- NAME
- Tcl_StandardChannels — How the Tcl library deals with the
standard channels
- DESCRIPTION
- APPLICATION
PROGRAMMING INTERFACES
- INITIALIZATION OF TCL STANDARD CHANNELS
- RE-INITIALIZATION OF TCL STANDARD CHANNELS
- SHELL-SPECIFIC DETAILS
- tclsh
- wish
- SEE
ALSO
- KEYWORDS
Tcl_StandardChannels — How the Tcl library deals with the standard
channels
This page explains the initialization and use of standard channels
in the Tcl library.
The term standard channels comes out of the Unix world
and refers to the three channels automatically opened by the OS for
each new application. They are stdin, stdout and stderr. The first is the
standard input an application can read from, the other two refer to
writable channels, one for regular output and the other for error
messages.
Tcl generalizes this concept in a cross-platform way and exposes
standard channels to the script level.
The public API procedures dealing directly with standard channels
are Tcl_GetStdChannel
and Tcl_SetStdChannel. Additional
public APIs to consider are Tcl_RegisterChannel,
Tcl_CreateChannel and
Tcl_GetChannel.
Standard channels are initialized by the Tcl library in three
cases: when explicitly requested, when implicitly required before
returning channel information, or when implicitly required during
registration of a new channel.
These cases differ in how they handle unavailable platform-
specific standard channels. (A channel is not “available” if it
could not be successfully opened; for example, in a Tcl application
run as a Windows NT service.)
- A single standard channel is initialized when it is
explicitly specified in a call to Tcl_SetStdChannel. The states of
the other standard channels are unaffected.
Missing platform-specific standard channels do not matter here.
This approach is not available at the script level.
- All uninitialized standard channels are initialized
to platform-specific default values:
- when open channels are listed with Tcl_GetChannelNames (or the
file channels script
command), or
- when information about any standard channel is
requested with a call to Tcl_GetStdChannel, or with a
call to Tcl_GetChannel which specifies
one of the standard names (stdin, stdout and stderr).
In case of missing platform-specific standard channels, the Tcl
standard channels are considered as initialized and then
immediately closed. This means that the first three Tcl channels
then opened by the application are designated as the Tcl standard
channels.
- All uninitialized standard channels are initialized
to platform-specific default values when a user-requested channel
is registered with Tcl_RegisterChannel.
In case of unavailable platform-specific standard channels the
channel whose creation caused the initialization of the Tcl
standard channels is made a normal channel. The next three Tcl
channels opened by the application are designated as the Tcl
standard channels. In other words, of the first four Tcl channels
opened by the application the second to fourth are designated as
the Tcl standard channels.
Once a Tcl standard channel is initialized through one of the
methods above, closing this Tcl standard channel will cause the
next call to Tcl_CreateChannel to make the
new channel the new standard channel, too. If more than one Tcl
standard channel was closed Tcl_CreateChannel will fill the
empty slots in the order stdin, stdout and stderr.
Tcl_CreateChannel
will not try to reinitialize an empty slot if that slot was not
initialized before. It is this behavior which enables an
application to employ method 1 of initialization, i.e. to create
and designate their own Tcl standard channels.
The Tcl shell (or rather the function Tcl_Main, which forms the core of
the shell's implementation) uses method 2 to initialize the
standard channels.
The windowing shell (or rather the function Tk_MainEx, which
forms the core of the shell's implementation) uses method 1 to
initialize the standard channels (See
Tk_InitConsoleChannels) on non-Unix platforms. On Unix
platforms, Tk_MainEx implicitly uses method 2 to initialize
the standard channels.
Tcl_CreateChannel,
Tcl_RegisterChannel,
Tcl_GetChannel,
Tcl_GetStdChannel,
Tcl_SetStdChannel,
Tk_InitConsoleChannels, tclsh, wish, Tcl_Main, Tk_MainEx
standard
channels
Copyright © 2001 ActiveState Corporation