[Arjen Markus] Part three of the tutorial [How to make a Tcl application]

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'''8. Platform issues'''

Tcl applications are highly portable in general:

   * Tcl itself deals gracefully with such trivial but annoying matters
as line-endings (the two characters carriage, return line feed versus line feed versus ... for instance)
   * The Tcl commands that deal with the operating system hide almost all platform-specific issues - file names and directory names for instance via the [[file]] command
   * the Tk package works on all supported platforms in much the same way - you need not to be concerned about platform-specific issues like window decorations most of the time

But despite of this, there are a few aspects that you do need to consider,
when your application or library is to run on several platforms - and believe
me: they will. This chapter is meant to introduce them and the means to
solve the problems they cause.

''8.1 The tcl_platform array''

Tcl collects all information about the platform in a global array, called
tcl_platform. Here is a typical listing of its contents:

 % parray tcl_platform
 tcl_platform(byteOrder) = littleEndian
 tcl_platform(machine)   = intel
 tcl_platform(os)        = Windows 95
 tcl_platform(osVersion) = 4.0
 tcl_platform(platform)  = windows
 tcl_platform(user)      = arjen
 tcl_platform(wordSize)  = 4

(Note the use of the array printing command ''parray'' - very useful in
an interactive shell)

The most useful of these for distinguishing between platforms is
tcl_platform(platform): it gives you a standard characterisation
of the OS. Typical values: windows, unix and macos.

''Tip:''

Put everything that is platform-specific in a central place - do not
scatter checks on the type of platform all over your code, as that
makes it difficult to maintain it. Probably the best approach is
to use one or two source files with platform-specific procedures whose
interface is essentially platform-'''in'''dependent.

Remember that in Tcl it is quite possible to use constructions like:

   if { $tcl_platform(platform) == "windows" } {
      proc getColours {} {
         ...
      }
   }

   if { $tcl_platform(platform) == "unix" } {
      proc getColours {} {
         ...
      }
   }

   if { $tcl_platform(platform) == "macos" } {
      proc getColours {} {
         ...
      }
   }

This way there is no need to do any run-time checking.

''8.2 File and directory names''

''8.3 Fonts and colours''

''8.4 Interacting with the desktop''
                                      
''8.5 Dealing with numbers''

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[Category Tutorial]
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