Standard Channels(3) Tcl Library Procedures Standard Channels(3)
Tcl_StandardChannels - How the Tcl library deals with the standard
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 applica‐
tion. 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 stan‐
dard 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 con‐
sider 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
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 immedi‐
ately closed. This means that the first three Tcl channels then
opened by the application are designated as the Tcl standard
3) All uninitialized standard channels are initialized to platform-
specific default values when a user-requested channel is regis‐
tered 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
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
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)
Tcl 7.5 Standard Channels(3)