Provided by: tcl8.6-doc_8.6.9+dfsg-2_all bug

NAME

       Tcl_CreateChannel,   Tcl_GetChannelInstanceData,  Tcl_GetChannelType,  Tcl_GetChannelName,
       Tcl_GetChannelHandle,            Tcl_GetChannelMode,             Tcl_GetChannelBufferSize,
       Tcl_SetChannelBufferSize,    Tcl_NotifyChannel,   Tcl_BadChannelOption,   Tcl_ChannelName,
       Tcl_ChannelVersion, Tcl_ChannelBlockModeProc, Tcl_ChannelCloseProc, Tcl_ChannelClose2Proc,
       Tcl_ChannelInputProc, Tcl_ChannelOutputProc, Tcl_ChannelSeekProc, Tcl_ChannelWideSeekProc,
       Tcl_ChannelTruncateProc,        Tcl_ChannelSetOptionProc,        Tcl_ChannelGetOptionProc,
       Tcl_ChannelWatchProc,            Tcl_ChannelGetHandleProc,           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)

       const 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. If the created
                                                        channel  is  assigned  to  one   of   the
                                                        standard   channels   (stdin,  stdout  or
                                                        stderr), the assigned channel  name  will
                                                        be the name of the standard channel.

       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 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  behavior  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 one and one million, allowing buffers of one byte 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  channel handlers 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 {
                      const 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, getHandleProc, and close2Proc,  in  addition  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_ChannelInputProc, Tcl_ChannelOutputProc,
       Tcl_ChannelSeekProc,         Tcl_ChannelWideSeekProc,         Tcl_ChannelThreadActionProc,
       Tcl_ChannelTruncateProc,        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 (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
       specify  the  wideSeekProc  member.   TCL_CHANNEL_VERSION_4  must  be  set  to specify the
       threadActionProc member (includes wideSeekProc).  TCL_CHANNEL_VERSION_5  must  be  set  to
       specify  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_CHANNEL_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 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
       unacceptable 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; 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.

       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 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.

   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 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 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  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.

   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 channel 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 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.

   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 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.

   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  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.

   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 truncated 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 value 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 {
                  const 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 {
                  const 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_VERSION_2.

SEE ALSO

       Tcl_Close(3tcl),   Tcl_OpenFileChannel(3tcl),   Tcl_SetErrno(3tcl),  Tcl_QueueEvent(3tcl),
       Tcl_StackChannel(3tcl), Tcl_GetStdChannel(3tcl)

KEYWORDS

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