Man Pages

---

Back to Software Index  Manpage Top Level

Name

Synopsis

#include <tcl.h>

Tcl_Channel
Tcl_CreateChannel(typePtr, channelName, instanceData, mask)

ClientData
Tcl_GetChannelInstanceData(channel)

Tcl_ChannelType *
Tcl_GetChannelType(channel)

CONST char *
Tcl_GetChannelName(channel)

int
Tcl_GetChannelHandle(channel, direction, handlePtr)

Tcl_ThreadId
Tcl_GetChannelThread(channel)

int
Tcl_GetChannelMode(channel)

int
Tcl_GetChannelBufferSize(channel)

Tcl_SetChannelBufferSize(channel, size)

Tcl_NotifyChannel(channel, mask)

int
Tcl_BadChannelOption(interp, optionName, optionList)

int
Tcl_IsChannelShared(channel)

int
Tcl_IsChannelRegistered(interp, channel)

int
Tcl_IsChannelExisting(channelName)

void
Tcl_CutChannel(channel)

void
Tcl_SpliceChannel(channel)

void
Tcl_ClearChannelHandlers(channel)

int
Tcl_ChannelBuffered(channel)

CONST char *
Tcl_ChannelName(typePtr)

Tcl_ChannelTypeVersion
Tcl_ChannelVersion(typePtr)

Tcl_DriverBlockModeProc *
Tcl_ChannelBlockModeProc(typePtr)

Tcl_DriverCloseProc *
Tcl_ChannelCloseProc(typePtr)

Tcl_DriverClose2Proc *
Tcl_ChannelClose2Proc(typePtr)

Tcl_DriverInputProc *
Tcl_ChannelInputProc(typePtr)

Tcl_DriverOutputProc *
Tcl_ChannelOutputProc(typePtr)

Tcl_DriverSeekProc *
Tcl_ChannelSeekProc(typePtr)

Tcl_DriverWideSeekProc *
Tcl_ChannelWideSeekProc(typePtr)

Tcl_DriverThreadActionProc *
Tcl_ChannelThreadActionProc(typePtr)

Tcl_DriverSetOptionProc *
Tcl_ChannelSetOptionProc(typePtr)

Tcl_DriverGetOptionProc *
Tcl_ChannelGetOptionProc(typePtr)

Tcl_DriverWatchProc *
Tcl_ChannelWatchProc(typePtr)

Tcl_DriverGetHandleProc *
Tcl_ChannelGetHandleProc(typePtr)

Tcl_DriverFlushProc *
Tcl_ChannelFlushProc(typePtr)

Tcl_DriverHandlerProc *
Tcl_ChannelHandlerProc(typePtr)

ArgumentsTcl_ChannelType   *typePtr    (in)
    Points to a structure containing the addresses of procedures that can be
called to perform I/O and other functions on the channel. CONST char   *channelName    (in)
    The name of this channel, such as file3; must not be in use by any other
channel. Can be NULL, in which case the channel is created without a name.
ClientData   instanceData    (in)
    Arbitrary one-word value to be associated with this channel.  This value
is passed to procedures in typePtr when they are invoked. int   mask    (in)
    OR-ed combination of TCL_READABLE and TCL_WRITABLE to indicate whether a
channel is readable and writable. Tcl_Channel   channel    (in)
    The channel to operate on. int   direction    (in)
    TCL_READABLE means the input handle is wanted; TCL_WRITABLE means the output
handle is wanted. ClientData   *handlePtr    (out)
    Points to the location where the desired OS-specific handle should be stored.
int   size    (in)
    The size, in bytes, of buffers to allocate in this channel. int   mask    (in)
    An OR-ed combination of TCL_READABLE, TCL_WRITABLE and TCL_EXCEPTION that
indicates events that have occurred on this channel. Tcl_Interp   *interp    (in)
    Current interpreter. (can be NULL) CONST char   *optionName    (in)
    Name of the invalid option. CONST char   *optionList    (in)
    Specific options list (space separated words, without "-")  to append to
the standard generic options list. Can be NULL for generic options error
message only.   DescriptionTcl uses a two-layered channel architecture. It
provides a generic upper layer to enable C and Tcl programs to perform
input and output using the same APIs for a variety of files, devices, sockets
etc. The generic C APIs are described in the manual entry for Tcl_OpenFileChannel.

The lower layer provides type-specific channel drivers for each type of
device supported on each platform.  This manual entry describes the C APIs
used to communicate between the generic layer and the type-specific channel
drivers.  It also explains how new types of channels can be added by providing
new channel drivers. 
Channel drivers consist of a number of components:
First, each channel driver provides a Tcl_ChannelType structure containing
pointers to functions implementing the various operations used by the generic
layer to communicate with the channel driver. The Tcl_ChannelType structure
and the functions referenced by it are described in the section TCL_CHANNELTYPE,
below. 
Second, channel drivers usually provide a Tcl command to create instances
of that type of channel. For example, the Tcl open command creates channels
that use the file and command channel drivers, and the Tcl socket command
creates channels that use TCP sockets for network communication. 
Third,
a channel driver optionally provides a C function to open channel instances
of that type. For example, Tcl_OpenFileChannel opens a channel that uses
the file channel driver, and Tcl_OpenTcpClient opens a channel that uses
the TCP network protocol.  These creation functions typically use Tcl_CreateChannel
internally to open the channel. 
To add a new type of channel you must implement
a C API or a Tcl command that opens a channel by invoking Tcl_CreateChannel.
When your driver calls Tcl_CreateChannel it passes in a Tcl_ChannelType
structure describing the driver’s I/O procedures. The generic layer will
then invoke the functions referenced in that structure to perform operations
on the channel. 
Tcl_CreateChannel opens a new channel and associates the
supplied typePtr and instanceData with it. The channel is opened in the
mode indicated by mask. For a discussion of channel drivers, their operations
and the Tcl_ChannelType structure, see the section TCL_CHANNELTYPE, below.

Tcl_CreateChannel interacts with the code managing the standard channels.
Once a standard channel was initialized either through a call to Tcl_GetStdChannel
or a call to Tcl_SetStdChannel closing this standard channel will cause
the next call to Tcl_CreateChannel to make the new channel the new standard
channel too. See Tcl_StandardChannels for a general treatise about standard
channels and the behaviour of the Tcl library with regard to them. 
Tcl_GetChannelInstanceData
returns the instance data associated with the channel in channel. This is
the same as the instanceData argument in the call to Tcl_CreateChannel
that created this channel. 
Tcl_GetChannelType returns a pointer to the Tcl_ChannelType
structure used by the channel in the channel argument. This is the same
as the typePtr argument in the call to Tcl_CreateChannel that created this
channel. 
Tcl_GetChannelName returns a string containing the name associated
with the channel, or NULL if the channelName argument to Tcl_CreateChannel
was NULL. 
Tcl_GetChannelHandle places the OS-specific device handle associated
with channel for the given direction in the location specified by handlePtr
and returns TCL_OK.  If the channel does not have a device handle for the
specified direction, then TCL_ERROR is returned instead.  Different channel
drivers will return different types of handle.  Refer to the manual entries
for each driver to determine what type of handle is returned. 
Tcl_GetChannelThread
returns the id of the thread currently managing the specified channel. This
allows channel drivers to send their file events to the correct event queue
even for a multi-threaded core. 
Tcl_GetChannelMode returns an OR-ed combination
of TCL_READABLE and TCL_WRITABLE, indicating whether the channel is open
for input and output. 
Tcl_GetChannelBufferSize returns the size, in bytes,
of buffers allocated to store input or output in channel. If the value was
not set by a previous call to Tcl_SetChannelBufferSize, described below,
then the default value of 4096 is returned. 
Tcl_SetChannelBufferSize sets
the size, in bytes, of buffers that will be allocated in subsequent operations
on the channel to store input or output. The size argument should be between
ten and one million, allowing buffers of ten bytes to one million bytes.
If size is outside this range, Tcl_SetChannelBufferSize sets the buffer
size to 4096. 
Tcl_NotifyChannel is called by a channel driver to indicate
to the generic layer that the events specified by mask have occurred on
the channel.  Channel drivers are responsible for invoking this function
whenever the channel handlers need to be called for the channel.  See WATCHPROC
below for more details. 
Tcl_BadChannelOption is called from driver specific
set or get option procs to generate a complete error message. 
Tcl_ChannelBuffered
returns the number of bytes of input currently buffered in the internal
buffer (push back area) of the channel itself. It does not report about
the data in the overall buffers for the stack of channels the supplied
channel is part of. 
Tcl_IsChannelShared checks the refcount of the specified
channel and returns whether the channel was shared among multiple interpreters
(result == 1) or not (result == 0). 
Tcl_IsChannelRegistered checks whether
the specified channel is registered in the given interpreter (result ==
1) or not (result == 0). 
Tcl_IsChannelExisting checks whether a channel
with the specified name is registered in the (thread)-global list of all
channels (result == 1) or not (result == 0). 
Tcl_CutChannel removes the
specified channel from the (thread)global list of all channels (of the
current thread). Application to a channel still registered in some interpreter
is not allowed. Also notifies the driver if the Tcl_ChannelType version
is TCL_CHANNEL_VERSION_4 (or higher), and Tcl_DriverThreadActionProc is
defined for it. 
Tcl_SpliceChannel adds the specified channel to the (thread)global
list of all channels (of the current thread). Application to a channel registered
in some interpreter is not allowed. Also notifies the driver if the Tcl_ChannelType
version is TCL_CHANNEL_VERSION_4 (or higher), and Tcl_DriverThreadActionProc
is defined for it. 
Tcl_ClearChannelHandlers removes all channelhandlers
and event scripts associated with the specified channel, thus shutting
down all event processing for this channel.  
Tcl_channeltypeA channel driver
provides a Tcl_ChannelType structure that contains pointers to functions
that implement the various operations on a channel; these operations are
invoked as needed by the generic layer.  The structure was versioned starting
in Tcl 8.3.2/8.4 to correct a problem with stacked channel drivers.  See the
OLD CHANNEL TYPES section below for details about the old structure. 
The
Tcl_ChannelType structure contains the following fields: 
typedef struct Tcl_ChannelType {
    char *typeName;
    Tcl_ChannelTypeVersion version;
    Tcl_DriverCloseProc *closeProc;
    Tcl_DriverInputProc *inputProc;
    Tcl_DriverOutputProc *outputProc;
    Tcl_DriverSeekProc *seekProc;
    Tcl_DriverSetOptionProc *setOptionProc;
    Tcl_DriverGetOptionProc *getOptionProc;
    Tcl_DriverWatchProc *watchProc;
    Tcl_DriverGetHandleProc *getHandleProc;
    Tcl_DriverClose2Proc *close2Proc;
    Tcl_DriverBlockModeProc *blockModeProc;    
    Tcl_DriverFlushProc *flushProc;    
    Tcl_DriverHandlerProc *handlerProc;    
    Tcl_DriverWideSeekProc *wideSeekProc;
    Tcl_DriverThreadActionProc *threadActionProc;
} Tcl_ChannelType;

It is not necessary to provide implementations for all channel operations.
 Those which are not necessary may be set to NULL in the struct: blockModeProc,
seekProc, setOptionProc, getOptionProc, and close2Proc, in addition to
flushProc, handlerProc, and threadActionProc.  Other functions that cannot
be implemented in a meaningful way should return EINVAL when called, to
indicate that the operations they represent are not available. Also note
that wideSeekProc can be NULL if seekProc is. 
The user should only use the
above structure for Tcl_ChannelType instantiation.  When referencing fields
in a Tcl_ChannelType structure, the following functions should be used
to obtain the values: Tcl_ChannelName, Tcl_ChannelVersion, Tcl_ChannelBlockModeProc,
Tcl_ChannelCloseProc, Tcl_ChannelClose2Proc, Tcl_ChannelInputProc, Tcl_ChannelOutputProc,
Tcl_ChannelSeekProc, Tcl_ChannelWideSeekProc, Tcl_ChannelThreadActionProc,
Tcl_ChannelSetOptionProc, Tcl_ChannelGetOptionProc, Tcl_ChannelWatchProc,
Tcl_ChannelGetHandleProc, Tcl_ChannelFlushProc, or Tcl_ChannelHandlerProc.

The change to the structures was made in such a way that standard channel
types are binary compatible.  However, channel types that use stacked channels
(ie: TLS, Trf) have new versions to correspond to the above change since
the previous code for stacked channels had problems.  
TypenameThe typeName
field contains a null-terminated string that identifies the type of the
device implemented by this driver, e.g. file or socket. 
This value can be
retrieved with Tcl_ChannelName, which returns a pointer to the string. 

Version The version field should be set to the version of the structure
that you require. TCL_CHANNEL_VERSION_2 is the minimum recommended. TCL_CHANNEL_VERSION_3
must be set to specifiy the wideSeekProc member. TCL_CHANNEL_VERSION_4 must
be set to specifiy the threadActionProc member (includes wideSeekProc).
If it is not set to any of these, then this Tcl_ChannelType is assumed
to have the original structure.  See OLD CHANNEL TYPES for more details.
 While Tcl will recognize and function with either structures, stacked
channels must be of at least TCL_CHANNEL_VERSION_2 to function correctly.

This value can be retrieved with Tcl_ChannelVersion, which returns one
of TCL_CHANNEL_VERSION_4, TCL_CHANNEL_VERSION_3, TCL_CHANNEL_VERSION_2,
or TCL_CHANNEL_VERSION_1.  
BlockmodeprocThe blockModeProc field contains
the address of a function called by the generic layer to set blocking and
nonblocking mode on the device. BlockModeProc should match the following
prototype: 

typedef int Tcl_DriverBlockModeProc(
    ClientData instanceData,
    int mode);

The instanceData is the same as the value passed to Tcl_CreateChannel when
this channel was created.  The mode argument is either TCL_MODE_BLOCKING
or TCL_MODE_NONBLOCKING to set the device into blocking or nonblocking
mode. The function should return zero if the operation was successful, or
a nonzero POSIX error code if the operation failed. 
If the operation is
successful, the function can modify the supplied instanceData to record
that the channel entered blocking or nonblocking mode and to implement
the blocking or nonblocking behavior. For some device types, the blocking
and nonblocking behavior can be implemented by the underlying operating
system; for other device types, the behavior must be emulated in the channel
driver. 
This value can be retrieved with Tcl_ChannelBlockModeProc, which
returns a pointer to the function. 
A channel driver not supplying a blockModeProc
has to be very, very careful. It has to tell the generic layer exactly which
blocking mode is acceptable to it, and should this also document for the
user so that the blocking mode of the channel is not changed to an inacceptable
value. Any confusion here may lead the interpreter into a (spurious and
difficult to find) deadlock.   
Closeproc and Close2procThe closeProc field
contains the address of a function called by the generic layer to clean
up driver-related information when the channel is closed. CloseProc must
match the following prototype: 

typedef int Tcl_DriverCloseProc(
    ClientData instanceData,
    Tcl_Interp *interp);

The instanceData argument is the same as the value provided to Tcl_CreateChannel
when the channel was created. The function should release any storage maintained
by the channel driver for this channel, and close the input and output
devices encapsulated by this channel. All queued output will have been flushed
to the device before this function is called, and no further driver operations
will be invoked on this instance after calling the closeProc. If the close
operation is successful, the procedure should return zero; otherwise it
should return a nonzero POSIX error code. In addition, if an error occurs
and interp is not NULL, the procedure should store an error message in
the interpreter’s result. 
Alternatively, channels that support closing the
read and write sides independently may set closeProc to TCL_CLOSE2PROC
and set close2Proc to the address of a function that matches the following
prototype: 

typedef int Tcl_DriverClose2Proc(
    ClientData instanceData,
    Tcl_Interp *interp,
    int flags);

The close2Proc will be called with flags set to an OR’ed combination of
TCL_CLOSE_READ or TCL_CLOSE_WRITE to indicate that the driver should close
the read and/or write side of the channel.  The channel driver may be invoked
to perform additional operations on the channel after close2Proc is called
to close one or both sides of the channel.  If flags is 0 (zero), the driver
should close the channel in the manner described above for closeProc.  No
further operations will be invoked on this instance after close2Proc is
called with all flags cleared.  In all cases, the close2Proc function should
return zero if the close operation was successful; otherwise it should
return a nonzero POSIX error code. In addition, if an error occurs and interp
is not NULL, the procedure should store an error message in the interpreter’s
result. 
These value can be retrieved with Tcl_ChannelCloseProc or Tcl_ChannelClose2Proc,
which returns a pointer to the respective function.  
InputprocThe inputProc
field contains the address of a function called by the generic layer to
read data from the file or device and store it in an internal buffer. InputProc
must match the following prototype: 

typedef int Tcl_DriverInputProc(
    ClientData instanceData,
    char *buf,
    int bufSize,
    int *errorCodePtr);

InstanceData is the same as the value passed to Tcl_CreateChannel when
the channel was created.  The buf argument points to an array of bytes in
which to store input from the device, and the bufSize argument indicates
how many bytes are available at buf. 
The errorCodePtr argument points to
an integer variable provided by the generic layer. If an error occurs, the
function should set the variable to a POSIX error code that identifies
the error that occurred. 
The function should read data from the input device
encapsulated by the channel and store it at buf.  On success, the function
should return a nonnegative integer indicating how many bytes were read
from the input device and stored at buf. On error, the function should return
-1. If an error occurs after some data has been read from the device, that
data is lost. 
If inputProc can determine that the input device has some
data available but less than requested by the bufSize argument, the function
should only attempt to read as much data as is available and return without
blocking. If the input device has no data available whatsoever and the channel
is in nonblocking mode, the function should return an EAGAIN error. If the
input device has no data available whatsoever and the channel is in blocking
mode, the function should block for the shortest possible time until at
least one byte of data can be read from the device; then, it should return
as much data as it can read without blocking. 
This value can be retrieved
with Tcl_ChannelInputProc, which returns a pointer to the function.  
OutputprocThe
outputProc field contains the address of a function called by the generic
layer to transfer data from an internal buffer to the output device. OutputProc
must match the following prototype: 

typedef int Tcl_DriverOutputProc(
    ClientData instanceData,
    CONST char *buf,
    int toWrite,
    int *errorCodePtr);

InstanceData is the same as the value passed to Tcl_CreateChannel when
the channel was created. The buf argument contains an array of bytes to
be written to the device, and the toWrite argument indicates how many bytes
are to be written from the buf argument. 
The errorCodePtr argument points
to an integer variable provided by the generic layer. If an error occurs,
the function should set this variable to a POSIX error code that identifies
the error. 
The function should write the data at buf to the output device
encapsulated by the channel. On success, the function should return a nonnegative
integer indicating how many bytes were written to the output device.  The
return value is normally the same as toWrite, but may be less in some cases
such as if the output operation is interrupted by a signal. If an error
occurs the function should return -1.  In case of error, some data may have
been written to the device. 
If the channel is nonblocking and the output
device is unable to absorb any data whatsoever, the function should return
-1 with an EAGAIN error without writing any data. 
This value can be retrieved
with Tcl_ChannelOutputProc, which returns a pointer to the function.  
Seekproc
and WideseekprocThe seekProc field contains the address of a function called
by the generic layer to move the access point at which subsequent input
or output operations will be applied. SeekProc must match the following
prototype: 

typedef int Tcl_DriverSeekProc(
    ClientData instanceData,
    long offset,
    int seekMode,
    int *errorCodePtr);

The instanceData argument is the same as the value given to Tcl_CreateChannel
when this channel was created.  Offset and seekMode have the same meaning
as for the Tcl_Seek procedure (described in the manual entry for Tcl_OpenFileChannel).

The errorCodePtr argument points to an integer variable provided by the
generic layer for returning errno values from the function.  The function
should set this variable to a POSIX error code if an error occurs. The function
should store an EINVAL error code if the channel type does not implement
seeking. 
The return value is the new access point or -1 in case of error.
If an error occurred, the function should not move the access point. 
If
there is a non-NULL seekProc field, the wideSeekProc field may contain the
address of an alternative function to use which handles wide (i.e. larger
than 32-bit) offsets, so allowing seeks within files larger than 2GB.  The
wideSeekProc will be called in preference to the seekProc, but both must
be defined if the wideSeekProc is defined.  WideSeekProc must match the
following prototype: 

typedef Tcl_WideInt Tcl_DriverWideSeekProc(
    ClientData instanceData,
    Tcl_WideInt offset,
    int seekMode,
    int *errorCodePtr);

The arguments and return values mean the same thing as with seekProc above,
except that the type of offsets and the return type are different. 
The seekProc
value can be retrieved with Tcl_ChannelSeekProc, which returns a pointer
to the function, and similarly the wideSeekProc can be retrieved with Tcl_ChannelWideSeekProc.
 
SetoptionprocThe setOptionProc field contains the address of a function
called by the generic layer to set a channel type specific option on a
channel. setOptionProc must match the following prototype: 

typedef int Tcl_DriverSetOptionProc(
    ClientData instanceData,
    Tcl_Interp *interp,
    CONST char *optionName,
    CONST char *newValue);

optionName is the name of an option to set, and newValue is the new value
for that option, as a string. The instanceData is the same as the value
given to Tcl_CreateChannel when this channel was created. The function should
do whatever channel type specific action is required to implement the new
value of the option. 
Some options are handled by the generic code and this
function is never called to set them, e.g. -blockmode. Other options are specific
to each channel type and the setOptionProc procedure of the channel driver
will get called to implement them. The setOptionProc field can be NULL,
which indicates that this channel type supports no type specific options.
 
If the option value is successfully modified to the new value, the function
returns TCL_OK. It should call Tcl_BadChannelOption which itself returns
TCL_ERROR if the optionName is unrecognized.  If newValue specifies a value
for the option that is not supported or if a system call error occurs,
the function should leave an error message in the result field of interp
if interp is not NULL. The function should also call Tcl_SetErrno to store
an appropriate POSIX error code. 
This value can be retrieved with Tcl_ChannelSetOptionProc,
which returns a pointer to the function.  
GetoptionprocThe getOptionProc
field contains the address of a function called by the generic layer to
get the value of a channel type specific option on a channel. getOptionProc
must match the following prototype: 

typedef int Tcl_DriverGetOptionProc(
    ClientData instanceData,
    Tcl_Interp *interp,
    CONST char *optionName,
    Tcl_DString *optionValue);

OptionName is the name of an option supported by this type of channel. If
the option name is not NULL, the function stores its current value, as
a string, in the Tcl dynamic string optionValue. If optionName is NULL,
the function stores in optionValue an alternating list of all supported
options and their current values. On success, the function returns TCL_OK.
 It should call Tcl_BadChannelOption which itself returns TCL_ERROR if
the optionName is unrecognized. If a system call error occurs, the function
should leave an error message in the result of interp if interp is not
NULL. The function should also call Tcl_SetErrno to store an appropriate
POSIX error code. 
Some options are handled by the generic code and this
function is never called to retrieve their value, e.g. -blockmode. Other options
are specific to each channel type and the getOptionProc procedure of the
channel driver will get called to implement them. The getOptionProc field
can be NULL, which indicates that this channel type supports no type specific
options. 
This value can be retrieved with Tcl_ChannelGetOptionProc, which
returns a pointer to the function.  
WatchprocThe watchProc field contains
the address of a function called by the generic layer to initialize the
event notification mechanism to notice events of interest on this channel.
WatchProc should match the following prototype: 

typedef void Tcl_DriverWatchProc(
    ClientData instanceData,
    int mask);

The instanceData is the same as the value passed to Tcl_CreateChannel when
this channel was created. The mask argument is an OR-ed combination of TCL_READABLE,
TCL_WRITABLE and TCL_EXCEPTION; it indicates events the caller is interested
in noticing on this channel. 
The function should initialize device type
specific mechanisms to notice when an event of interest is present on the
channel.  When one or more of the designated events occurs on the channel,
the channel driver is responsible for calling Tcl_NotifyChannel to inform
the generic channel module.  The driver should take care not to starve other
channel drivers or sources of callbacks by invoking Tcl_NotifyChannel too
frequently.  Fairness can be insured by using the Tcl event queue to allow
the channel event to be scheduled in sequence with other events.  See the
description of Tcl_QueueEvent for details on how to queue an event. 
This
value can be retrieved with Tcl_ChannelWatchProc, which returns a pointer
to the function.  
GethandleprocThe getHandleProc field contains the address
of a function called by the generic layer to retrieve a device-specific
handle from the channel. GetHandleProc should match the following prototype:


typedef int Tcl_DriverGetHandleProc(
    ClientData instanceData,
    int direction,
    ClientData *handlePtr);

InstanceData is the same as the value passed to Tcl_CreateChannel when
this channel was created. The direction argument is either TCL_READABLE
to retrieve the handle used for input, or TCL_WRITABLE to retrieve the
handle used for output. 
If the channel implementation has device-specific
handles, the function should retrieve the appropriate handle associated
with the channel, according the direction argument.  The handle should be
stored in the location referred to by handlePtr, and TCL_OK should be returned.
 If the channel is not open for the specified direction, or if the channel
implementation does not use device handles, the function should return
TCL_ERROR. 
This value can be retrieved with Tcl_ChannelGetHandleProc, which
returns a pointer to the function.  
FlushprocThe flushProc field is currently
reserved for future use. It should be set to NULL. FlushProc should match
the following prototype: 

typedef int Tcl_DriverFlushProc(
    ClientData instanceData);

This value can be retrieved with Tcl_ChannelFlushProc, which returns a
pointer to the function.  
HandlerprocThe handlerProc field contains the
address of a function called by the generic layer to notify the channel
that an event occurred.  It should be defined for stacked channel drivers
that wish to be notified of events that occur on the underlying (stacked)
channel. HandlerProc should match the following prototype: 

typedef int Tcl_DriverHandlerProc(
    ClientData instanceData,
    int interestMask);

InstanceData is the same as the value passed to Tcl_CreateChannel when
this channel was created.  The interestMask is an OR-ed combination of TCL_READABLE
or TCL_WRITABLE; it indicates what type of event occurred on this channel.

This value can be retrieved with Tcl_ChannelHandlerProc, which returns
a pointer to the function.  
ThreadactionprocThe threadActionProc field contains
the address of the function called by the generic layer when a channel
is created, closed, or going to move to a different thread, i.e. whenever
thread-specific driver state might have to initialized or updated. It can
be NULL. The action TCL_CHANNEL_THREAD_REMOVE is used to notify the driver
that it should update or remove any thread-specific data it might be maintaining
for the channel. 
The action TCL_CHANNEL_THREAD_INSERT is used to notify
the driver that it should update or initialize any thread-specific data
it might be maintaining using the calling thread as the associate. See Tcl_CutChannel
and Tcl_SpliceChannel for more detail. 

typedef void Tcl_DriverThreadActionProc(
    ClientData instanceData,
      int        action);

InstanceData is the same as the value passed to Tcl_CreateChannel when
this channel was created. 
These values can be retrieved with Tcl_ChannelThreadActionProc,
which returns a pointer to the function.  
Tcl_badchanneloptionThis procedure
generates a "bad option" error message in an (optional) interpreter.  It
is used by channel drivers when  a invalid Set/Get option is requested.
Its purpose is to concatenate the generic options list to the specific
ones and factorize the generic options error message string. 
It always return
TCL_ERROR 
An error message is generated in interp’s result object to indicate
that a command was invoked with the a bad option The message has the form

    bad option "blah": should be one of 
    <...generic options...>+<...specific options...>
so you get for instance:
    bad option "-blah": should be one of -blocking,
    -buffering, -buffersize, -eofchar, -translation,
    -peername, or -sockname
when called with optionList="peername sockname"
‘‘blah’’ is the optionName argument and ‘‘<specific options>’’ is a space separated
list of specific option words. The function takes good care of inserting
minus signs before each option, commas after, and an ‘‘or’’ before the last
option.  
Old Channel Types The original (8.3.1 and below) Tcl_ChannelType
structure contains the following fields: 
typedef struct Tcl_ChannelType {
    char *typeName;
    Tcl_DriverBlockModeProc *blockModeProc;    
    Tcl_DriverCloseProc *closeProc;
    Tcl_DriverInputProc *inputProc;
    Tcl_DriverOutputProc *outputProc;
    Tcl_DriverSeekProc *seekProc;
    Tcl_DriverSetOptionProc *setOptionProc;
    Tcl_DriverGetOptionProc *getOptionProc;
    Tcl_DriverWatchProc *watchProc;
    Tcl_DriverGetHandleProc *getHandleProc;
    Tcl_DriverClose2Proc *close2Proc;
} Tcl_ChannelType;

It is still possible to create channel with the above structure.  The internal
channel code will determine the version.  It is imperative to use the new
Tcl_ChannelType structure if you are creating a stacked channel driver,
due to problems with the earlier stacked channel implementation (in 8.2.0
to 8.3.1). 
Prior to 8.4.0 (i.e. during the later releases of 8.3 and early part
of the 8.4 development cycle) the Tcl_ChannelType structure contained the
following fields: 

typedef struct Tcl_ChannelType {
    char *typeName;
    Tcl_ChannelTypeVersion version;
    Tcl_DriverCloseProc *closeProc;
    Tcl_DriverInputProc *inputProc;
    Tcl_DriverOutputProc *outputProc;
    Tcl_DriverSeekProc *seekProc;
    Tcl_DriverSetOptionProc *setOptionProc;
    Tcl_DriverGetOptionProc *getOptionProc;
    Tcl_DriverWatchProc *watchProc;
    Tcl_DriverGetHandleProc *getHandleProc;
    Tcl_DriverClose2Proc *close2Proc;
    Tcl_DriverBlockModeProc *blockModeProc;    
    Tcl_DriverFlushProc *flushProc;    
    Tcl_DriverHandlerProc *handlerProc;    
} Tcl_ChannelType;

When the above structure is registered as a channel type, the version field
should always be TCL_CHANNEL_VERSION_2.  
See AlsoTcl_Close(3), Tcl_OpenFileChannel(3),
Tcl_SetErrno(3), Tcl_QueueEvent(3), Tcl_StackChannel(3), Tcl_GetStdChannel(3)
 Keywordsblocking, channel driver, channel registration, channel type,
nonblocking