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FAQ: comp.lang.tcl Tcl Language Usage Questions And Answers


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See reader questions & answers on this topic! - Help others by sharing your knowledge
                   TCL LANGUAGE USAGE QUESTIONS AND ANSWERS 
                  ------------------------------------------

INTRODUCTION

   This FAQ file is intended to address some of the most commonly asked
   questions regarding the Tcl programming language. It is hoped that by
   addressing these here in this document, that the postings to
   comp.lang.tcl will be of a less repetitive and more informative nature.
   It is also hoped that via this and other available resources, more and
   more people will be able to utilize Tcl and Tcl based applications.

   This FAQ is maintained by Joe Moss (joe@morton.rain.com) and is posted
   monthly to comp.lang.tcl and news.answers and can also be obtained from
   the Tcl archive at ftp.neosoft.com/languages/tcl (198.64.191.10) in the
   /languages/tcl/alcatel/docs directory or via the World Wide Web as
   http://route.psg.com/tcl.html.

   Note that this FAQ deals with Tcl, not with Tk. For Tk Toolkit Q&As see
   the FAQ posting by Thomas J. Accardo (tja@cpu.com) (see the question
   below, entitled "Where can I get further info about Tcl" for pointers to
   other Tcl FAQs). However, there is some inevitable overlap. Some of the
   most commonly asked questions about usage of the Tk widgets are due to
   misunderstandings of how the Tcl interpreter works. These issues may
   still be addressed here. Also, some examples of Tcl usage may involve
   the use of the Tk widgets.

   Questions and Answers about the use of non-Tk related extensions (such
   as Extended Tcl and Expect, but not TkX, expectk, BLT, etc.) are also
   appropriate here.

   The source of this document is maintained in HTML. It is then run
   through a script (written in Tcl, of course) to automagically generate
   the Index of Questions, number the questions, generate the URL list, and
   add the From and Subject lines to the text version. The version
   available via the WWW will be updated whenever there are changes, while
   the ASCII text version will be generated and posted once a month.

   Last modified: Sat Jun 01 16:34:01 PDT 1996
  ---------------------------------------------------------------------------

INDEX OF QUESTIONS:

SECTION A - GENERAL INFORMATION:
     * Q.A1- Where can I get further info about Tcl?
     * Q.A2- Where can I find help getting Tcl to work on my machine?
     * Q.A3- How can I combine Tcl and C?
     * Q.A4- How do I combine several extensions to provide all the
             features I want?
     * Q.A5- What about dynamic loading?
     * Q.A6- How do I create a stand alone program in case Tcl isn't
             installed?
     * Q.A7- How do I make my script executable regardless of the location
             of tclsh?
     * Q.A8- Why isn't <some extension> part of the standard distribution?

SECTION B - PROGRAMMING RELATED QUESTIONS AND ANSWERS:
     * Q.B1- How can I create/use association lists or property lists?
     * Q.B2- How can I generate a random number in Tcl?
     * Q.B3- How can I call one proc with the multi parameter value
             returned by another proc?
     * Q.B4- How can I pass an array into a proc?
     * Q.B5- How can I run an external command and read its output via a
             pipe?
     * Q.B6- How can I delete a procedure from within a script?
     * Q.B7- How can I get more than 7 digits of double precision?
     * Q.B8- How can I grab the command line when a non-built-in call is
             made?
     * Q.B9- How can I get or set an environment variable?
     * Q.B10- How can I use numbers with leading zeroes?
     * Q.B11- How can I find the command line arguments to my application?
     * Q.B12- I found a bug in the handling of comments!
     * Q.B13- How can I input and output binary data?
     * Q.B14- How can I trap signals, and perform other more Unix specific
              functions?
     * Q.B15- How can I do a double indirect? - Why doesn't $$var work?
     * Q.B16- How can I get quoted strings to work the way I want?
     * Q.B17- How can I share procedures between multiple tcl applications?
     * Q.B18- How can I get items inserted into a list?
     * Q.B19- How can I perform a non-blocking read on a file identifier?
     * Q.B20- How can I read and write from a pipe without locking up?
     * Q.B21- How can I find what version I am running?
     * Q.B22- How can I use variables to hold array names?
     * Q.B23- How can I make variables static, like in C?
     * Q.B24- How can I convert between a character and its ASCII value?

SECTION C - EXPLANATION OF ERROR MESSAGES:
     * Q.C1- "not found" or "Command not found"
     * Q.C2- invalid command name "}"
     * Q.C3- missing close-brace
     * Q.C4- X server insecure (must use xauth-style authorization)
     * Q.C5- expected integer but got ...
     * Q.C6- "Undefined symbol: main" or similar

SECTION D - COMPILING AND OTHER PLATFORM SPECIFIC QUESTIONS:
     * Q.D1- Why does "glob" cut off the first two characters of each
             filename?
     * Q.D2- How do I get Tcl working on MS Windows (3.1, 95, or NT)?
     * Q.D3- How can I build the Tcl library as a shared libary?
  ---------------------------------------------------------------------------

GENERAL INFORMATION:


------------------

From: Tcl Q&A
Subject: Q.A1- Where can I get further info about Tcl?

   The newsgroup comp.lang.tcl is for discussion of the Tcl language and
   packages based on it. There are several FAQ documents related to Tcl
   available from ftp.neosoft.com in the /languages/tcl/alcatel/docs
   directory that provide further information and pointers to yet more
   reading material, both on-line and in print. They can also be accessed
   via the World Wide Web: http://www.teraform.com/~lvirden/tcl-faq/

   Three other good starting points, if you are surfing the Web for
   information, are:
   http://www.sco.com/Technology/tcl/Tcl.html
   http://web.cs.ualberta.ca/~wade/HyperTcl/
   http://www.sunlabs.com/research/tcl/

   Of course, the source distribution itself includes a great deal of
   documentation. The latest version is 7.5. It can be obtained from:
   ftp://ftp.smli.com/pub/tcl/


------------------

From: Tcl Q&A
Subject: Q.A2- Where can I find help getting Tcl to work on my machine?

   Look in the Tcl distribution for the file called "porting.notes". This
   will contain a collection of notes that various people have provided
   about porting Tcl to various machines and operating systems. There is
   also a file called "README" which should be read *FIRST* - before doing
   anything else with the code (this should always be one's first step with
   any package).

   You should also check the on-line database of porting information
   available via the Sun Tcl/Tk Web pages.

   There are also some questions and answers related to building the
   interpreter on various platforms at the end of this document.


------------------

From: Tcl Q&A
Subject: Q.A3- How can I combine Tcl and C?

   Tcl was intended to be used along with C code. Because of this, there
   are several ways to combine the two. Here is a list of some of them
   (with man page references for further info):
     * Adding Commands to Tcl (Tcl_CreateCommand)
     * Command Traces (Tcl_CreateTrace)
     * Variable Traces (Tcl_TraceVar)
     * Getting/Setting Variables (Tcl_GetVar/Tcl_SetVar)
     * Linking Variables (Tcl_LinkVar)
     * Asynchronous Event Handlers (Tcl_AsyncCreate)
     * Adding Functions to "expr" Command (Tcl_CreateMathFunc)
     * Custom "tclAppInit" Routine (Tcl_AppInit)
     * Custom "main" Routine (Tcl_Main)
     * Via Pipes (open)
     * "exec" Command (exec)

   The last two do not provide nearly the same level of integration as
   those near the top of the list, but they are necessary if you do not
   have access to the source code.

   There is plenty of documentation available that explains this in further
   detail. See the "TclCommandWriting" man page, included with Extended Tcl
   or available via the WWW at http://psg.com/~joem/CmdWrite.html

   See also part two of Larry's Tcl FAQ and the Tcl Bibliography for other
   references.

   In addition to all the ways possible with standard Tcl, several
   extensions add others, notably via pseudo-ttys with expect, and via X
   event, file event, timer, and idle callback handlers with Tk. Also check
   out "Embedded Tk", available from ftp://ftp.vnet.net/pub/users/drh/


------------------

From: Tcl Q&A
Subject: Q.A4- How do I combine several extensions to provide all the
               features I want?

   The simple answer is to create a custom Tcl_AppInit function to call the
   Init procedures for each of the desired extensions. This can be done by
   copying the tclAppInit.c (tkAppInit.c for Tk apps) file from Tcl source
   distribution, modifying it, and then linking your Tcl_AppInit function
   with the Tcl library. For example, the standard Tcl_AppInit looks like
   this:

    int
    Tcl_AppInit(interp)
        Tcl_Interp *interp; /* Interpreter for application. */
    {
        /* ... */

        if (Tcl_Init(interp) == TCL_ERROR) {
            return TCL_ERROR;
        }

        /* ... */
    }

   In order to add the "foo" extension, you would add a call to the foo
   init function, like this:

    int
    Tcl_AppInit(interp)
        Tcl_Interp *interp; /* Interpreter for application. */
    {
        /* ... */

        if (Tcl_Init(interp) == TCL_ERROR) {
            return TCL_ERROR;
        }

        if (Foo_Init(interp) == TCL_ERROR) {
            return TCL_ERROR;
        }

        /* ... */
    }

   For more information, see the Tcl_AppInit man page and the tclAppInit.c
   file.

   The more complete answer is that the simple answer doesn't always work.
   Many extensions require more extensive modifications and the way to
   combine them depends on the specific extensions involved.

   If you wish to include Extended Tcl, you should start with the
   Tcl_AppInit function that comes with it. Also make sure when you link
   the program that you include the Extended Tcl library *before* the
   standard Tcl library, for example:

    cc -o mytclsh mytclXAppInit.c -ltclx -ltcl ...

   or for an interpreter including the Tk extensions:

    cc -o mywish mytkXAppInit.c -ltkx -ltk -ltclx -ltcl ...

   On the positive side, there are also several packages available that can
   aid in combining extensions, such as Tcl-my-fancy, or extensions that
   include configuration files for combining them with other popular
   extensions. For example, several extensions written by Sven Delmas allow
   you to specify options to configure to generate a Makefile that will
   build an interpreter containing the specified extensions. See part five
   of Larry Virden's FAQ for the location of these and other packages.


------------------

From: Tcl Q&A
Subject: Q.A5- What about dynamic loading?

   The current version of Tcl supports dynamic loading on many platforms.


------------------

From: Tcl Q&A
Subject: Q.A6- How do I create a stand alone program in case Tcl isn't
               installed?

   The "Plus" patches from Jan Nijtmans (nijtmans@nici.kun.nl) include a
   make target for building a standalone interpreter. See
   http://www.nici.kun.nl/tkpvm/pluspatch.html

   Additionally, Earle Lowe (lowee@cpsc.ucalgary.ca) has replied that:


      If you want to create a stand-alone program that will run anywhere
      regardless of the presence/absence of TCL/Tk, than you need to go
      through some more work.

      Basically, this involves converting the TCL init files to C strings,
      and then rather than calling Tcl_Init() and Tk_Init(), you call
      Tcl_Eval() with the converted C strings as arguments.

      I suggest getting the wish_compiler package by Alexei Rodriguez
      (alexei@cis.ufl.edu) from
      ftp://ftp.neosoft.com/languages/tcl/alcatel/code/
      z

      This package contains a tcl2c converter, and instructions as to its
      use.

      With some Makefile magic, you can use using TCL/Tk as intended (as an
      interpreted language using some version of wish) and when the code
      works, you can create a stand-alone compiled program.

   Another option that is available is "Embedded Tk". You can get it from
   ftp://ftp.vnet.net/pub/users/drh/


------------------

From: Tcl Q&A
Subject: Q.A7- How do I make my script executable regardless of the
               location of tclsh?

   There are several tricks you can use, but the most common is to make use
   of the fact that, in Tcl, the backslash character works as a line
   continuation indicator, even within comments. So instead of:

    #! /usr/local/bin/tclsh

    puts "Hello World"

   You could use:

    #! /bin/sh
    # The next line is executed by /bin/sh, but not Tcl \
      exec tclsh $0 ${1+"$@"}

    puts "Hello World"

   and the script will run regardless of where tclsh is located (as long as
   it is in one of the directories in the user's PATH).


------------------

From: Tcl Q&A
Subject: Q.A8- Why isn't <some extension> part of the standard
               distribution?

   Often people ask why some language extension that they use often (in
   fact, they may find it indispensible) isn't merged into the core
   language.

   It must be remembered that a lot of people use Tcl on widely differing
   systems and in vastly different ways. Also, Tcl was originally intended
   as an embedded language that would provide the minimal programming
   constructs and framework upon which a programmer could base the
   scripting language for their application.

   Having said that, the core Tcl language _has_ grown over the years. Some
   of the added features have come from extensions written by others. When
   the capabilities provided by extensions are deemed to be of sufficient
   value to all users of Tcl, John has added them to the base language.
   Some examples are associative arrays, the file I/O commands (and file
   handles), and the "unknown" procedure, all of which appeared in Extended
   Tcl before becoming part of standard Tcl. Tk 4.0 and later contain the
   equivalent functionality of the addinput and photo widget extensions
   that were written by others for Tcl 7.3/Tk 3.6.
  ---------------------------------------------------------------------------

PROGRAMMING RELATED QUESTIONS AND ANSWERS:


------------------

From: Tcl Q&A
Subject: Q.B1- How can I create/use association lists or property lists?

   Use arrays or Extended Tcl keyed lists.

   For example, if you did a:

    keylset ttyFields ttyName tty1a
    keylset ttyFields baudRate 57600
    keylset ttyFields parity strip

   And then an "echo $ttyFields", you'd get:

    {ttyName tty1a} {baudRate 57600} {parity strip}

   Or using arrays:

    set ttyFields(ttyName)  tty1a
    set ttyFields(baudRate) 57600
    set ttyFields(parity)   strip


------------------

From: Tcl Q&A
Subject: Q.B2- How can I generate a random number in Tcl?

   Extended Tcl includes a command named "random" that is an interface to
   the random number generator in the system's standard C library.

   For example, to generate a random number between 0 and 9 inclusive:

    set random_number [random 10]

   It also allows you to specify a seed value.

   To get a value you can use as a seed, you might try one or a combination
   of these (on Unix-like systems):

    [pid]
    [file atime /dev/kmem]
    [getclock]        (Extended Tcl only)

   Several pseudo-random number generator functions, written in straight
   Tcl, have been posted to comp.lang.tcl, see Larry's tcl-faq/part4 for a
   list.

   Here is one that is syntax compatible with the one in Extended Tcl, but
   implemented in standard Tcl. The constants are from Don Libes. Be aware
   that there is little checking for valid arguments.

    proc random {args} {
        global RNG_seed
    
        set max 259200
        set argcnt [llength $args]
        if { $argcnt < 1 || $argcnt > 2 } {
            error "wrong # args: random limit | seed ?seedval?"
        }
        if ![string compare [lindex $args 0] seed] {
            if { $argcnt == 2 } {
                set RNG_seed [lindex $args 1]
            } else {
                set RNG_seed [expr \
                    ([pid]+[file atime /dev/kmem])%$max]
            }
            return
        }
        if ![info exists RNG_seed] {
            set RNG_seed [expr ([pid]+[file atime /dev/kmem])%$max]
        }
        set RNG_seed [expr ($RNG_seed*7141+54773) % $max]
        return [expr int(double($RNG_seed)*[lindex $args 0]/$max)]
    }


------------------

From: Tcl Q&A
Subject: Q.B3- How can I call one proc with the multi parameter value
               returned by another proc?

   Assuming y requires multiple args and x returns multiple words, use
   Tcl's eval command:

    eval y [x]


------------------

From: Tcl Q&A
Subject: Q.B4- How can I pass an array into a proc?

   Use upvar rather than try to use global variables when possible. If the
   function is event driven, you are forced to use global variables.

    # print elements of an array
    proc show_array arrayName {
        upvar $arrayName myArray

        foreach element [array names myArray] {
           puts stdout "${arrayName}($element) =  $myArray($element)"
        }
    }

    set arval(0) zero
    set arval(1) one
    show_array arval

   To return an array from a procedures, just take the array name in as an
   argument, as above. Any changes you make in the array will be made in
   the parent's array as well.

   Extended Tcl introduces a concept called keyed lists which are arrays
   made out of lists of key-value pairs and can be passed by value to
   routines, over networks, etc.


------------------

From: Tcl Q&A
Subject: Q.B5- How can I run an external command and read its output via a
               pipe?

   For example, to grep a pattern out of a range of files, one might do:

   Karl Lehenbauer (karl@NeoSoft.com) writes:

    set files [glob /home/cole/stats/*]

    proc parseInfo { site } {
       global files

    #
    # site is chosen from a listbox earlier
    #
       set in [open [concat "|/usr/bin/grep $site $files"] r]

       while {[gets $in line]>-1} {
          puts stderr $line
       }
       catch {close $in}
    }

   One thing: the matching strings are *not* returned in directory order.

   But what if I want to check the return code AND use the output of the
   command? Kevin B. Kenny (kennykb@dssv01.crd.ge.com) writes:

    if [catch {exec ls} data] {
        # The exec got an error, and $errorCode has its termination status
    } else {
        # The exec succeeded
    }
    # In any case, `data' contains all the output from the child process.

   Note that Karl Lehenbauer (karl@NeoSoft.com) adds that errorCode will be
   a list containing three elements, the string "CHILDSTATUS", the process
   ID of the child, and the exit status of the child.


------------------

From: Tcl Q&A
Subject: Q.B6- How can I delete a procedure from within a script?

   Rename the procedure to have no name, for example:

    rename procedureName ""


------------------

From: Tcl Q&A
Subject: Q.B7- How can I get more than 7 digits of double precision?

   In Tcl 7.x, set the global variable `tcl_precision' to a value in the
   range 1-17. For example:

    % expr 4*atan(1)
    3.14159
    % set tcl_precision 0
    can't set "tcl_precision": improper value for precision
    % set tcl_precision 3
    3
    % expr 4*atan(1)
    3.14
    % set tcl_precision 16
    16
    % expr 4*atan(1)
    3.141592653589793
    % set tcl_precision 18
    can't set "tcl_precision": improper value for precision

   In Tcl 6.x, you must modify the tclExpr.c module to use "%lf" instead of
   "%g".


------------------

From: Tcl Q&A
Subject: Q.B8- How can I grab the command line when a non-built-in call is
               made?

   The procedure "unknown" is called automatically with arguments
   containing the command and its arguments for any command that couldn't
   be found. In fact, Tcl and Extended Tcl use this feature to provide
   demand loaded commands, and even entire libraries. Also, when using the
   "tclsh" or "tcl" shells interactively, the "unknown" procedure enables
   the shell to run external programs (i.e. without typing "exec" first).

   So by modifying the unknown procedure you can provide your own extended
   functionality, or even remove the demand loading capability if you so
   desire.


------------------

From: Tcl Q&A
Subject: Q.B9- How can I get or set an environment variable?

   By using something like the following.

    set olddisplay $env(DISPLAY)
    set env(DISPLAY) unix:0

   Thanks to Joel Fine (joel@cs.berkeley.edu) for the answer.

   You can also check if an environment variable exists, like this:

    if [info exists env(VARNAME)] {
        # okay, it's there, use it
        set value $env(VARNAME)
    } else {
        # the environment var isn't set, use a default
        set value "the default value"
    }


------------------

From: Tcl Q&A
Subject: Q.B10- How can I use numbers with leading zeroes?

   In Tcl, numbers that start with a zero are interpreted as octal values
   (or hexadecimal, if the zero is immediately followed by an 'x'). Often,
   this is a useful feature, but there are some problems:
     * What if the number has invalid octal (hexadecimal) digits?

       Prior to Tcl 7.4b3, the interpreter would try to interpret a number
       such as 08 as a floating point value. Starting with 7.4b3, the
       interpreter generates an error message when presented with such a
       value.
     * What if you want the value interpreted as a decimal number (as if
       the leading zeros weren't there)?

       You have to strip off the zeros. Here are two possible
       implementations of a function "stripzeros":

    proc stripzeros {value} {
        regsub ^0+(.+) $value \\1 retval
        return $retval
    }

     or

    proc stripzeros {value} {
        set retval [string trimleft $value 0]
        if { ![string length $retval] } {
            return 0
        }
        return $retval
    }

     Then, whenever you are passing a value to a command that requires an
     integer value, and there is the possibility that the value has a
     leading zero, just pass it through this function first, i.e. instead
     of:

    set dayofweek [expr $days%7]

     use:

    set dayofweek [expr [stripzeros $days]%7]



------------------

From: Tcl Q&A
Subject: Q.B11- How can I find the command line arguments to my
                application?

   The program name is assigned to the global variable "argv0" and any
   arguments to it are placed in the global variable "argv" as a list. The
   variable "argc" is set to the number of elements in the list "argv". As
   an example:

    #! /usr/local/bin/tclsh

    if { $argc != 2 } {
        puts stderr "$argv0: Usage: $argv0 <infile> <outfile>"
        exit 1
    }

    set infile  [lindex $argv 0]
    set outfile [lindex $argv 1]


------------------

From: Tcl Q&A
Subject: Q.B12- I found a bug in the handling of comments!

   Sometimes people find that Tcl behaves differently than they expect
   (often because of the way some other language acts in a similar
   situation). They then think this unexpected behavior is a bug. Probably
   the most common occurance of this is in regard to comments.

   In Tcl, everything passed to the parser must have proper list structure,
   even comments (and yes comments are passed to the parser, unlike in some
   languages where they are stripped at an earlier stage). Generally, this
   means you need to make sure your braces are evenly matched, even though
   they may be on a line that is commented out.

   So, for example, you have an "if" statement that tests a certain
   condition, but you want to try testing a different condition. You
   comment out the old condition and type a new "if" statement. This code
   will cause an error that there is a missing close-brace:

    ## WRONG
    if { $newflag } {
    # if { $oldflag } {
        puts hello
    }

   In this case, you have to balance the braces, for example:

    ## CORRECT
    if { $newflag } {
    # if { $oldflag } {
        puts hello
    # }
    }

   Another interesting point about comments in Tcl is that the line
   continuation mechanism still applies, so:

    # This is a comment line that ends with a backslash \
      and this line is still part of the comment


------------------

From: Tcl Q&A
Subject: Q.B13- How can I input and output binary data?

   Internally, the Tcl interpreter stores nearly everything in
   null-terminated strings. This procludes the possibility of directly
   storing binary data (or more specifically, anything with embedded
   nulls).

   However, certain operations can be performed without the data being
   stored in Tcl variables. For example, file handles can be attached
   directly to external programs, like this:

    set infp [open "|compress -dc $fileName"]

    exec gzip -c $newFileName <@ $infp

   or in Extended Tcl:

    set infp [open "|compress -dc $fileName"]
    set outfp [open "|gzip -c $newFileName" w]

    copyfile $infp $outfp

   In Tcl 7.5, you can copy binary data between open channels using the
   command "unsupported0". However, as is indicated by the name, it is not
   a method which should be relied on - it will disappear in a future
   release, although proper support for handling binary data should be
   added at that time.

   Alternatively, you can in some way convert the data to a form that is
   usable from Tcl:
     * Reading from or writing to a pipe that is connected to an external
       filter, such as atob/btoa, uuencode/uudecode, or even od.
     * A method of handling binary data one character at a time using
       standard Tcl can be found in:
       ftp://ftp.neosoft.com/languages/tcl/alcatel/code/
     * There are also two different extensions available that enable the
       handling of binary data: tclbin and BinarIO. The former is better
       for handling structured data, while the latter works better for
       binary data streams.

   Thanks to Wayne Throop throop@aur.alcatel.com for his contribution to
   this section.


------------------

From: Tcl Q&A
Subject: Q.B14- How can I trap signals, and perform other more Unix
                specific functions?

   Extended Tcl offers many of these types of functions. For instance,
   extended Tcl has the "signal" command:


      signal action siglist [command]

      where action is one of "default", "ignore", "error", "trap", "get",
      plus the POSIX "block" and "unblock" actions (available only on POSIX
      systems, of course). Siglist is a list of either the symbolic or
      numeric Unix signal (the SIG prefix is optional). Command is your
      error handler (or a simple {puts stdout "Don't press *that* key!"}
      :-) "trap" does what you expect, and I find "error" and "get" to be
      extremely useful in interactive programs which demand keyboard
      traversal.

      Extended Tcl also has things like fork, etc.

   Answer by Brad Morrison (brad@NeoSoft.com).


------------------

From: Tcl Q&A
Subject: Q.B15- How can I do a double indirect? - Why doesn't $$var work?

   There is one and only one level of substitution possible with every pass
   through the interpreter. Also, when doing variable substitution, the
   interpreter finds the dollar sign and then takes everything following it
   up until the next invalid character (where invalid is defined as
   anything other than a letter, digit, or underscore) as the name of the
   variable - well, that is, unless it finds array notation or the
   "${varname}" form.

   In the case of "$$var", the character after the first dollar sign is an
   invalid character (another dollar sign), so there is no variable name
   and variable substitution is not performed (the dollar sign is left as
   is) and scanning starts again for any dollar signs and a following
   variable name. It is immediately found at that second dollar sign, the
   substitution is performed, and scanning for dollar signs resumes with
   whatever was after the variable name. Since there isn't anything else,
   substition is done for this pass through the interpreter (remember it's
   only done once).

   The "eval" command runs its arguments through the interpreter, so you
   could use "eval" to cause a second pass through the interpreter, and
   thus, have "$$var" work:

    % set a 5
    5
    % set var a
    a
    % puts $$var              ;# This doesn't work
    $a
    % eval puts $$var         ;# This does  - but it's dangerous
    5

   However, if the contents of "var" contain any _special_ characters (e.g.
   whitespace, semicolon) you'll run into problems.

   A better method is to take advantage of the behaviour of the "set"
   command when given only one argument, and combine command substitution
   with variable substitution:

    % puts [set $var]         ;# This works safely
    5

   or, in fact, you could use just command substitution (which is performed
   once for each "[ ]" pair):

    % puts [set [set var]]    ;# as does this
    5

   Similarly, to print the values of `var1', `var2', and `var3':

    set var1 3.14159
    set var2 hello
    set var3 13
    foreach num {1 2 3} {
	puts "var$num = [set var$num]"
    }

   will output:

    var1 = 3.14159
    var2 = hello
    var3 = 13

   The "upvar" command can also be used to derefence variables.

   In addition, starting with version 7.4, the interpreter includes the
   command "subst" which can be used to perform substitutions.

   Note that all of the above applies to array variables also.


------------------

From: Tcl Q&A
Subject: Q.B16- How can I get quoted strings to work the way I want?

   A long article dealing with the issues can be found at
   ftp://ftp.neosoft.com/languages/tcl/alcatel/docs/

   Here are some short answers:

   Q. I'm trying to build up a command for later execution but am having
   trouble with variable values that include whitespace or special
   characters.

   A. The safest way to build up commands is to use the list command so
   that you can keep track of the list structure. Avoid using double quotes
   because you can end up with an extra trip through the evaluator. We'll
   illustrate this with a command to create a button that prints out the
   label on the button when you click it.


      Wrong answer #1:

    button $myname -text $label -command "puts stdout $label"

      Why? because if $label has whitespace then the puts command will be
      passed the wrong number of arguments. If $label has $ or [ ]
      characters, they will be interpreted instead of printed.

      Good answer #2:

    button $myname -text $label -command [list puts stdout $label]

      Why? because list will properly quote the value of $label

   Q. I'm trying to build up a command for later execution but am having
   trouble getting some variables to evaluate now, and some to evaluate
   later when the command is run.

   A. The cleanest way to do this is to define a procedure that hides the
   use of the variables at run time, and then build up a call to that
   procedure using the list command as described previously. (You can even
   define the procedure on the fly. It will have global scope even it if is
   created within another procedure.)


      Wrong answer #1:

    button $myname -text $label -command \
        [list puts stdout $ArrayOfDynamicStuff($label)]

      Why? The array value will be substituted when the button is created,
      not later on when the button is clicked. Also, note that the command
      is executed at the global scope, so it is not necessary to include a
      "global ArrayOfDynamicStuff" in the command.

      Wrong answer #2 (backquotes and list):

    button $myname -text $label -command \
        [list puts stdout \$ArrayOfDynamicStuff($label)]

      Why? Here the list command and the backquote of $ are fighting with
      each other. The command ends up being something like:

    puts stdout {$ArrayOfDynamicStuff(foo)}

      which prevents the substitution of the value of the array element.

      Dubious answer #3 (backquotes and double-quotes):

    button $myname -text $label -command \
        "puts stdout \$ArrayOfDynamicStuff($label)"

      Why? This only works if the value of $label has no special characters
      or whitespace.

      Clean answer #4 (proc):

    proc doit { i } {
        global ArrayOfDynamicStuff
        puts stdout $ArrayOfDynamicStuff($i)
    }
    button $myname -text $label -command [list doit $label]

      Why? Using little TCL procs for your button commands is a good habit
      because it eliminates most needs for fancy quoting, and it makes it
      easier to tweak the button command later on.

   Q. I'm trying to pass along a variable number of args to another
   procedure but I'm having trouble getting the $args to expand right.

   A. Avoid using eval and double quotes because that results in an extra
   trip through the interpreter. The eval command will do a concat of its
   arguments if there are more than one, so that pretty much eliminates the
   need to group things with double quotes. Let's extend the button
   example:


      Wrong answer #1:

    proc mybutton { myname label args } {
        button $myname -text $label -command [list puts stdout $label] $args
    }

      Why? All the extra arguments to mybutton are grouped into one list
      element that is but into the value of $args. However, the button
      command expects to see individual arguments, not a sub-list.

      Wrong answer #2:

    proc mybutton { myname label args } {
        eval "button $myname -text $label \
                 -command [list puts stdout $label] $args"
    }

      Why? The double quotes allow expansion of $label as well as $args, so
      if $label has any whitespace, the button command will be malformed

      Good answer #3:

    proc mybutton { myname label args } {
        set cmd {button $myname -text $label -command [list puts stdout $label]}
        eval $cmd $args
    }

      Why? Eval will first concatenate its two arguments and then run the
      result through the interpreter. Think of this as stripping off the
      outer curly braces from $cmd and $arg and making a single list with
      all the elements of both. $label will be evaluated exactly once, so
      the puts command will remain good, and whatever went into args will
      also be processed exactly one time.

   Q. Why do I get a syntax error in an if/while/for statement?

   A. You may have written something like

    wish: set foo bar
    wish: if {$foo == bar} {puts stdout bar}
    syntax error in expression "$foo == bar"

   in which bar is interpreted as neither a string nor a variable, since
   strings as operands in expressions MUST be surrounded by double quotes
   or braces.

   Change to

    wish: if {$foo == "bar"} {puts stdout bar}

   or

    wish: if {$foo == {bar}} {puts stdout bar}

   always in expressions, depending on if you want expansion performed or
   not.

   Contributed by Jesper Blommaskog (d9jesper@dtek.chalmers.se).


------------------

From: Tcl Q&A
Subject: Q.B17- How can I share procedures between multiple tcl
                applications?

   Rather than sourcing the files explicitly, build a tcl library:

   Step 1. Put the files in a common directory

   Step 2. Build the tclIndex for the "library". I use a Makefile with a
   convention like:

    install.index:
        (cd ${DESTDIR}/tclscripts/lib; \
        echo 'source /usr/local/lib/tcl/init.tcl;\
        auto_mkindex . *.tk' | tcl ; exit 0)

   Step 3. Modify your tcl scripts to reference the library:

   e.g.:

    # local additions
    lappend auto_path /usr/local/lib/tcl_local $env(RDS_TCL_SCRIPTS)/lib

   Now, as soon as your script tried to reference a procedure in the
   library, the "unknown" command autoloads the procedure for you.

   Contributed by Joe VanAndel (vanandel@ncar.ucar.edu).


------------------

From: Tcl Q&A
Subject: Q.B18- How can I get items inserted into a list?

   A user asked why

    linsert $list 0 ..

   did not result in .. being inserted into list.

   Jesper Blommaskog (d9jesper@dtek.chalmers.se) replied:

   When doing list operations other than "lappend", you must save the
   returned value. This applies to "list", "lindex", "lrange", and
   "lreplace" at least.

   In this example, you would perhaps want to do something like:

    set list [ linsert $list 0 .. ] 


------------------

From: Tcl Q&A
Subject: Q.B19- How can I perform a non-blocking read on a file identifier?

   From Frank Smith (frank@arraysystems.nstn.ns.ca) we are told that if you
   have Extended Tcl, you can

    read $fileId [fstat $fileId size]

   This will read only the number of bytes currently available on `fileId'
   and consequently will not block.


------------------

From: Tcl Q&A
Subject: Q.B20- How can I read and write from a pipe without locking up?

   The stdio package has an optimization for speed that buffers characters
   so that you can avoid un-necessary system calls. This isn't very good
   for interactive use, so stdio changes its behaviour if the file is a
   terminal. When you write to a pipe you are not writing to a terminal and
   so your output is buffered, similarly when the program at the other end
   of the pipe writes its response it is also buffered. The end result is
   the programs at both end of the pipe stall waiting for input. It is
   possible to force the buffers to be written using "flush" in Tcl and
   fflush() in C, often though, you don't have control over the program at
   the other end of the pipeline and in that case your only solution is to
   use a pseudo-teletype (pty). Unfortunately this isn't all that easy.

   While this isn't directly supported in Tcl, the "spawn" command in
   expect opens a pty and starts a command.


------------------

From: Tcl Q&A
Subject: Q.B21- How can I find what version I am running?

   Well, there is no one answer to this. Tcl itself, will tell you its
   version, if you type:

    info tclversion

   or with recent versions:

    puts $tcl_version

   On the other hand, to get the Tk version, you must use:

    puts $tk_version

   Other extensions use other means (for example, Extended Tcl uses "infox
   version" to return its version number).

   Starting with the 7.4 version, the tclsh executable is, by default,
   installed with the version number appended - making it rather obvious
   which version you are running. However, if it is not installed like that
   on your system, you can find out the version from the shell prompt with
   something like:

    echo 'puts $tcl_version;exit 0' | tclsh

   (the ";exit 0" part is only needed for wish and its derivatives, but
   doesn't hurt anything when it isn't needed)


------------------

From: Tcl Q&A
Subject: Q.B22- How can I use variables to hold array names?

   Eric Bleeker (ericbl@paramount.nikhefk.nikhef.nl) has written:


      You may have written something like:

    % set foo "bar baz"
    bar baz
    % foreach aap $foo {
        set $aap(1) "something"
    }
    can't read "aap(1)": variable isn't array

      This means Tcl tries to substitute the array element `aap(1)' which
      doesn't exist. To fix this use:

    % foreach aap $foo {
        set [set aap](1) "something"
    }

      In this case two arrays `bar' and `baz' are created.

   An alternative format is:

    % foreach aap $foo {
        set ${aap}(1) "something"
    }


------------------

From: Tcl Q&A
Subject: Q.B23- How can I make variables static, like in C?

   The "static" keyword in C is used for different purposes. One of the
   most common is when you wish to have a variable that retains its value
   across invocations of the same function (but without making the variable
   globally available). Karl Lehenbauer (karl@NeoSoft.com) wrote the
   following procedure to emulate this behaviour:

    proc static {args} {
        set procName [lindex [info level -1] 0]
        foreach varName $args {
	    uplevel 1 "upvar #0 staticvars($procName:$varName) $varName"
        }
    }

   Another common use, is when you want a global variable that is only
   visible to functions within that particular source file. This is best
   handled by a namespace facility. Tcl does not yet have a built-in
   namespace facility, but several extensions are available the provide
   one. See the software catalog in Larry's FAQ for more info.


------------------

From: Tcl Q&A
Subject: Q.B24- How can I convert between a character and its ASCII value?

   To get the ASCII value of a character:

    proc toASCII { char } {
        scan $char %c value
        return $value
    }

    puts "The ASCII value of 'A' is [toASCII A]"

   To convert from an ASCII value to a character:

    proc toChar { value } {
        return [format %c $value]
    }

    puts "The ASCII value 65 corresponds to the character '[toChar 65]'"

  ---------------------------------------------------------------------------

EXPLANATION OF ERROR MESSAGES:

   This section lists some error messages and some possible (and
   non-obvious) reasons why you may be getting them.


------------------

From: Tcl Q&A
Subject: Q.C1- "not found" or "Command not found"

   This error message comes from your shell, not Tcl. The script probably
   starts with a "#!" that is followed by a path to the Tcl interpreter
   that is too long for your system. Many Unix systems are limited to 32
   characters (including the "#!") in this regard. So, something like this
   will fail:

    #! /usr/home/homedir/very/long/path/tclsh

    # rest of script

   You can either shorten the path by moving the tclsh executable to a
   different directory or by using symbolic links. Another option is to not
   specify the path at all. See the question "How do I make my script
   executable regardless of the location of tclsh?" for an example of how
   to do this.


------------------

From: Tcl Q&A
Subject: Q.C2- invalid command name "}"

   You have probably commented out a line that ends with an open curly
   brace. See Question B12 for more info.


------------------

From: Tcl Q&A
Subject: Q.C3- missing close-brace

   Your braces aren't balanced. Again, one likely, though perhaps
   non-obvious, reason is improperly commented lines. See Question B12.


------------------

From: Tcl Q&A
Subject: Q.C4- X server insecure (must use xauth-style authorization)

   Tk requires you to have a secure X server before you can use the send
   command. See the question 2.A.7 "How can I get Tk 3.3 to even start - I
   get security error messages." in Thomas Accardo's Tk Toolkit Usage FAQ
   as well as http://ce-toolkit.crd.ge.com/tkxauth/ for instructions on how
   to make your server secure.


------------------

From: Tcl Q&A
Subject: Q.C5- expected integer but got ...

   This error occurs when trying to perform some mathematical operation
   that requires an integer value, but given a non-integer value. While
   this is pretty obvious when the value supplied is a floating point value
   or an alphabetic string, this error can be confusing when the value is
   something that may look like a valid integer. Specifically numbers that
   have leading zeros. See the question How can I use numbers with leading
   zeros? for an explanation.


------------------

From: Tcl Q&A
Subject: Q.C6- "Undefined symbol: main" or similar

   If when linking an application you get an error saying that the "main"
   function was not found, it is probably because the application was
   written for a version of Tcl prior to Tcl 7.4 and you are linking it
   with version 7.4 (or later).

   Older versions of Tcl included a "main" function in the library
   ("libtcl.a"). This caused various problems, in particular with C++
   programs, and it was removed starting with version 7.4b1. The
   application being linked with the Tcl library must now provide the
   "main" routine itself.
  ---------------------------------------------------------------------------

COMPILING AND OTHER PLATFORM SPECIFIC QUESTIONS:

   This section contains some hints on compiling the Tcl distribution on
   various platforms and other notes regarding platform specific problems.

   For building Tcl 7.4 and newer releases, there is now an on-line
   database, maintained by Sun, for people to query, if they have problems
   building the standard distribution. If the software did not build
   automatically for you, check at http://www.sunlabs.com/research/tcl/ for
   an entry regarding your platform. And please share your experience, if
   there is no current entry and you were able to get the distribution to
   build somehow.

   Notes that were previously listed here, but which apply primarily to
   older releases are now available in a separate document at:
   http://psg.com/~joem/tcl/PlatformNotes.html

   See also Question A2


------------------

From: Tcl Q&A
Subject: Q.D1- Why does "glob" cut off the first two characters of each
               filename?

   This problem can occur on SVR4, Solaris 2.x, and similar systems that
   have both System V and BSD libraries. It happens when the interpreter is
   compiled with include files that don't match the libraries to which it
   is linked (specifically, readdir returns a different structure than is
   expected). You need to modify the Makefile to make sure the include
   files correspond to the libraries.


------------------

From: Tcl Q&A
Subject: Q.D2- How do I get Tcl working on MS Windows (3.1, 95, or NT)?

   For Microsoft specific information see the FAQ maintained by Eric
   Johnson (ejohnson@camax.com). It is available at:
   http://ourworld.compuserve.com:80/homepages/efjohnson/tclwin.htm


------------------

From: Tcl Q&A
Subject: Q.D3- How can I build the Tcl library as a shared libary?

   This is now supported for some platforms in the standard distribution.

   You can also get the "Plus" patches, which include support for
   additional platforms. For more information, see
   http://www.nici.kun.nl/tkpvm/pluspatch.html

   Additionally, Extended Tcl includes instructions for building shared
   libraries on many Unix systems. You may be able to use this information,
   if your platform is not supported by the plus patches.
  ---------------------------------------------------------------------------
   End of FAQ
---
set Email "joe@morton.rain.com (Joe Moss)";set URL http://psg.com/~joem/
 ======================================================================
"Folk, if you are not checking the Frequently Asked Questions documents
before you ask questions, you are taking longer to resolve problems,
and annoying more folk, than necessary."   -- Larry W. Virden

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Last Update March 27 2014 @ 02:12 PM