Tcl Language Usage Questions And Answers

Extended Tcl includes a command named random to do this. For example, to generate a random number between 0 and 9 inclusive:

    set random_number [random 10]

It also has a srandom command to seed the random number generator.

Several pseudo-random number generator functions, written in straight Tcl, have been posted to comp.lang.tcl and are available from alcatel.

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

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

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

    eval y [x]

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.

Q6- 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.

Q7- How can I merge extended Tcl with wish, expect, etc.?

Extended Tcl, as of version 6.7c, was enhanced to make it easier to incorporate into applications.

Q8- How can I delete a procedure from within a script?

Rename the procedure to have no name, for example rename procedureName ""

Q9- How can I get parray to recognize an array created via upvar?

Right now (June, 1992) upvar doesn't allow you to attach to an individual element of an array. This is considered a bug by Mr. Ousterhout and has been place on a bug list.

Q10- How can I get more than 7 digits of double precision ?

In Tcl 6.x, modify the tclExpr.c module to use %lf instead of %g. In Tcl 7.x, set the global variable tcl_precision instead.

Q11- 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.

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

Q12- 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 $end(VARNAME)
    } else {
	    # the environment var isn't set, use a default
	    set value "the default value"
    }

Dave Morriss was recently having problems because he was trying to do something like:

    set index [expr [exec date +%W]%[llength $pop_server_list]]

but during the 8th and 9th week of the year, he got errors - 08 and 09 are not valid octal numbers in Tcl.

Some of the solutions provided were:

From George A. Howlett (george.howlett@att.com), we got:

    set wknum [format "%g" [exec date +%W]]
    set index [expr [exec $wknum%[llength $pop_server_list]]

From Fred Feirtag (feirtag@wave.nrl.navy.mil):

    set index [expr (1[exec date +%W]-100)%[llength $pop_server_list]]

From Dan R. Schenck (schendr@Texaco.com):

    set index [expr [string trimleft [exec date +%W] 0]%[llength $pop_server_list]]

Jay Goldberg jayg@accessware.com points out that while all of the above work in this case (dealing with a date) they don't work in the general case.

Instead you can use the statement:

    regsub 0+(.+) orig_string \\1 new_string

to strip off leading zeros. So, applying this to the example above:

    regsub 0+(.+) [exec date +%W] \\1 weeknum
    set index [expr $weeknum%[llength $pop_server_list]]

If you are using extended Tcl or Expect, you will find the parameters in the Tcl variable argv as a list. Note that in extended Tcl, the name of the program is in the Tcl variable programName and NOT in argv[0]. Thanks to Brad Morrison (brad@NeoSoft.com) and Bud Bach (bachww@rtsg.mot.com) for this answer.

Q15- How can I put comments in my script, e.g. in a case statement?

You can't have comments where you have them. Move the comments inside of the { for the case that you want. Your code should read:

    case 1 {
      -1    {
            #
            # Cannot find information sought
            #
            exit 2
            }
       0    {
            #
            # Error in arguments
            #
            exit 1
            }
       default    {
            #
            # Desired information found
            #
            exit 0
            }
    }

Thanks to Gerald W. Lester (gwlester@cpu.com).

Q16- How can I input and output binary data?

With Extended Tcl you can safely do stuff like:

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

    copyfile $infp $outfp

Thanks to Karl Lehenbauer (karl@NeoSoft.com) for the code example.

A method of handling binary data one character at a time using standard Tcl can be found in: ftp.aud.alcatel.com:/tcl/code/binary-io-hack.shar.gz

Q17- How can I trap signals, and 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!"} :-)

Extended Tcl also has things like fork, etc.

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

Q18- 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.aud.alcatel.com/tcl/docs/README.programmer.gz

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).

Q19- 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).

Q20- 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 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.

Q22- 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 it's 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.

Q23- How can I find what version I am running?

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

    info tclversion

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

    puts $tk_version

Other extensions use other means.

Q24- How do I create a stand alone program in case Tcl isn't installed?

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.aud.alcatel.com/tcl/code/wish_compiler.shar.gz 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.

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.

Dov Grobgeld (dov@menora.weizmann.ac.il) provided info on creating Tcl and Tk shared libraries under AIX 3.1.5:

For Tcl:

    cc -o tkshar.o *.o -bE:tclshar.exp -bM:SRE -berok -lX11 -lm
    ar r libtclshr tclshar.o

For Tk:

    cc -o tkshar.o *.o -bE:tkshar.exp -bM:SRE -berok -Ltcl -lX11 -lm -ltclshr
    ar r libtkshr tkshar.o

where tckshar.exp and tkshar.exp had lists of the external functions.

There are also problems with fonts on AIX and the IBM. A patch is needed from IBM to fix the X server so that fonts are working

See the porting notes. Also, be aware that there have been some reports of Configure assuming that you should use the compatibility version of opendir() and that you have no dirent.h . This causes glob-ing to fail, as well as TclX's readdir, etc.

Changing tcl7.3 to remove the opendir compatibility file and removing the -DNO_DIRENT, -DUSE_DIRENT2 fixed it all up.

Q28- Has anyone gotten Tcl to compile under VMS?

Information from John Kimball (jkimball@src.honeywell.com) on May 4, 1993 was that he had gotten Tcl 6.7 and Tk 3.2 ported to VMS 5.5. See the catalog for the file information.

Q29- What does it take to get Tcl to compile under SCO Unix?

Add a "#undef select" to tkEvent.c, and remove the reference to TK_EXCEPTION around line 460 of main.c.

Tk uses its own scheme for allocating the border colors for its 3D widgets, which causes problems when running TK on a system with "PseudoColor" display class, and a 16-cell colormap.

If you can't go to eight bitplanes, you can instead start the server with a "-static" (Xsco) or "-analog" (Xsight) option, making the display class become "StaticColor". This makes the entire colormap read-only, and it will return the color that most closely maps to the desired color as possible.

This information is from Keith Amann (Keith_Amann@stortek.com).

Q30- Why do I get format and scan errors when I run tclTest?

That's a problem (scanf/printf) many systems seem to have. Don't worry too much about it - just don't use these 'advanced' features. If you're hacking C, you'll have the same problems.

For instance, printf("%Ng, double_value) and strtod("+",&terminal) produce incorrect results.

Q31- Why do I get lots of errors under Irix 4.0.1 when I run tclTest?

There's a bug in the 4.0.1 optimizer that's fixed in 4.0.2. Compile tclVar.c using -O0 (no optimization).

Q32- Does anyone else have problems with Tcl on a Cray?

See the porting notes for a set of changes mentioned. Also, Booker C. Bense (benseb@grumpy.sdsc.edu) reports that version 3.0.1.6 has some real problems with char pointers, causing Tcl to crash. Using version 3.0.2.1, things are much better, except for a minor formatting problem and serious problems with scan.

Q33- How do I install Tcl 7.1/Tk 3.4 on NeXTSTEP 3.1?

I have indicated in the past that this particular question seems to generate controversy - I have replaced previous explanations with the latest msg from someone who sounds authoritative...

From: Robert Nicholson (robert@steffi.demon.co.uk)

This is for TCL7.1 and TK3.4

This is a variation on Thomas Funke's entry.

Installing TCL7.1 on NeXT

To install tcl7.1 on NeXTSTEP 3.1 you must:

• run configure with predefined CPP:
  type "sh" to run a Bourne shell. then type

      CPP='cc -E' ./configure
  

• edit Makefile:
  add tmpnam.o to COMPAT_OBJS:

    COMPAT_OBJS =      getcwd.o waitpid.o strtod.o tmpnam.o
  

At this point you probably want to rename the all calls to strtod and tmpnam to something else in order to distinguish them from those that are supplied in the NeXT libraries libsys_a.a. You can do this by adding the following line to AC_FLAGS

    -Dstrtod=newstrtod -Dtmpnam=newtmpnam

and renaming the names of the definitions in the appropriate compat/*.c files.

To check everything is working correctly open a tclsh

    % expr {"0" == "+"} 
    0

Using the wrong strtod function will cause this test to return 1 which is a serious error. Ignore the precision errors.

Installing TK3.4 on NEXTSTEP 3.1

There is a call to strtod in TK3.4 so you should link against ../tcl-7.1/compat/strtod.o and apply the -Dstrtod=newstrtod to AC_FLAGS in Tk also.

Note:Tk's raise test will fail when running the tvtwm window manager. Changing to either twm or even better fvwm ensures that this test will succeed.

From John Ousterhout (ouster@cs.Berkeley.edu):

Here's my form letter that seems to explain most of the problems people have had printing parts of the book:

Some old versions of the Transcript spooler software cannot properly handle encapsulated Postscript files within another Postscript file when they do page reversal. They tend to garble the page structure of the file, causing an error at the point of the first EPS file. Part II of the Tcl book has lots of EPS files embedded in it, one for each screen dump. If your printer cannot print this file I suggest checking to see if page reversal is enabled for your printer. If so, try asking your local system wizard to disable it for you; this should allow the file to print. Or, find some other way to dump the Postscript file directly to the printer without going through the spooling software (e.g. perhaps you can simply cat it to the printer's serial port).

Others have suggested:

The embedded pictures have CR as the line separator instead of LF and this may be causing the problem. Try translating them to LFs ...

    tr '\015' '\012' <book.p2.ps >fixedbook.p2.ps

and:

This is not the original poster's problem but in countries using A4 paper and on a particular printer, the Dataproducts LZR1260E, the frame size causes the printing to be stretched vertically. This happens with other Framemaker generated documents by the way. A PS interpreter bug no doubt. The fix is to edit the dimensions for A4 paper. This shell script does both fixes.

    #!/bin/sh
    cat $1 | tr '\015' '\012' | sed '/FMDOCUMENT$/s/612 792/595 842/'

From Gordon Lack (gml4410@ggr.co.uk) we are told that the SGI c compiler has some bugs with variable arguments.

tclVar.c must be compiled with -O0 at IRIX C 4.0.1 because of a compiler bug with varargs.

Done by placing specific rule into Makefile.

    # GGR SG needs -O0 for varargs at 4.0.1

    CC_SWITCHES0 =  -O0 -I. -I${SRC_DIR} ${AC_FLAGS} ${MATH_FLAGS} \
          ${GENERIC_FLAGS} ${PROTO_FLAGS} ${MEM_DEBUG_FLAGS} \
          -DTCL_LIBRARY=\"${TCL_LIBRARY}\"

    tclVar.o: tclVar.c
          $(CC) -c $(CC_SWITCHES0) $

Peter Neelin (neelin@pet.mni.mcgill.ca) also noted that:

I get tclX 7.3a to compile on an SGI (irix 4.0.5) with the following
changes to the Config.mk file:
 
    71c71
    < CFLAGS=-cckr -D__GNU_LIBRARY__
    ---
    > #CFLAGS=
    106,107c106,107
    < TCL_PLUS_BUILD=TCL_PLUS
    < CCPLUS=g++
    ---
    > #TCL_PLUS_BUILD=TCL_PLUS
    > CCPLUS=CC
    191,193c191
    < MAN_DIR_SEPARATOR=
    < 
    < LIBOBJS=strftime.o
    ---
    > #MAN_DIR_SEPARATOR=.

 
The -D__GNU_LIBRARY solves the srandom problem and I think that the -cckr gets
around the prototype error with waitpid (it's crude, but it works).
I've forgotten why I needed the strftime.
 
If you want the version of Config.mk that worked for me, send me mail.
 
Q36- How do I build expect on Solaris 2.3?

From Jeff Abramson (jra@hrcms.jazz.att.com) we are told that,


using either SunPro cc 2.0.1 or gcc 2.5.8 with
no problems.  For SunPro I do:
 
    CC=cc ./configure --prefix=directory_of_your_choice
    make CC=cc

 
For gcc I do:
 
    CC="gcc -fwritable-strings" ./configure \
        --prefix=directory_of_your_choice
    make CC="gcc -fwritable-strings"


 
Q37- How do I port Tcl and Tk to a Sequent?
 
From Andrew Swan (aswan@soda.berkeley.edu) we find:

For both Tcl and Tk, I used gcc rather than the Sequent cc which is
not ansi.

For Tcl, there were a couple of problems with the math library.  First,
the Sequent math library doesn't include the 'fmod' function.  I got
the source for fmod from ftp.uu.net, put it in the compat subdirectory,
and added it to the Makefile.  fmod also wanted functions 'isnan' and
'finite' but I just commented those out since I don't believe tcl
supports infinity and nan.

The other problem with the math library was that it has two copies
of the 'tanh' function in it.  This is easy to fix by using "ar"
to separate the library in to object files and then reconstruct
it, leaving one of the copies of tanh out.

With those changes, Tcl compiled cleanly and passed all the tests
except the scanning tests.  Apparently, the *scanf functions are
broken in Dynix.  The problem is with recognizing the end of octal
numbers, and I just let this problem go.  Fixing it would probably
involve finding source to the *scanf functions and including it.

To compile Tk, I had to make a new version of stddef.h including
things like wchar_t.  I just copied it in to another include directory
and put that directory first and then tweaked the copy.  Tk also had
the math library problem (tanh).  Other than those, however, everything
compiled cleanly and passed all the tests.


Q38- How do I get around the symbol table full msg in AU/X 3.0?

After getting things compiled on AU/X, you have to be sure
to do the links with the -A {factor} arguments which expand the default
table allocations.  {factor} is the amount by which the compiler multiplies
the default allocation.  Try -A 2.  Contact
Walter B. Kulecz, PhD. (wkulecz@medics.jsc.nasa.gov) for more details
of his port.

Q39- How do I compile Tcl on MS-DOS, MacOS, or other non-UNIX system?

Refer to Larry Virden's FAQ
(see tcl-faq/part1) or
(see tcl-faq/part4)
for details concerning tools, finished
ports, or mailing lists relating to this topic.

Q40- How do I get Tcl to compile on my QNX system?

Steve Furr (furr@qnx.com) indicates:

For the record, to get TCL to work under QNX, I had to:


Make the _POSIX_SOURCE change indicated for pathname lengths
Change a definition of VARARGS, so the Watcom ANSI C compiler
would accept varargs functions used.
Resolve a conflict with error codes defined by QNX.