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Manual page for STREAMIO(4)

streamio - STREAMS ioctl commands

SYNOPSIS

#include <stropts.h>
int ioctl (fd, command, arg)
int fd, command;

DESCRIPTION

STREAMS (see intro.2 ioctl commands are a subset of ioctl.2 commands that perform a variety of control functions on STREAMS. The arguments command and arg are passed to the file designated by fd and are interpreted by the streamhead. Certain combinations of these arguments may be passed to a module or driver in the stream.

fd is an open file descriptor that refers to a stream. command determines the control function to be performed as described below. arg represents additional information that is needed by this command. The type of arg depends upon the command, but it is generally an integer or a pointer to a command-specific data structure.

Since these STREAMS commands are a subset of ioctl, they are subject to the errors described there. In addition to those errors, the call will fail with errno set to EINVAL, without processing a control function, if the stream referenced by fd is linked below a multiplexor, or if command is not a valid value for a stream.

Also, as described in ioctl, STREAMS modules and drivers can detect errors. In this case, the module or driver sends an error message to the stream head containing an error value. Subsequent system calls will fail with errno set to this value.

IOCTLS

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 streamhead. It then calls the open routine of the newly-pushed module.

I_PUSH will fail if one of the following occurs:

EINVAL
The module name is invalid.
EFAULT
arg points outside the allocated address space.
ENXIO
The open routine of the new module failed.
ENXIO
A hangup is received on the stream referred to by fd.
I_POP
Removes the module just below the stream head of the stream pointed to by fd. arg should be 0 in an I_POP request.

I_POP will fail if one of the following occurs:

EINVAL
No module is present on stream.
ENXIO
A hangup is received on the stream referred to by fd.
I_LOOK
Retrieves the name of the module just below the stream head of the stream pointed to by fd, and places it in a null-terminated character string pointed at by arg. The buffer pointed to by arg should be at least FMNAMESZ+1 bytes long. An `#include <sys/conf.h>' declaration is required.

I_LOOK will fail if one of the following occurs:

EFAULT
arg points outside the allocated address space of the process.
EINVAL
No module is present on stream.

I_FLUSH
This request flushes all input and/or output queues, depending on the value of arg. Legal arg values are:
FLUSHR
Flush read queues.
FLUSHW
Flush write queues.
FLUSHRW
Flush read and write queues.

I_FLUSH will fail if one of the following occurs:

EAGAIN
No buffers could be allocated for the flush message.
EINVAL
The value of arg is invalid.
ENXIO
A hangup is received on the stream referred to by fd.
I_SETSIG
Informs the stream head that the user wishes the kernel to issue the SIGPOLL signal (see sigvec.2 when a particular event has occurred on the stream associated with fd. I_SETSIG supports an asynchronous processing capability in STREAMS. The value of arg is a bitmask that specifies the events for which the user should be signaled. It is the bitwise-OR of any combination of the following constants:
S_INPUT
A non-priority message has arrived on a stream head read queue, and no other messages existed on that queue before this message was placed there. This is set even if the message is of zero length.
S_HIPRI
A priority message is present on the stream head read queue. This is set even if the message is of zero length.
S_OUTPUT
The write queue just below the stream head is no longer full. This notifies the user that there is room on the queue for sending (or writing) data downstream.
S_MSG
A STREAMS signal message that contains the SIGPOLL signal has reached the front of the stream head read queue.

A user process may choose to be signaled only of priority messages by setting the arg bitmask to the value S_HIPRI.

Processes that wish to receive SIGPOLL signals must 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 will be signaled when the event occurs.

If the value of arg is zero, the calling process will be unregistered and will not receive further SIGPOLL signals.

I_SETSIG will fail if one of the following occurs:

EINVAL
The value of arg is invalid or arg is zero and the process is not registered to receive the SIGPOLL signal.
EAGAIN
A data structure could not be allocated 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 pointed to by arg, where the events are those specified in the description of I_SETSIG above.

I_GETSIG will fail if one of the following occurs:

EINVAL
The process is not registered to receive the SIGPOLL signal.
EFAULT
arg points outside the allocated address space of the process.
I_FIND
This request 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. It returns 0 if the named module is not present.

I_FIND will fail if one of the following occurs:

EFAULT
arg points outside the allocated address space of the process.
EINVAL
arg does not point to a valid module name.
I_PEEK
This request allows a user to retrieve the information in the first message on the stream head read queue without taking the message off the queue. arg points to a strpeek structure which contains the following members:

	struct strbuf	ctlbuf;
	struct strbuf	databuf;
	long		flags;

The maxlen field in the ctlbuf and databuf strbuf structures (see getmsg.2 must be set to the number of bytes of control information and/or data information, respectively, to retrieve. If the user sets flags to RS_HIPRI, I_PEEK will only look for a 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 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 0 or RS_HIPRI.

I_PEEK will fail if one of the following occurs:

EFAULT
arg points, or the buffer area specified in ctlbuf or databuf is, outside the allocated address space of the process.
I_SRDOPT
Sets the read mode using the value of the argument arg. Legal arg values are:
RNORM
Byte-stream mode, the default.
RMSGD
Message-discard mode.
RMSGN
Message-nondiscard mode.

Read modes are described in read.2v

I_SRDOPT will fail if one of the following occurs:

EINVAL
arg is not one of the above legal values.
I_GRDOPT
Returns the current read mode setting in an int pointed to by the argument arg. Read modes are described in read.2v

I_GRDOPT will fail if one of the following occurs:

EFAULT
arg points outside the allocated address space of the process.
I_NREAD
Counts the number of data bytes in data blocks in the first message on the stream head read queue, and places this value in the location pointed to by arg. The return value for the command is the number of messages on the stream head read queue. For example, if zero is returned in arg, but the ioctl return value is greater than zero, this indicates that a zero-length message is next on the queue.

I_NREAD will fail if one of the following occurs:

EFAULT
arg points outside the allocated address space of the process.
I_FDINSERT
creates a message from user 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.

arg points to a strfdinsert structure which contains the following members:

	struct strbuf	ctlbuf;
	struct strbuf	databuf;
	long		flags;
	int		fd;
	int		offset;

The len field in the ctlbuf strbuf structure (see putmsg.2 must be set to the size of a pointer plus the number of bytes of control information to be sent with the message. fd specifies the file descriptor of the other stream and offset, which must be word-aligned, specifies the number of bytes beyond the beginning of the control buffer where I_FDINSERT will store a pointer to the fd stream's driver read queue structure. The len field in the databuf strbuf structure must be set to the number of bytes of data information to be sent with the message or zero if no data part is to be sent.

flags specifies the type of message to be created. A non-priority message is created if flags is set to 0, and a priority message is created if flags is set to RS_HIPRI. For non-priority messages, I_FDINSERT will 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_NDELAY 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_NDELAY has been specified. No partial message is sent.

I_FDINSERT will fail if one of the following occurs:

EAGAIN
A non-priority message was specified, the O_NDELAY flag is set, and the stream write queue is full due to internal flow control conditions.
EAGAIN
Buffers could not be allocated for the message that was to be created.
EFAULT
arg points, or the buffer area specified in ctlbuf or databuf is, outside the allocated address space of the process.
EINVAL
fd in the strfdinsert structure is not a valid, open stream file descriptor; the size of a pointer plus offset is greater than the len field for the buffer specified through ctlptr; offset does not specify a properly-aligned location in the data buffer; an undefined value is pointed to by flags.
ENXIO
A hangup is received on the stream referred to by fd.
ERANGE
The len field 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 field for the buffer specified through databuf is larger than the maximum configured size of the data part of a message, or the len field 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 permit a process to specify timeouts and variable-sized amounts of data when sending an ioctl request to downstream modules and drivers. It allows information to be sent with the ioctl, and will return to the user any information sent upstream by the downstream recipient. I_STR blocks 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 fails with errno set to ETIME.

At most, one I_STR can be active on a stream. Further I_STR calls will block until the active I_STR completes at the stream head. The default timeout interval for these requests is 15 seconds. The O_NDELAY (see open.2v flag has no effect on this call.

To send requests downstream, arg must point to a strioctl structure which contains the following members:

	int	ic_cmd;		/* downstream command */
	int	ic_timout;	/* ACK/NAK timeout */
	int	ic_len;		/* length of data arg */
	char	*ic_dp;		/* ptr to data arg */

ic_cmd is the internal ioctl command intended for a downstream module or driver and ic_timout is the number of seconds (-1 = infinite, 0 = use default, >0 = as specified) an I_STR request will 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 field 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 user (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 will convert the information pointed to by the strioctl structure to an internal ioctl command message and send it downstream.

I_STR will fail if one of the following occurs:

EAGAIN
Buffers could not be allocated for the ioctl message.
EFAULT
arg points, or the buffer area specified by ic_dp and ic_len (separately for data sent and data returned) is, outside the allocated address space of the process.
EINVAL
ic_len is less than 0 or ic_len is larger than the maximum configured size of the data part of a message or ic_timout is less than -1.
ENXIO
A hangup is received on the stream referred to by fd.
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 streamhead. 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 will fail with errno set to the value in the message.

I_SENDFD
Requests the stream associated with fd to send a message, containing a file pointer, to the stream head at the other end of a stream pipe. The file pointer corresponds to arg, which must be an integer file descriptor.

I_SENDFD converts arg into the corresponding system file pointer. It allocates a message block and inserts the file pointer in the block. The user id and group id associated with the sending process are also inserted. This message is placed directly on the read queue (see intro.2 of the stream head at the other end of the stream pipe to which it is connected.

I_SENDFD will fail if one of the following occurs:

EAGAIN
The sending stream is unable to allocate a message block to contain the file pointer.
EAGAIN
The read queue of the receiving stream head is full and cannot accept the message sent by I_SENDFD.
EBADF
arg is not a valid, open file descriptor.
EINVAL
fd is not connected to a stream pipe.
ENXIO
A hangup is received on the stream referred to by fd.
I_RECVFD
Retrieves the file descriptor associated with the message sent by an I_SENDFD ioctl over a stream pipe. arg is a pointer to a data buffer large enough to hold an strrecvfd data structure containing the following members:

	int fd;
	unsigned short uid;
	unsigned short gid;
	char fill[8];

fd is an integer file descriptor. uid and gid are the user ID and group ID, respectively, of the sending stream.

If O_NDELAY is not set (see open.2v I_RECVFD will block until a message is present at the streamhead. If O_NDELAY is set, I_RECVFD will fail with errno set to EAGAIN if no message is present at the streamhead.

If the message at the stream head is a message sent by an I_SENDFD, a new user file descriptor is allocated for the file pointer contained in the message. The new file descriptor is placed in the fd field of the strrecvfd structure. The structure is copied into the user data buffer pointed to by arg.

I_RECVFD will fail if one of the following occurs:

EAGAIN
A message was not present at the stream head read queue, and the O_NDELAY flag is set.
EBADMSG
The message at the stream head read queue was not a message containing a passed file descriptor.
EFAULT
arg points outside the allocated address space of the process.
EMFILE
Too many descriptors are active.
ENXIO
A hangup is received on the stream referred to by fd.

The following four commands are used for connecting and disconnecting multiplexed STREAMS configurations.

I_LINK
Connects two streams, where fd 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 gets connected below the multiplexing driver. I_LINK causes the multiplexing driver to send an acknowledgement message to the stream head regarding the linking operation. This call returns a multiplexor ID number (an identifier used to disconnect the multiplexor, see I_UNLINK) on success, and a -1 on failure.

I_LINK will fail if one of the following occurs:

ENXIO
A hangup is received on the stream referred to by fd.
ETIME
The ioctl timed out before an acknowledgement was received.
EAGAIN
Storage could not be allocated to perform the I_LINK.
EBADF
arg is not a valid, open file descriptor.
EINVAL
The stream referred to by fd does not support multiplexing.
EINVAL
arg is not a stream, or is already linked under a multiplexor.
EINVAL
The specified link operation would cause a "cycle" in the resulting configuration; that is, if a given stream head is linked into a multiplexing configuration in more than one place.

An I_LINK can also fail while waiting for the multiplexing driver to acknowledge the link request, if a message indicating an error or a hangup is received at the stream head of fd. In addition, an error code can be returned in the positive or negative acknowledgement message. For these cases, I_LINK will fail with errno set to the value in the message.

I_UNLINK
Disconnects the two streams specified by fd and arg. fd is the file descriptor of the stream connected to the multiplexing driver. arg is the multiplexor ID number that was returned by the ioctl I_LINK command when a stream was linked below the multiplexing driver. If arg is -1, then all streams which were linked to fd are disconnected. As in I_LINK, this command requires the multiplexing driver to acknowledge the unlink.

I_UNLINK will fail if one of the following occurs:

ENXIO
A hangup is received on the stream referred to by fd.
ETIME
The ioctl timed out before an acknowledgement was received.
EAGAIN
Buffers could not be allocated for the acknowledgement message.
EINVAL
The multiplexor ID number was invalid.

An I_UNLINK can also fail while waiting for the multiplexing driver to acknowledge the link request, if a message indicating an error or a hangup is received at the stream head of fd. In addition, an error code can be returned in the positive or negative acknowledgement message. For these cases, I_UNLINK will fail with errno set to the value in the message.

SEE ALSO

close.2v fcntl.2v getmsg.2 intro.2 ioctl.2 open.2v poll.2 putmsg.2 read.2v sigvec.2 write.2v

STREAMS Programmer's Guide
STREAMS Primer


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Created by unroff & hp-tools. © by Hans-Peter Bischof. All Rights Reserved (1997).

Last modified 21/April/97