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