http://purl.org/tcl/home/man/tcl8.4/TkCmd/messageBox.htm

Note: Unlike most Tk widgets, this widget's name (messageBox) is not the same as its creation command (tk_messageBox).  Even more confusing to someone is that the source for the tk/tcl version is located in msgbox.tcl .  
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[incr Widgets] also has a messageBox - once their DNS problems are resolved, a URL to their doc will be inserted in place of this part of the msg.

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With recent versions, the Windows messageBox is "native", while
the Unix one continues to rely on the implementation visible in
./library/msgbox.tcl.  Before version 8.something, both Win* and
Unix used msgbox.tcl (what about Mac?).  Notice that this means
that, if there's something about messageBox you don't like, you
have the source code readily available for your enhancements.

[[Detail example of how to do this.]]

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[[Give example of how to use [option database] with messageBox.]]

On Unix, the '''default font''' for message boxes is Times 18, which is way too fat for longer text. Before creating a messageBox, just add the line
 option add *Dialog.msg.font {Times 12} 
to get a better readable font (won't hurt in Windows, where native message boxes are used anyway). ([RS])

the wraplength also defaults to 3 inches but can be overriden the
same way for long messages ([BBH])
 option add *DIalog.msg.wrapLength 6i  
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[Martin Lemburg]
Because of the need to have a messagebox in the tclsh, I wrote a 
tclsh version ... tcl_messageBox.
To download the stubs enabled version (tcl v8.1 and higher) use the URL:
    ftp://ftp.dcade.de/pub/ml/tcl/packages/tcl_messageBox10.zip
    (package only)
    ftp://ftp.dcade.de/pub/ml/tcl/packages/tcl_messageBox10.all.zip
    (inclusive the source, project files, etc.)
The syntax is simular to the tk_messageBox:
   tcl_messageBox ?option optionArg option optionArg ...?
options:
   * -align: left or right
   * -default: depends on the given type
   * -icon: exclamation, warning, info(rmation), asterisk, question, stop, error or hand
   * -message: string, may be empty
   * -modal: app(lication), window, system or task
   * -title: string, may be empty
   * -type: abortretryignore, ok, okcancel, retrycancel, yesno or yesnocancel
   * -help: this text
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