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Standard Channels(3) Tcl Library Procedures Standard Channels(3)
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NAME
Tcl_StandardChannels - How the Tcl library deals with the standard channels
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DESCRIPTION
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.
APPLICATION PROGRAMMING INTERFACES
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.
INITIALIZATION OF TCL STANDARD CHANNELS
Standard channels are initialized by the Tcl library in three cases: when explicitly
requested, when implicitly required before returning channel information, or when implic-
itly 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.)
1) 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.
2) All uninitialized standard channels are initialized to platform-specific default
values:
(a) when open channels are listed with Tcl_GetChannelNames (or the file channels
script command), or
(b) 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.
3) 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.
RE-INITIALIZATION OF 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_Creat-
eChannel 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 initial-
ized before. It is this behavior which enables an application to employ method 1 of ini-
tialization, i.e. to create and designate their own Tcl standard channels.
SHELL-SPECIFIC DETAILS
tclsh
The Tcl shell (or rather the function Tcl_Main, which forms the core of the shell's imple-
mentation) uses method 2 to initialize the standard channels.
wish
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_InitConsoleChan-
nels) on non-Unix platforms. On Unix platforms, Tk_MainEx implicitly uses method 2 to
initialize the standard channels.
SEE ALSO
Tcl_CreateChannel(3), Tcl_RegisterChannel(3), Tcl_GetChannel(3), Tcl_GetStdChannel(3),
Tcl_SetStdChannel(3), Tk_InitConsoleChannels(3), tclsh(1), wish(1), Tcl_Main(3),
Tk_MainEx(3)
KEYWORDS
standard channels
Tcl 7.5 Standard Channels(3)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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