refchan

refchan(3tcl)                 Tcl Built-In Commands                refchan(3tcl)



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NAME
       refchan - command handler API of reflected channels

SYNOPSIS
       cmdPrefix option ?arg arg ...?
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DESCRIPTION
       The Tcl-level handler for a reflected channel has to be a command with
       subcommands (termed an ensemble, as it is a command such as that created
       by namespace ensemble create, though the implementation of handlers for
       reflected channel is not tied to namespace ensembles in any way; see
       EXAMPLE below for how to build an oo::class that supports the API). Note
       that cmdPrefix is whatever was specified in the call to chan create, and
       may consist of multiple arguments; this will be expanded to multiple
       words in place of the prefix.

       Of all the possible subcommands, the handler must support initialize,
       finalize, and watch. Support for the other subcommands is optional.

   MANDATORY SUBCOMMANDS
       cmdPrefix initialize channelId mode
              An invocation of this subcommand will be the first call the
              cmdPrefix will receive for the specified new channelId. It is the
              responsibility of this subcommand to set up any internal data
              structures required to keep track of the channel and its state.

              The return value of the method has to be a list containing the
              names of all subcommands supported by the cmdPrefix. This also
              tells the Tcl core which version of the API for reflected channels
              is used by this command handler.

              Any error thrown by the method will abort the creation of the
              channel and no channel will be created. The thrown error will
              appear as error thrown by chan create. Any exception other than an
              error (e.g., break, etc.) is treated as (and converted to) an
              error.

              Note: If the creation of the channel was aborted due to failures
              here, then the finalize subcommand will not be called.

              The mode argument tells the handler whether the channel was opened
              for reading, writing, or both. It is a list containing any of the
              strings read or write. The list will always contain at least one
              element.

              The subcommand must throw an error if the chosen mode is not
              supported by the cmdPrefix.

       cmdPrefix finalize channelId
              An invocation of this subcommand will be the last call the
              cmdPrefix will receive for the specified channelId. It will be
              generated just before the destruction of the data structures of
              the channel held by the Tcl core. The command handler must not
              access the channelId anymore in no way. Upon this subcommand being
              called, any internal resources allocated to this channel must be
              cleaned up.

              The return value of this subcommand is ignored.

              If the subcommand throws an error the command which caused its
              invocation (usually chan close) will appear to have thrown this
              error. Any exception beyond error (e.g., break, etc.) is treated
              as (and converted to) an error.

              This subcommand is not invoked if the creation of the channel was
              aborted during initialize (See above).

       cmdPrefix watch channelId eventspec
              This subcommand notifies the cmdPrefix that the specified
              channelId is interested in the events listed in the eventspec.
              This argument is a list containing any of read and write. The list
              may be empty, which signals that the channel does not wish to be
              notified of any events. In that situation, the handler should
              disable event generation completely.

              Warning: Any return value of the subcommand is ignored. This
              includes all errors thrown by the subcommand, break, continue, and
              custom return codes.

              This subcommand interacts with chan postevent. Trying to post an
              event which was not listed in the last call to watch will cause
              chan postevent to throw an error.

   OPTIONAL SUBCOMMANDS
       cmdPrefix read channelId count
              This optional subcommand is called when the user requests data
              from the channel channelId. count specifies how many bytes have
              been requested. If the subcommand is not supported then it is not
              possible to read from the channel handled by the command.

              The return value of this subcommand is taken as the requested data
              bytes. If the returned data contains more bytes than requested, an
              error will be signaled and later thrown by the command which
              performed the read (usually gets or read). However, returning
              fewer bytes than requested is acceptable.

              Note that returning nothing (0 bytes) is a signal to the higher
              layers that EOF has been reached on the channel. To signal that
              the channel is out of data right now, but has not yet reached EOF,
              it is necessary to throw the error "EAGAIN", i.e. to either

                     return -code error EAGAIN
              or
                     error EAGAIN

              For extensibility any error whose value is a negative integer
              number will cause the higher layers to set the C-level variable
              "errno" to the absolute value of this number, signaling a system
              error.  However, note that the exact mapping between these error
              numbers and their meanings is operating system dependent.

              For example, while on Linux both

                     return -code error -11
              and
                     error -11

              are equivalent to the examples above, using the more readable
              string "EAGAIN", this is not true for BSD, where the equivalent
              number is -35.

              The symbolic string however is the same across systems, and
              internally translated to the correct number. No other error value
              has such a mapping to a symbolic string.

              If the subcommand throws any other error, the command which caused
              its invocation (usually gets, or read) will appear to have thrown
              this error. Any exception beyond error, (e.g., break, etc.) is
              treated as and converted to an error.

       cmdPrefix write channelId data
              This optional subcommand is called when the user writes data to
              the channel channelId. The data argument contains bytes, not
              characters. Any type of transformation (EOL, encoding) configured
              for the channel has already been applied at this point. If this
              subcommand is not supported then it is not possible to write to
              the channel handled by the command.

              The return value of the subcommand is taken as the number of bytes
              written by the channel. Anything non-numeric will cause an error
              to be signaled and later thrown by the command which performed the
              write. A negative value implies that the write failed. Returning a
              value greater than the number of bytes given to the handler, or
              zero, is forbidden and will cause the Tcl core to throw an error.

              To signal that the channel is not able to accept data for writing
              right now, it is necessary to throw the error "EAGAIN", i.e. to
              either

                     return -code error EAGAIN
              or
                     error EAGAIN

              For extensibility any error whose value is a negative integer
              number will cause the higher layers to set the C-level variable
              "errno" to the absolute value of this number, signaling a system
              error.  However, note that the exact mapping between these error
              numbers and their meanings is operating system dependent.

              For example, while on Linux both

                     return -code error -11
              and
                     error -11

              are equivalent to the examples above, using the more readable
              string "EAGAIN", this is not true for BSD, where the equivalent
              number is -35.

              The symbolic string however is the same across systems, and
              internally translated to the correct number. No other error value
              has such a mapping to a symbolic string.

              If the subcommand throws any other error the command which caused
              its invocation (usually puts) will appear to have thrown this
              error.  Any exception beyond error (e.g., break, etc.) is treated
              as and converted to an error.

       cmdPrefix seek channelId offset base
              This optional subcommand is responsible for the handling of chan
              seek and chan tell requests on the channel channelId. If it is not
              supported then seeking will not be possible for the channel.

              The base argument is the same as the equivalent argument of the
              builtin chan seek, namely:

              start     Seeking is relative to the beginning of the channel.

              current   Seeking is relative to the current seek position.

              end       Seeking is relative to the end of the channel.

              The offset is an integer number specifying the amount of bytes to
              seek forward or backward. A positive number should seek forward,
              and a negative number should seek backward.  A channel may provide
              only limited seeking. For example sockets can seek forward, but
              not backward.

              The return value of the subcommand is taken as the (new) location
              of the channel, counted from the start. This has to be an integer
              number greater than or equal to zero.  If the subcommand throws an
              error the command which caused its invocation (usually chan seek,
              or chan tell) will appear to have thrown this error. Any exception
              beyond error (e.g., break, etc.) is treated as and converted to an
              error.

              The offset/base combination of 0/current signals a chan tell
              request, i.e., seek nothing relative to the current location,
              making the new location identical to the current one, which is
              then returned.

       cmdPrefix configure channelId option value
              This optional subcommand is for setting the type-specific options
              of channel channelId. The option argument indicates the option to
              be written, and the value argument indicates the value to set the
              option to.

              This subcommand will never try to update more than one option at a
              time; that is behavior implemented in the Tcl channel core.

              The return value of the subcommand is ignored.

              If the subcommand throws an error the command which performed the
              (re)configuration or query (usually fconfigure or chan configure)
              will appear to have thrown this error. Any exception beyond error
              (e.g., break, etc.) is treated as and converted to an error.

       cmdPrefix cget channelId option
              This optional subcommand is used when reading a single type-
              specific option of channel channelId. If this subcommand is
              supported then the subcommand cgetall must be supported as well.

              The subcommand should return the value of the specified option.

              If the subcommand throws an error, the command which performed the
              (re)configuration or query (usually fconfigure or chan configure)
              will appear to have thrown this error. Any exception beyond error
              (e.g., break, etc.) is treated as and converted to an error.

       cmdPrefix cgetall channelId
              This optional subcommand is used for reading all type-specific
              options of channel channelId. If this subcommand is supported then
              the subcommand cget has to be supported as well.

              The subcommand should return a list of all options and their
              values.  This list must have an even number of elements.

              If the subcommand throws an error the command which performed the
              (re)configuration or query (usually fconfigure or chan configure)
              will appear to have thrown this error. Any exception beyond error
              (e.g., break, etc.) is treated as and converted to an error.

       cmdPrefix blocking channelId mode
              This optional subcommand handles changes to the blocking mode of
              the channel channelId. The mode is a boolean flag. A true value
              means that the channel has to be set to blocking, and a false
              value means that the channel should be non-blocking.

              The return value of the subcommand is ignored.

              If the subcommand throws an error the command which caused its
              invocation (usually fconfigure or chan configure) will appear to
              have thrown this error. Any exception beyond error (e.g., break,
              etc.) is treated as and converted to an error.

NOTES
       Some of the functions supported in channels defined in Tcl's C interface
       are not available to channels reflected to the Tcl level.

       The function Tcl_DriverGetHandleProc is not supported; i.e., reflected
       channels do not have OS specific handles.

       The function Tcl_DriverHandlerProc is not supported. This driver function
       is relevant only for stacked channels, i.e., transformations.  Reflected
       channels are always base channels, not transformations.

       The function Tcl_DriverFlushProc is not supported. This is because the
       current generic I/O layer of Tcl does not use this function anywhere at
       all. Therefore support at the Tcl level makes no sense either. This may
       be altered in the future (through extending the API defined here and
       changing its version number) should the function be used at some time in
       the future.

EXAMPLE
       This demonstrates how to make a channel that reads from a string.

              oo::class create stringchan {
                  variable data pos
                  constructor {string {encoding {}}} {
                      if {$encoding eq ""} {set encoding [encoding system]}
                      set data [encoding convertto $encoding $string]
                      set pos 0
                  }

                  method initialize {ch mode} {
                      return "initialize finalize watch read seek"
                  }
                  method finalize {ch} {
                      my destroy
                  }
                  method watch {ch events} {
                      # Must be present but we ignore it because we do not
                      # post any events
                  }

                  # Must be present on a readable channel
                  method read {ch count} {
                      set d [string range $data $pos [expr {$pos+$count-1}]]
                      incr pos [string length $d]
                      return $d
                  }

                  # This method is optional, but useful for the example below
                  method seek {ch offset base} {
                      switch $base {
                          start {
                              set pos $offset
                          }
                          current {
                              incr pos $offset
                          }
                          end {
                              set pos [string length $data]
                              incr pos $offset
                          }
                      }
                      if {$pos < 0} {
                          set pos 0
                      } elseif {$pos > [string length $data]} {
                          set pos [string length $data]
                      }
                      return $pos
                  }
              }

              # Now we create an instance...
              set string "The quick brown fox jumps over the lazy dog.\n"
              set ch [chan create read [stringchan new $string]]

              puts [gets $ch];   # Prints the whole string

              seek $ch -5 end;
              puts [read $ch];   # Prints just the last word

SEE ALSO
       chan(3tcl), transchan(3tcl)

KEYWORDS
       API, channel, ensemble, prefix, reflection



Tcl                                    8.5                         refchan(3tcl)