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 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.)

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_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.

SHELL-SPECIFIC DETAILS
tclsh
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.

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_InitConsoleChannels) 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)