refchan(n) Tcl Built-In Commands refchan(n)
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NAME
refchan - Command handler API of reflected channels, version 1
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). 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 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 argu-
ment 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 cus-
tom 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. This means that both
return -code error -11
and
error -11
are equivalent to the examples above, using the more readable string "EAGAIN". No other error value has such a mapping to a sym-
bolic 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 charac-
ters. Any type of transformation (EOL, encoding) configured for the channel has already been applied at this point. If this subcom-
mand 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 seek and tell requests on the channel channelId. If it is not supported
then seeking will not be possible for the channel.
The base argument is one of
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 base argument of the builtin chan seek command takes the same names.
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 seek, or 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 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) 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) 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) 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.
SEE ALSO
chan(n)
KEYWORDS
channel, reflection
Tcl 8.5 refchan(n)