Provided by: tcl9.0-doc_9.0.1+dfsg-1_all 
NAME
refchan - command handler API of reflected channels
SYNOPSIS
chan create mode cmdPrefix
cmdPrefix blocking channel mode
cmdPrefix cget channel option
cmdPrefix cgetall channel
cmdPrefix configure channel option value
cmdPrefix finalize channel
cmdPrefix initialize channel mode
cmdPrefix read channel count
cmdPrefix seek channel offset base
cmdPrefix watch channel eventspec
cmdPrefix write channel data
________________________________________________________________________________________________________________
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 channel mode
An invocation of this subcommand will be the first call the cmdPrefix will receive for the
specified new channel. 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 may be empty, but will usually
contain at least one element.
The subcommand must throw an error if the chosen mode is not supported by the cmdPrefix.
cmdPrefix finalize channel
An invocation of this subcommand will be the last call the cmdPrefix will receive for the
specified channel. 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 channel 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 channel eventspec
This subcommand notifies the cmdPrefix that the specified channel 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 channel count
This optional subcommand is called when the user requests data from the channel channel. 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 channel data
This optional subcommand is called when the user writes data to the channel channel. 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 channel offset base
This optional subcommand is responsible for the handling of chan seek and chan tell requests on
the channel channel. 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 channel option value
This optional subcommand is for setting the type-specific options of channel channel. 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 channel option
This optional subcommand is used when reading a single type-specific option of channel channel. 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 channel
This optional subcommand is used for reading all type-specific options of channel channel. 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 channel mode
This optional subcommand handles changes to the blocking mode of the channel channel. 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.
cmdPrefix truncate channel length
This optional subcommand handles changing the length of the underlying data stream for the channel
channel. Its length gets set to length.
If the subcommand throws an error the command which caused its invocation (usually chan truncate)
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)