg is not the above value.
Returns the current write mode setting, as described above, in the int that is pointed to by the argument arg.
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:

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.
The arg argument is not a valid, open file descriptor.
The fildes argument is not connected to a STREAM pipe.
Hangup received on fildes.

The ioctl() function with the I_SENDFD command may fail if:

The arg argument is equal to the fildes argument.
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:

A message is not present at the STREAM head read queue and the O_NONBLOCK flag is set.
The message at the STREAM head read queue is not a message containing a passed file descriptor.
All file descriptors available to the process are currently open.
Hangup received on fildes.
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:

The sl_nmods member is less than 1.

Unable to allocate buffers.
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:
Check if the message is marked.
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:

Invalid arg value.
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:

Invalid arg value.
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:

No message on the STREAM head read queue.
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:

Invalid arg value.
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:

Invalid arg value.
Returns the close time delay in the integer pointed to by arg.

The following commands are used for connecting and disconnecting multiplexed STREAMS configurations. These commands use an implementation-defined default timeout interval.

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:

Hangup received on fildes.
Timeout before acknowledgement message was received at STREAM head.

Unable to allocate STREAMS storage to perform the I_LINK.
The arg argument is not a valid, open file descriptor.
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.

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:

Hangup received on fildes.
Timeout before acknowledgement message was received at STREAM head.

Unable to allocate buffers for the acknowledgement message.
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.

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:

Hangup received on fildes.
Timeout before acknowledgement message was received at STREAM head.

Unable to allocate STREAMS storage to perform the I_PLINK.
The arg argument is not a valid, open file descriptor.
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.

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:

Hangup received on fildes.
Timeout before acknowledgement message was received at STREAM head.

Unable to allocate buffers for the acknowledgement message.
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.

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.

Under the following general conditions, ioctl() shall fail if:

The fildes argument is not a valid open file descriptor.
A signal was caught during the ioctl() operation.
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:

The request or arg argument is not valid for this device.
Some physical I/O error has occurred.
The file associated with the fildes argument is not a STREAMS device that accepts control functions.
The request and arg arguments are valid for this device driver, but the service requested cannot be performed on this particular sub-device.
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.

None.

The implementation-defined timeout interval for STREAMS has historically been 15 seconds.

None.

The ioctl() function may be removed in a future version.

Section 2.6, STREAMS, close(), fcntl(), getmsg(), open(), pipe(), poll(), putmsg(), read(), sigaction(), write()

The Base Definitions volume of POSIX.1‐2017, <stropts.h>

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 .

2017 IEEE/The Open Group