• 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

 % 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

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

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

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

• If you read the directory name from an external source, on Windows it might contain backslashes. This is a bit tricky: backslashes can turn into escape characters if you naively use strings containing backslashes as if they were lists:

   # read line from file: c:\myapp c:\mysecondapp c:\mydocs

   set firstdir [lindex $line 0]
   puts $firstdir
   ==> c:myapp

   (Note that the backslash is gone!)

• If you pass a full path like "c:/my documents/myfile.txt" as an argument (via exec) to an external program that does not understand "/". In that case you will need:

   file nativename "c:/my documents/myfile.txt"
   ==> c:\my documents\myfile.txt

• Sometimes with names containing spaces you will get into trouble because of the awkward quoting rules that are suddenly involved. This is a common situation on Windows. Use:

   file attributes "c:/my documents" -shortname
   ==> C:/MYDOCU~1

• Use, if at all possible, only the platform's default font or the font families Times, Helvetica and Courier. These are guaranteed to be present with any Tcl/Tk installation.
• Fonts on UNIX/Linux generally appear smaller than on Windows. Typically add about 20% (2 points) to the font size to get it right: so on UNIX/Linux a 12-points font looks the same size as a 10-points font on Windows.

• Beware of special characters, such as mathematical symbols: on some platforms, such as UNIX/Linux, Tcl/Tk will inspect all or almost all fonts available to see if any contain such special characters. This can take a long while!
• There is currently no way to detect from a script if a particular character can be drawn on the screen or not. This should not matter with the Latin letters, but if you need to use Chinese characters, for instance, things can be problematic.

• Platforms differ greatly in styles and colouring. If possible, use the defaults provided by Tk. This is especially true on Windows - Tk gets most things (regarding colours and border width and such) right by default.
• But on UNIX/Linux you may want to fiddle with the defaults: Tk's defaults generally make windows appear "gross". A good starting point is gtklook.tcl, here on the Wiki.
• Be careful to set both the background and the foreground colours if you want to display text: on some UNIX environments the default text colour is white or nearly white, so if you then set the background for the widget holding the text to white, it is very likely your users will have trouble reading it!

• Most computers nowadays use the same internal representation for floating-point data, so that the main difference is big-endianness and little-endianness. Use tcl_platform(byteOrder) if you are confronted with that problem, for instance, when dealing with binary data files.
• Tcl is very good at catching numerical errors:

   catch {
      set x 0.0
      set y [expr {1.0/$x}]
   } msg
   puts $msg

   ==> divide by zero

   The global variables errorInfo and errorCode hold more detailed information:

   % puts $errorInfo
   divide by zero
       while executing
   "expr {1.0/$x}"
   % puts $errorCode
   ARITH DIVZERO {divide by zero}

• One of the few things that are platform-dependent is the error handling by such functions as atan2(). If atan2() is called with two arguments 0, then this results in a domain error on some platforms and not on others. This is because Tcl depends on the underlying mathematical processing to do the "right thing". Formally, atan2(0,0) is not defined, so a domain error is reasonable, but it can be annoying nonetheless.
• Most computers will not complain if an integer overflow occurs, so here is a problem that might go unnoticed for some time. Usually, the problem is obvious because positive values all of a sudden become negative. If you need large integers, consider using the wide() function to force ordinary integers into 64-bits integers. This type is available even on platforms that do not natively support 64-bits integers!

   set x 1.1
   set y [expr $x*2.3]

   set x 1.1
   set y [expr {$x*2.3}]

• expr needs to parse the expression only once and can store the result for later reuse. This can not be done in the first case.
• Tcl will not convert the value of x to a string and substitute its result in the expression. This means that time is saved there as well and that the exact value is used.

   puts [time {set x 1.1; set y [expr $x*2.3]}   10000
   ===> 135 microseconds per iteration

   puts [time {set x 1.1; set y [expr {$x*2.3}]} 10000
   ===> 14 microseconds per iteration

   for { set i 0 } { $i < 100 } { incr i } {
      set x [expr {$i*0.01}]
      set y [expr {$x*100.0-$i}]
      puts "$x $y"
   }

   0.0 0.0
   0.01 0.0
   0.02 0.0
   0.03 0.0
   0.04 0.0
   0.05 0.0
   0.06 0.0
   0.07 8.881784197e-016
   0.08 0.0
   0.09 0.0
   0.1 0.0
   0.11 0.0
   0.12 0.0
   0.13 0.0
   0.14 1.7763568394e-015
   0.15 0.0
   ...
   0.27 0.0
   0.28 3.5527136788e-015
   0.29 -3.5527136788e-015
   0.3 0.0
   ...
   0.54 0.0
   0.55 7.1054273576e-015
   0.56 7.1054273576e-015
   0.57 7.1054273576e-015
   0.58 -7.1054273576e-015
   0.59 0.0
   ...
   0.97 0.0
   0.98 0.0
   0.99 0.0

   set x 1.0e30
   set y 1.0e-30
   set z 1.0e30

   puts [expr {$x+$y-$z}]
   ===> 0.0

   puts [expr {$x-$z+$y}]
   ===> 1e-030

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