tclvars.n

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the directory it names is checked first.
If \fBTCL_LIBRARY\fR isn't set or doesn't refer to an appropriate
directory, then Tcl checks several other directories based on a
compiled-in default location, the location of the binary containing
the application, and the current working directory.
.TP
\fBtcl_patchLevel\fR
When an interpreter is created Tcl initializes this variable to
hold a string giving the current patch level for Tcl, such as
\fB7.3p2\fR for Tcl 7.3 with the first two official patches, or
\fB7.4b4\fR for the fourth beta release of Tcl 7.4.
The value of this variable is returned by the \fBinfo patchlevel\fR
command.
.VS 8.0 br
.TP
\fBtcl_pkgPath\fR
This variable holds a list of directories indicating where packages are
normally installed.  It is not used on Windows.  It typically contains
either one or two entries; if it contains two entries, the first is
normally a directory for platform-dependent packages (e.g., shared library
binaries) and the second is normally a directory for platform-independent
packages (e.g., script files). Typically a package is installed as a
subdirectory of one of the entries in \fB$tcl_pkgPath\fR. The directories
in \fB$tcl_pkgPath\fR are included by default in the \fBauto_path\fR
variable, so they and their immediate subdirectories are automatically
searched for packages during \fBpackage require\fR commands.  Note:
\fBtcl_pkgPath\fR it not intended to be modified by the application.  Its
value is added to \fBauto_path\fR at startup; changes to \fBtcl_pkgPath\fR
are not reflected in \fBauto_path\fR.  If you want Tcl to search additional
directories for packages you should add the names of those directories to
\fBauto_path\fR, not \fBtcl_pkgPath\fR.
.VE
.TP
\fBtcl_platform\fR
This is an associative array whose elements contain information about
the platform on which the application is running, such as the name of
the operating system, its current release number, and the machine's
instruction set.  The elements listed below will always
be defined, but they may have empty strings as values if Tcl couldn't
retrieve any relevant information.  In addition, extensions
and applications may add additional values to the array.  The
predefined elements are:

.RS
.VS
.TP
\fBbyteOrder\fR
The native byte order of this machine: either \fBlittleEndian\fR or
\fBbigEndian\fR. 
.VE
.TP
\fBdebug\fR
If this variable exists, then the interpreter
was compiled with debugging symbols enabled.  This variable will only
exist on Windows so extension writers can specify which package to load
depending on the C run-time library that is loaded.
.TP
\fBmachine\fR
The instruction set executed by this machine, such as
\fBintel\fR, \fBPPC\fR, \fB68k\fR, or \fBsun4m\fR.  On UNIX machines, this
is the value returned by \fBuname -m\fR.
.TP
\fBos\fR 
The name of the operating system running on this machine,
such as \fBWindows 95\fR, \fBWindows NT\fR, \fBMacOS\fR, or \fBSunOS\fR.
On UNIX machines, this is the value returned by \fBuname -s\fR.
On Windows 95 and Windows 98, the value returned will be \fBWindows
95\fR to provide better backwards compatibility to Windows 95; to
distinguish between the two, check the \fBosVersion\fR.
.TP
\fBosVersion\fR
The version number for the operating system running on this machine.
On UNIX machines, this is the value returned by \fBuname -r\fR.  On
Windows 95, the version will be 4.0; on Windows 98, the version will
be 4.10.
.TP
\fBplatform\fR
Either \fBwindows\fR, \fBmacintosh\fR, or \fBunix\fR.  This identifies the
general operating environment of the machine.
.TP
\fBthreaded\fR
If this variable exists, then the interpreter
was compiled with threads enabled.
.TP
\fBuser\fR
This identifies the
current user based on the login information available on the platform.
This comes from the USER or LOGNAME environment variable on Unix,
and the value from GetUserName on Windows and Macintosh.
.TP
\fBwordSize\fR
.VS 8.4
This gives the size of the native-machine word in bytes (strictly, it
is same as the result of evaluating \fIsizeof(long)\fR in C.)
.VE 8.4
.RE
.TP
\fBtcl_precision\fR
.VS
This variable controls the number of digits to generate
when converting floating-point values to strings.  It defaults
to 12.
17 digits is ``perfect'' for IEEE floating-point in that it allows
double-precision values to be converted to strings and back to
binary with no loss of information.  However, using 17 digits prevents
any rounding, which produces longer, less intuitive results.  For example,
\fBexpr 1.4\fR returns 1.3999999999999999 with \fBtcl_precision\fR
set to 17, vs. 1.4 if \fBtcl_precision\fR is 12.
.RS
All interpreters in a process share a single \fBtcl_precision\fR value:
changing it in one interpreter will affect all other interpreters as
well.  However, safe interpreters are not allowed to modify the
variable.
.RE
.VE
.TP
\fBtcl_rcFileName\fR
This variable is used during initialization to indicate the name of a
user-specific startup file.  If it is set by application-specific
initialization, then the Tcl startup code will check for the existence
of this file and \fBsource\fR it if it exists.  For example, for \fBwish\fR
the variable is set to \fB~/.wishrc\fR for Unix and \fB~/wishrc.tcl\fR
for Windows.
.TP
\fBtcl_rcRsrcName\fR
This variable is only used on Macintosh systems.  The variable is used
during initialization to indicate the name of a user-specific
\fBTEXT\fR resource located in the application or extension resource
forks.  If it is set by application-specific initialization, then the
Tcl startup code will check for the existence of this resource and
\fBsource\fR it if it exists.  For example, the Macintosh \fBwish\fR
application has the variable is set to \fBtclshrc\fR.
.TP
\fBtcl_traceCompile\fR
The value of this variable can be set to control
how much tracing information
is displayed during bytecode compilation.
By default, tcl_traceCompile is zero and no information is displayed.
Setting tcl_traceCompile to 1 generates a one line summary in stdout
whenever a procedure or top level command is compiled.
Setting it to 2 generates a detailed listing in stdout of the
bytecode instructions emitted during every compilation.
This variable is useful in
tracking down suspected problems with the Tcl compiler.
It is also occasionally useful when converting
existing code to use Tcl8.0.

This variable and functionality only exist if
TCL_COMPILE_DEBUG was defined during Tcl's compilation.
.TP
\fBtcl_traceExec\fR
The value of this variable can be set to control
how much tracing information
is displayed during bytecode execution.
By default, tcl_traceExec is zero and no information is displayed.
Setting tcl_traceExec to 1 generates a one line trace in stdout
on each call to a Tcl procedure.
Setting it to 2 generates a line of output
whenever any Tcl command is invoked
that contains the name of the command and its arguments.
Setting it to 3 produces a detailed trace showing the result of
executing each bytecode instruction.
Note that when tcl_traceExec is 2 or 3,
commands such as set and incr
that have been entirely replaced by a sequence
of bytecode instructions are not shown.
Setting this variable is useful in
tracking down suspected problems with the bytecode compiler
and interpreter.
It is also occasionally useful when converting
code to use Tcl8.0.

This variable and functionality only exist if
TCL_COMPILE_DEBUG was defined during Tcl's compilation.
.TP
\fBtcl_wordchars\fR
The value of this variable is a regular expression that can be set to
control what are considered ``word'' characters, for instances like
selecting a word by double-clicking in text in Tk.  It is platform
dependent.  On Windows, it defaults to \fB\\S\fR, meaning anything
but a Unicode space character.  Otherwise it defaults to \fB\\w\fR,
which is any Unicode word character (number, letter, or underscore).
.TP
\fBtcl_nonwordchars\fR
The value of this variable is a regular expression that can be set to
control what are considered ``non-word'' characters, for instances like
selecting a word by double-clicking in text in Tk.  It is platform
dependent.  On Windows, it defaults to \fB\\s\fR, meaning any Unicode space
character.  Otherwise it defaults to \fB\\W\fR, which is anything but a
Unicode word character (number, letter, or underscore).
.TP
\fBtcl_version\fR
When an interpreter is created Tcl initializes this variable to
hold the version number for this version of Tcl in the form \fIx.y\fR.
Changes to \fIx\fR represent major changes with probable
incompatibilities and changes to \fIy\fR represent small enhancements and
bug fixes that retain backward compatibility.
The value of this variable is returned by the \fBinfo tclversion\fR
command.

.SH "SEE ALSO"
eval(n)

.SH KEYWORDS
arithmetic, bytecode, compiler, error, environment, POSIX, precision, subprocess, variables