streamio(7) streamio(7)
NAME
streamio - STREAMS ioctl commands
SYNOPSIS
#include <sys/types.h>
#include <stropts.h>
int ioctl (int fildes, int command, ... /* arg */);
DESCRIPTION
STREAMS [see intro(2)] ioctl commands are a subset of the ioctl(2)
system calls which perform a variety of control functions on streams.
fildes 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. The
command and arg are interpreted by the stream head. Certain
combinations of these arguments may be passed to a module or driver
in the stream.
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 fildes 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. This causes subsequent
system calls to fail with errno set to this value.
COMMAND FUNCTIONS
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. If the stream is a pipe, the module will be
inserted between the stream heads of both ends of the
pipe. It then calls the open routine of the newly-
pushed module. On failure, errno is set to one of the
following values:
EINVAL Invalid module name.
EFAULT arg points outside the allocated address
space.
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ENXIO Open routine 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. To remove a module from a
pipe requires that the module was pushed on the side it
is being removed from. arg should be 0 in an I_POP
request. On failure, errno is set to one of the
following values:
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 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. On failure, errno is set to
one of the following values:
EFAULT arg points outside the allocated address
space.
EINVAL No module present in 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.
If a pipe or FIFO does not have any modules pushed, the
read queue of the stream head on either end is flushed
depending on the value of arg.
If FLUSHR is set and fildes is a pipe, the read queue
for that end of the pipe is flushed and the write queue
for the other end is flushed. If fildes is a FIFO, both
queues are flushed.
If FLUSHW is set and fildes is a pipe and the other end
of the pipe exists, the read queue for the other end of
the pipe is flushed and the write queue for this end is
flushed. If fildes is a FIFO, both queues of the FIFO
are flushed.
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streamio(7) streamio(7)
If FLUSHRW is set, all read queues are flushed, that is,
the read queue for the FIFO and the read queue on both
ends of the pipe are flushed.
Correct flush handling of a pipe or FIFO with modules
pushed is achieved via the pipemod module. This module
should be the first module pushed onto a pipe so that it
is at the midpoint of the pipe itself.
On failure, errno is set to one of the following values:
ENOSR Unable to allocate buffers for flush
message due to insufficient STREAMS memory
resources.
EINVAL Invalid arg value.
ENXIO Hangup received on fildes.
I_FLUSHBAND Flushes a particular band of messages. arg points to a
bandinfo structure that has the following members:
unsigned char bi_pri;
int bi_flag;
The bi_flag field may be one of FLUSHR, FLUSHW, or
FLUSHRW as described earlier.
I_SETSIG Informs the stream head that the user wishes the kernel
to issue the SIGPOLL signal [see signal(2)] 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 user
should be signaled. It is the bitwise-OR of any
combination of the following constants:
S_INPUT Any message other than an M_PCPROTO has
arrived on a stream head read queue. This
event is maintained for compatibility with
prior UNIX System V releases. This is set
even if the message is of zero length.
S_RDNORM An ordinary (non-priority) message has
arrived on a stream head read queue. This
is set even if the message is of zero
length.
S_RDBAND A priority band message (band > 0) has
arrived on a stream head read queue. This
is set even if the message is of zero
length.
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streamio(7) streamio(7)
S_HIPRI A high 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_WRNORM This event is the same as S_OUTPUT.
S_WRBAND A priority band greater than 0 of a queue
downstream exists and is writable. This
notifies the user 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 An M_ERROR message has reached the stream
head.
S_HANGUP An M_HANGUP message 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.
A user process may choose to be signaled only of high
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. On failure, errno is set to one of the
following values:
EINVAL arg value is invalid or arg is zero and
process is not registered to receive the
SIGPOLL signal.
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streamio(7) streamio(7)
EAGAIN Allocation of a data structure to store the
signal request failed.
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. On failure, errno is set to one of
the following values:
EINVAL Process not registered to receive the
SIGPOLL signal.
EFAULT arg points outside the allocated address
space.
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. It
returns 0 if the named module is not present. On
failure, errno is set to one of the following values:
EFAULT arg points outside the allocated address
space.
EINVAL arg does not contain a valid module name.
I_PEEK Allows a user to retrieve the information in the first
message on the stream head read queue without taking the
message off the queue. I_PEEK is analogous to getmsg(2)
except that it does not remove the message from the
queue. arg points to a strpeek structure which contains
the following members:
struct strbufctlbuf;
struct strbufdatabuf;
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. flags may be set to RS_HIPRI
or 0. If RS_HIPRI is set, I_PEEK will look for a high
priority message on the stream head read queue.
Otherwise, I_PEEK will look for the first 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.
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. On failure,
7/91 Page 5
streamio(7) streamio(7)
errno is set to the following value:
EFAULT arg points, or the buffer area specified in
ctlbuf or databuf is, outside the allocated
address space.
EBADMSG Queued message to be read is not valid for
I_PEEK
EINVAL Illegal value for flags.
I_SRDOPT Sets the read mode [see read(2)] 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.
In addition, treatment of control messages by the stream
head may be changed by setting the following flags in
arg:
RPROTNORM Fail read() with EBADMSG if a control
message is at the front of the stream head
read queue.
RPROTDAT Deliver the control portion of a message as
data when a user issues read(). This is
the default behavior.
RPROTDIS Discard the control portion of a message,
delivering any data portion, when a user
issues a read().
On failure, errno is set to the following value:
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(2). On failure, errno is set to the following
value:
EFAULT arg points outside the allocated address
space.
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
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streamio(7) streamio(7)
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. On failure, errno is set to the
following value:
EFAULT arg points outside the allocated address
space.
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 strbufctlbuf;
struct strbufdatabuf;
long flags;
int fildes;
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. fildes in the strfdinsert structure
specifies the file descriptor of the other stream.
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. This pointer
will be the address of the read queue structure of the
driver for the stream corresponding to fildes in the
strfdinsert 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. An
ordinary (non-priority) message is created if flags is
set to 0, a high priority message is created if flags is
set to RS_HIPRI. For normal messages, I_FDINSERT will
block if the stream write queue is full due to internal
flow control conditions. For high priority messages,
I_FDINSERT does not block on this condition. For normal
messages, I_FDINSERT does not block when the write queue
is full and O_NDELAY or O_NONBLOCK is set. Instead, it
fails and sets errno to EAGAIN.
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streamio(7) streamio(7)
I_FDINSERT also blocks, unless prevented by lack of
internal resources, waiting for the availability of
message blocks, regardless of priority or whether
O_NDELAY or O_NONBLOCK has been specified. No partial
message is sent. On failure, errno is set to one of the
following values:
EAGAIN A non-priority message was specified, the
O_NDELAY or O_NONBLOCK flag is set, and the
stream write queue is full due to internal
flow control conditions.
ENOSR Buffers could not be allocated for the
message that was to be created due to
insufficient STREAMS memory resources.
EFAULT arg points, or the buffer area specified in
ctlbuf or databuf is, outside the allocated
address space.
EINVAL One of the following: fildes 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 stored in
flags.
ENXIO Hangup received on fildes of the ioctl call
or fildes in the strfdinsert structure.
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_FDINSERT can also fail if an error message was
received by the stream head of the stream corresponding
to fildes in the strfdinsert structure. In this case,
errno will be set to the value in the message.
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streamio(7) streamio(7)
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 user ioctl requests
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 and
O_NONBLOCK [see open(2)] flags have no effect on this
call.
To send requests downstream, arg must point to a
strioctl structure which contains the following members:
int ic_cmd;
int ic_timout;
int ic_len;
char *ic_dp;
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. The default timeout
is infinite. 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. On failure, errno is
set to one of the following values:
ENOSR Unable to allocate buffers for the ioctl
message due to insufficient STREAMS memory
resources.
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streamio(7) streamio(7)
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.
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 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 will fail
with errno set to the value in the message.
I_SWROPT Sets the write mode using the value of the argument arg.
Legal 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, this bit must not be set in arg.
On failure, errno may be set to the following value:
EINVAL arg is the the above legal 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 Requests the stream associated with fildes 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 open 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
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streamio(7) streamio(7)
intro(2)] of the stream head at the other end of the
stream pipe to which it is connected. On failure, errno
is set to one of the following values:
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 fildes is not connected to a stream pipe.
ENXIO Hangup received on fildes.
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;
uid_t uid;
gid_t 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 and O_NONBLOCK are clear [see open(2)],
I_RECVFD will block until a message is present at the
stream head. If O_NDELAY or O_NONBLOCK is set, I_RECVFD
will 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 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. On failure, errno is set to
one of the following values:
EAGAIN A message is not present at the stream head
read queue, and the O_NDELAY or O_NONBLOCK
flag is set.
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streamio(7) streamio(7)
EBADMSG The message at the stream head read queue
is not a message containing a passed file
descriptor.
EFAULT arg points outside the allocated address
space.
EMFILE NOFILES file descriptors are currently
open.
ENXIO Hangup received on fildes.
EOVERFLOW uid or gid is too large to be stored in the
structure pointed to by arg.
I_LIST Allows the user to list all the module names on the
stream, up to and including the topmost driver name. If
arg is NULL, the return value is the number of modules,
including the driver, that are on the stream pointed to
by fildes. This allows the user to allocate enough
space for the module names. If arg is non-NULL, it
should point to an str_list structure that has the
following members:
int sl_nmods;
struct str_mlist *sl_modlist;
The str_mlist structure has the following member:
char l_name[FMNAMESZ+1];
sl_nmods indicates the number of entries the user has
allocated in the array and on return, sl_modlist
contains the list of module names. The return value
indicates the number of entries that have been filled
in. On failure, errno may be set to one of the
following values:
EINVAL The sl_nmods member is less than 1.
EAGAIN Unable to allocate buffers
I_ATMARK Allows the user to see if the current message on the
stream head read queue is "marked" by some module
downstream. arg determines how the checking is done
when there may be multiple marked messages on the stream
head read queue. It may take the following values:
ANYMARK Check if the message is marked.
LASTMARK Check if the message is the last one marked
on the queue.
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The return value is 1 if the mark condition is satisfied
and 0 otherwise. On failure, errno may be set to the
following value:
EINVAL Invalid arg value.
I_CKBAND Check if the message of a given priority band exists on
the stream head read queue. This returns 1 if a message
of a given priority exists, or -1 on error. arg should
be an integer containing the value of the priority band
in question. On failure, errno may be set to the
following value:
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.
On failure, errno may be set to the following value:
ENODATA No message on the stream head read queue.
I_CANPUT Check if a certain band is writable. arg is set to the
priority band in question. The return value is 0 if the
priority band arg is flow controlled, 1 if the band is
writable, or -1 on error. On failure, errno may be set
to the following value:
EINVAL Invalid arg value.
I_SETCLTIME Allows the user to set the time the stream head will
delay when a stream is closing and there are data on the
write queues. Before closing each module and driver,
the stream head will delay for the specified amount of
time to allow the data to drain. If, after the delay,
data are still present, data will be flushed. arg is
the number of milliseconds to delay, rounded up to the
nearest legal value on the system. The default is
fifteen seconds. On failure, errno may be set to the
following value:
EINVAL Invalid arg value.
I_GETCLTIME Returns the close time delay in the integer pointed by
arg.
The following four commands are used for connecting and disconnecting
multiplexed STREAMS configurations.
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
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streamio(7) streamio(7)
connected to another driver. The stream designated by
arg gets connected below the multiplexing driver.
I_LINK requires 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. On failure, errno is set to one of the
following values:
ENXIO Hangup received on fildes.
ETIME Time out before acknowledgement message was
received at stream head.
EAGAIN Temporarily unable to allocate storage to
perform the I_LINK.
ENOSR Unable to allocate storage to perform the
I_LINK due to insufficient STREAMS memory
resources.
EBADF arg is not a valid, open file descriptor.
EINVAL fildes stream 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 driver is linked into a
multiplexing configuration in more than one
place.
EINVAL fildes is the file descriptor of a pipe or
FIFO.
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 fildes. 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 fildes and arg.
fildes is the file descriptor of the stream connected to
the multiplexing driver. arg is the multiplexor ID
number that was returned by the I_LINK. If arg is -1,
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then all Streams which were linked to fildes are
disconnected. As in I_LINK, this command requires the
multiplexing driver to acknowledge the unlink. On
failure, errno is set to one of the following values:
ENXIO Hangup received on fildes.
ETIME Time out before acknowledgement message was
received at stream head.
ENOSR Unable to allocate storage to perform the
I_UNLINK due to insufficient STREAMS memory
resources.
EINVAL arg is an invalid multiplexor ID number or
fildes is not the stream on which the
I_LINK that returned arg was performed.
EINVAL fildes is the file descriptor of a pipe or
FIFO.
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 fildes. 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.
I_PLINK 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 gets connected via a persistent link below the
multiplexing driver. I_PLINK requires the multiplexing
driver to send an acknowledgement message to the stream
head regarding the linking operation. This call creates
a persistent link which can exist even if the file
descriptor fildes associated with the upper stream to
the multiplexing driver is closed. This call returns a
multiplexor ID number (an identifier that may be used to
disconnect the multiplexor, see I_PUNLINK) on success,
and a -1 on failure. On failure, errno may be set to
one of the following values:
ENXIO Hangup received on fildes.
ETIME Time out before acknowledgement message was
received at the stream head.
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EAGAIN Unable to allocate STREAMS storage to
perform the I_PLINK.
EBADF arg is not a valid, open file descriptor.
EINVAL fildes 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.
EINVAL fildes is the file descriptor of a pipe or
FIFO.
An I_PLINK can also fail while waiting for the
multiplexing driver to acknowledge the link request, if
a message indicating an error on 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 will
fail with errno set to the value in the message.
I_PUNLINK Disconnects the two streams specified by fildes and arg
that are connected with a persistent link. fildes is
the file descriptor of the stream connected to the
multiplexing driver. arg is the multiplexor ID number
that was returned by I_PLINK when a stream was linked
below the multiplexing driver. If arg is MUXID_ALL then
all streams which are persistent links to fildes are
disconnected. As in I_PLINK, this command requires the
multiplexing driver to acknowledge the unlink. On
failure, errno may be set to one of the following
values:
ENXIO Hangup received on fildes.
ETIME Time out before acknowledgement message was
received at the stream head.
EAGAIN Unable to allocate buffers for the
acknowledgement message.
EINVAL Invalid multiplexor ID number.
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EINVAL fildes is the file descriptor of a pipe or
FIFO.
An I_PUNLINK 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 fildes. In addition, an error code
can be returned in the positive or negative
acknowledgement message. For these cases, I_PUNLINK
will fail with errno set to the value in the message.
SEE ALSO
close(2), fcntl(2), getmsg(2), intro(2), ioctl(2), open(2), poll(2),
putmsg(2), read(2), signal(2), write(2), signal(5).
Programmer's Guide: STREAMS.
DIAGNOSTICS
Unless specified otherwise above, the return value from ioctl is 0
upon success and -1 upon failure with errno set as indicated.
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