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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 subcom-
mands 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 chan-
nel 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 cmd-
Prefix.
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 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 com-
mand.
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). How-
ever, 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 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 chan-
nelId. 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 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 chan-
nelId. The option argument indicates the option to be written, and the value argu-
ment 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)configura-
tion 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 sup-
ported 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 chan-
nelId. 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)
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