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.)
Missing platform-specific standard channels do not matter here. This approach is not available at the script level.
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 Tcl_Main) uses method 2 to initialize the standard channels.
The windowing shell (or rather Tk_MainEx) 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.