Author: Jan Nijtmans <[email protected]>
State: Final
Type: Project
Vote: Done
Created: 17-Oct-2022
Tcl-Version: 8.7
Tcl-branch: tip-646
Vote-Summary: Accepted 3/0/0
Votes-For: JN, KBK, KW, SL
Votes-Against: none
Votes-Present: none
Abstract
This TIP is inspired by Bug #5bfe3de008,
suggesting to remove the -eofchar handling for writable channels. So,
assuming $chan is a writable channel,
fconfigure $chan -eofchar \\x1A
close $chan
, the eof character must now be explicitly written:
puts -nonewline $chan \\x1A
close $chan
In other words, the close command no longer writes any byte to the channel as part
of the close process. If you want any byte to be written, it should be done
explicitly just before closing the channel.
Also, this TIP suggests to change the default -eofchar for channels on Windows
in Tcl 9.0 to be the same as on UNIX: -eofchar {}.
The only feature -eofchar "\\x1A {}" is currently used for is for starpacks, where
a script file and some archive is glue'd together in a single file, using \\x1A as
separation. That functionality will be kept as-is in Tcl 9.0.
Background & Rationale
Tcl channels have the configuration option -eofchar {$inChar $outChar}. The current functionality
of inChar (for channels which are readable) is that - as soon as this character is encountered
in the input stream - it behaves as EOF. The current functionality of outChar is that - as soon
as the channel is closed - the character is written to the output stream. This last behavior is
problematic: We don't know where the cursor is when the file is closed, no check is done and no seek
to the end of the channel is done. For this reason, no extension or application is known using
this feature. Whenever -eofchar is set, it is usually either set to {} or to "\\x1A {}".
The default for channels on Windows is -eofchar "\\x1A {}", which is different from the default
on UNIX. This is causing problem with a lot of extensions, which need to set -eofchar {}
explicitly for channels, preventing that the character \\x1A somewhere in the input
stream causes the channel to be closed. The motivation for this was that old DOS files
sometimes have the \x1A character at the end, but this is highly unlikely now: Current
editors on Windows don't do that any more. Therefore, the default for channels on Windows
will be changed to -eofchar {} in Tcl 9.0.
Specification
For Tcl 9.0, remove the outChar behavior of -eofchar completely. Any attempt to
set the outChar to something else than the empty string will result in an error-message.
For Tcl 9.0, change the default -eofchar value on Windows to be the same as it is on UNIX.
For Tcl 8.7, we can do a little bit to reduce the chance of errors. Currently, setting a single character:
fconfigure $chan -eofchar \x1A
is equivalent to
fconfigure $chan -eofchar "\x1A \x1A"
For Tcl 8.7, this will be changed to be equivalent to:
fconfigure $chan -eofchar "\x1A {}"
This prevents the - most likely unintended - behavior that a \\x1A
character is written to the channel (if it is writable) whenever
it is closed.
Implementation
Implementation is available in the tip-646 branch of the Tcl repository.
Copyright
This document has been placed in the public domain.