standard channels(3)
NAME
Tcl_StandardChannels - How the Tcl library deals with the
standard channels
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.
APIs
The public API procedures dealing directly with standard
channels are Tcl_GetStdChannel and Tcl_SetStdChannel.
Additional public APIs to consider are Tcl_RegisterChan
nel, 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 plat
form- 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 ser
vice.)
- 1) A single standard channel is initialized when it is
- explicitly specified in a call to Tcl_SetStdChan
nel. The state 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_GetStdChan nel, 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 chan
nels, 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 userrequested channel is registered with Tcl_Register Channel.
- In case of unavailable platform-specific standard
channels the channel whose creation caused the ini
tialization of the Tcl standard channels is made a
normal channel. The next three Tcl channels opened
by the application are designated as the Tcl stan
dard channels. In other words, of the first four
Tcl channels opened by the application the second
to fourth are designated as the Tcl standard chan
nels.
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. - tclsh
The Tcl shell (or rather Tcl_Main) uses method 2 to ini tialize the standard channels. - wish
The windowing shell (or rather Tk_MainEx) 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 chan
nels.
SEE ALSO
Tcl_CreateChannel(3), Tcl_RegisterChannel(3), Tcl_GetChan
nel(3), Tcl_GetStdChannel(3), Tcl_SetStdChannel(3),
Tk_InitConsoleChannels(3), tclsh(1), wish(1), Tcl_Main(3),
Tk_MainEx(3)
KEYWORDS
- standard channels