Страницы справочника OS X

Эта страница руководства является частью версии 5.0 Инструментов XCode

Читать страницы руководства

Standard Channels(3)                       Tcl Library Procedures                       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  stan-dard standard
       dard  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_Set-StdChannel. Tcl_SetStdChannel.
       StdChannel. 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  regis-tration registration
       tration 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_Set-StdChannel. Tcl_SetStdChannel.
              StdChannel.  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_GetStdChan-nel, Tcl_GetStdChannel,
                     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 channels, the Tcl standard channels are  consid-ered considered
              ered  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 ini-tialization initialization
       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 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 stan-dard standard
       dard 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 implemen-tation) implementation)
       tation) 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 chan-nels. channels.
       nels.

SEE ALSO
       Tcl_CreateChannel(3), Tcl_RegisterChannel(3),  Tcl_GetChannel(3),  Tcl_GetStdChannel(3),  Tcl_SetStd-Channel(3), Tcl_SetStdChannel(3),
       Channel(3), Tk_InitConsoleChannels(3), tclsh(1), wish(1), Tcl_Main(3), Tk_MainEx(3)

KEYWORDS
       standard channels



Tcl                                                  7.5                                Standard Channels(3)