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Tcl_CreateChannel(3)        Tcl Library Procedures        Tcl_CreateChannel(3)



______________________________________________________________________________________________________________________

NAME
       Tcl_CreateChannel,  Tcl_GetChannelInstanceData,  Tcl_GetChannelType,  Tcl_GetChannelName, Tcl_GetChannelHandle,
       Tcl_GetChannelMode, Tcl_GetChannelBufferSize,  Tcl_SetChannelBufferSize,  Tcl_NotifyChannel,  Tcl_BadChannelOp-
       tion,   Tcl_ChannelName,   Tcl_ChannelVersion,   Tcl_ChannelBlockModeProc,  Tcl_ChannelCloseProc,  Tcl_Channel-
       Close2Proc,   Tcl_ChannelInputProc,   Tcl_ChannelOutputProc,   Tcl_ChannelSeekProc,    Tcl_ChannelWideSeekProc,
       Tcl_ChannelTruncateProc, Tcl_ChannelSetOptionProc, Tcl_ChannelGetOptionProc, Tcl_ChannelWatchProc, Tcl_Channel-
       GetHandleProc, Tcl_ChannelFlushProc, Tcl_ChannelHandlerProc, Tcl_ChannelThreadActionProc,  Tcl_IsChannelShared,
       Tcl_IsChannelRegistered,  Tcl_CutChannel,  Tcl_SpliceChannel,  Tcl_IsChannelExisting, Tcl_ClearChannelHandlers,
       Tcl_GetChannelThread, Tcl_ChannelBuffered - procedures for creating and manipulating channels

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_DriverTruncateProc *                                                                                        |
       Tcl_ChannelTruncateProc(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)


ARGUMENTS
       const Tcl_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.

______________________________________________________________________________________________________________________


DESCRIPTION
       Tcl  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 struc-
       ture containing pointers to functions implementing the various operations used by the generic layer to communi-
       cate 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  describ-
       ing 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  initial-
       ized  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 argu-
       ment. 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 chan-
       nel 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 setOptionProc or getOptionProc 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_CHANNELTYPE
       A 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_DriverTruncateProc *truncateProc;                                                            |
              } 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  addi-
       tion to flushProc, handlerProc, threadActionProc, and truncateProc.  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_ChannelInput-
       Proc,   Tcl_ChannelOutputProc,   Tcl_ChannelSeekProc,   Tcl_ChannelWideSeekProc,   Tcl_ChannelThreadActionProc, |
       Tcl_ChannelTruncateProc, Tcl_ChannelSetOptionProc, Tcl_ChannelGetOptionProc, Tcl_ChannelWatchProc, Tcl_Channel-
       GetHandleProc, Tcl_ChannelFlushProc, or Tcl_ChannelHandlerProc.

       The change to the structures was made in such a way that standard channel types are  binary  compatible.   How-
       ever,  channel  types  that  use  stacked channels (i.e. TLS, Trf) have new versions to correspond to the above
       change since the previous code for stacked channels had problems.

   TYPENAME
       The 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_VER-
       SION_4 must be set to specifiy the threadActionProc member (includes wideSeekProc).  TCL_CHANNEL_VERSION_5 must |
       be set to specifiy the truncateProc members (includes wideSeekProc and threadActionProc).  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_5, TCL_CHAN- |
       NEL_VERSION_4, TCL_CHANNEL_VERSION_3, TCL_CHANNEL_VERSION_2 or TCL_CHANNEL_VERSION_1.

   BLOCKMODEPROC
       The blockModeProc field contains the address of a function called by the generic layer to set blocking and non-
       blocking 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  block-
       ing  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 CLOSE2PROC
       The 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; oth-
       erwise 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.

       The  closeProc and close2Proc values can be retrieved with Tcl_ChannelCloseProc or Tcl_ChannelClose2Proc, which
       return a pointer to the respective function.

   INPUTPROC
       The 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 argu-
       ment 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  suc-
       cess,  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.

   OUTPUTPROC
       The 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 argu-
       ment 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 WIDESEEKPROC
       The  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  val-
       ues  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 func-
       tion 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 sim-
       ilarly the wideSeekProc can be retrieved with Tcl_ChannelWideSeekProc.

   SETOPTIONPROC
       The 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.

   GETOPTIONPROC
       The 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 chan-
       nel type supports no type specific options.

       This value can be retrieved with Tcl_ChannelGetOptionProc, which returns a pointer to the function.

   WATCHPROC
       The 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 pro-
       totype:

              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 respon-
       sible 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.

   GETHANDLEPROC
       The  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  direc-
       tion, 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.

   FLUSHPROC
       The  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.

   HANDLERPROC
       The 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 interest-
       Mask  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.


   THREADACTIONPROC
       The 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.

   TRUNCATEPROC
       The truncateProc field contains the address of the function called by the generic layer when a channel is trun-
       cated to some length. It can be NULL.

              typedef int Tcl_DriverTruncateProc(
                      ClientData instanceData,
                      Tcl_WideInt length);

       InstanceData is the same as the value passed to Tcl_CreateChannel when this channel was created, and length  is
       the  new  length  of the underlying file, which should not be negative. The result should be 0 on success or an
       errno code (suitable for use with Tcl_SetErrno) on failure.

       These values can be retrieved with Tcl_ChannelTruncateProc, which returns a pointer to the function.

TCL_BADCHANNELOPTION
       This procedure generates a "bad option" error message in an (optional) interpreter.   It  is  used  by  channel
       drivers  when an 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 returns TCL_ERROR

       An error message is generated in interp's result object to indicate that a  command  was  invoked  with  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 equal to "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_DriverTruncateProc *truncateProc;
              } Tcl_ChannelType;

       When the above structure is registered as a channel type, the version field should always  be  TCL_CHANNEL_VER-
       SION_2.


SEE ALSO
       Tcl_Close(3),  Tcl_OpenFileChannel(3), Tcl_SetErrno(3), Tcl_QueueEvent(3), Tcl_StackChannel(3), Tcl_GetStdChan-
       nel(3)


KEYWORDS
       blocking, channel driver, channel registration, channel type, nonblocking



Tcl                                   8.4                 Tcl_CreateChannel(3)