1tclvars(n) Tcl Built-In Commands tclvars(n)
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8 tclvars - Variables used by Tcl
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12 The following global variables are created and managed automatically by
13 the Tcl library. Except where noted below, these variables should nor‐
14 mally be treated as read-only by application-specific code and by
15 users.
16 env This variable is maintained by Tcl as an array whose elements
18 are the environment variables for the process. Reading an ele‐
19 ment will return the value of the corresponding environment
20 variable. Setting an element of the array will modify the cor‐
21 responding environment variable or create a new one if it does
22 not already exist. Unsetting an element of env will remove the
23 corresponding environment variable. Changes to the env array
24 will affect the environment passed to children by commands like
25 exec. If the entire env array is unset then Tcl will stop moni‐
26 toring env accesses and will not update environment variables.
27 Under Windows, the environment variables PATH and COMSPEC in any
29 capitalization are converted automatically to upper case. For
30 instance, the PATH variable could be exported by the operating
31 system as “path”, “Path”, “PaTh”, etc., causing otherwise simple
32 Tcl code to have to support many special cases. All other envi‐
33 ronment variables inherited by Tcl are left unmodified. Setting
34 an env array variable to blank is the same as unsetting it as
35 this is the behavior of the underlying Windows OS. It should be
36 noted that relying on an existing and empty environment variable
37 will not work on Windows and is discouraged for cross-platform
38 usage.
39 The following elements of env are special to Tcl:
41 env(HOME)
43 This environment variable, if set, gives the location of
44 the directory considered to be the current user's home
45 directory, and to which a call of cd without arguments or
46 with just “~” as an argument will change into. Most plat‐
47 forms set this correctly by default; it does not normally
48 need to be set by user code.
49 env(TCL_LIBRARY)
51 If set, then it specifies the location of the directory
52 containing library scripts (the value of this variable
53 will be assigned to the tcl_library variable and there‐
54 fore returned by the command info library). If this
55 variable is not set then a default value is used.
56 Note that this environment variable should not normally
58 be set.
59 env(TCLLIBPATH)
61 If set, then it must contain a valid Tcl list giving
62 directories to search during auto-load operations.
63 Directories must be specified in Tcl format, using “/” as
64 the path separator, regardless of platform. This vari‐
65 able is only used when initializing the auto_path vari‐
66 able.
67 env(TCL_INTERP_DEBUG_FRAME)
69 If existing, it has the same effect as running interp
70 debug {} -frame 1 as the very first command of each new
71 Tcl interpreter.
72 errorCode
74 This variable holds the value of the -errorcode return option
75 set by the most recent error that occurred in this interpreter.
76 This list value represents additional information about the
77 error in a form that is easy to process with programs. The
78 first element of the list identifies a general class of errors,
79 and determines the format of the rest of the list. The follow‐
80 ing formats for -errorcode return options are used by the Tcl
81 core; individual applications may define additional formats.
82 ARITH code msg
84 This format is used when an arithmetic error occurs (e.g.
85 an attempt to divide zero by zero in the expr command).
86 Code identifies the precise error and msg provides a
87 human-readable description of the error. Code will be
88 either DIVZERO (for an attempt to divide by zero), DOMAIN
89 (if an argument is outside the domain of a function, such
90 as acos(-3)), IOVERFLOW (for integer overflow), OVERFLOW
91 (for a floating-point overflow), or UNKNOWN (if the cause
92 of the error cannot be determined).
93 Detection of these errors depends in part on the underly‐
95 ing hardware and system libraries.
96 CHILDKILLED pid sigName msg
98 This format is used when a child process has been killed
99 because of a signal. The pid element will be the
100 process's identifier (in decimal). The sigName element
101 will be the symbolic name of the signal that caused the
102 process to terminate; it will be one of the names from
103 the include file signal.h, such as SIGPIPE. The msg ele‐
104 ment will be a short human-readable message describing
105 the signal, such as “write on pipe with no readers” for
106 SIGPIPE.
107 CHILDSTATUS pid code
109 This format is used when a child process has exited with
110 a non-zero exit status. The pid element will be the
111 process's identifier (in decimal) and the code element
112 will be the exit code returned by the process (also in
113 decimal).
114 CHILDSUSP pid sigName msg
116 This format is used when a child process has been sus‐
117 pended because of a signal. The pid element will be the
118 process's identifier, in decimal. The sigName element
119 will be the symbolic name of the signal that caused the
120 process to suspend; this will be one of the names from
121 the include file signal.h, such as SIGTTIN. The msg ele‐
122 ment will be a short human-readable message describing
123 the signal, such as “background tty read” for SIGTTIN.
124 NONE This format is used for errors where no additional infor‐
126 mation is available for an error besides the message
127 returned with the error. In these cases the -errorcode
128 return option will consist of a list containing a single
129 element whose contents are NONE.
130 POSIX errName msg
132 If the first element is POSIX, then the error occurred
133 during a POSIX kernel call. The errName element will
134 contain the symbolic name of the error that occurred,
135 such as ENOENT; this will be one of the values defined in
136 the include file errno.h. The msg element will be a
137 human-readable message corresponding to errName, such as
138 “no such file or directory” for the ENOENT case.
139 To set the -errorcode return option, applications should use
141 library procedures such as Tcl_SetObjErrorCode, Tcl_SetReturnOp‐
142 tions, and Tcl_PosixError, or they may invoke the -errorcode
143 option of the return command. If none of these methods for set‐
144 ting the error code has been used, the Tcl interpreter will
145 reset the variable to NONE after the next error.
146 errorInfo
148 This variable holds the value of the -errorinfo return option
149 set by the most recent error that occurred in this interpreter.
150 This string value will contain one or more lines identifying the
151 Tcl commands and procedures that were being executed when the
152 most recent error occurred. Its contents take the form of a
153 stack trace showing the various nested Tcl commands that had
154 been invoked at the time of the error.
155 tcl_library
157 This variable holds the name of a directory containing the sys‐
158 tem library of Tcl scripts, such as those used for auto-loading.
159 The value of this variable is returned by the info library com‐
160 mand. See the library manual entry for details of the facili‐
161 ties provided by the Tcl script library. Normally each applica‐
162 tion or package will have its own application-specific script
163 library in addition to the Tcl script library; each application
164 should set a global variable with a name like $app_library
165 (where app is the application's name) to hold the network file
166 name for that application's library directory. The initial
167 value of tcl_library is set when an interpreter is created by
168 searching several different directories until one is found that
169 contains an appropriate Tcl startup script. If the TCL_LIBRARY
170 environment variable exists, then the directory it names is
171 checked first. If TCL_LIBRARY is not set or doesn't refer to an
172 appropriate directory, then Tcl checks several other directories
173 based on a compiled-in default location, the location of the
174 binary containing the application, and the current working
175 directory.
176 tcl_patchLevel
178 When an interpreter is created Tcl initializes this variable to
179 hold a string giving the current patch level for Tcl, such as
180 8.4.16 for Tcl 8.4 with the first sixteen official patches, or
181 8.5b3 for the third beta release of Tcl 8.5. The value of this
182 variable is returned by the info patchlevel command.
183 tcl_pkgPath
185 This variable holds a list of directories indicating where pack‐
186 ages are normally installed. It is not used on Windows. It
187 typically contains either one or two entries; if it contains two
188 entries, the first is normally a directory for platform-depen‐
189 dent packages (e.g., shared library binaries) and the second is
190 normally a directory for platform-independent packages (e.g.,
191 script files). Typically a package is installed as a subdirec‐
192 tory of one of the entries in $tcl_pkgPath. The directories in
193 $tcl_pkgPath are included by default in the auto_path variable,
194 so they and their immediate subdirectories are automatically
195 searched for packages during package require commands. Note:
196 tcl_pkgPath is not intended to be modified by the application.
197 Its value is added to auto_path at startup; changes to tcl_pkg‐
198 Path are not reflected in auto_path. If you want Tcl to search
199 additional directories for packages you should add the names of
200 those directories to auto_path, not tcl_pkgPath.
201 tcl_platform
203 This is an associative array whose elements contain information
204 about the platform on which the application is running, such as
205 the name of the operating system, its current release number,
206 and the machine's instruction set. The elements listed below
207 will always be defined, but they may have empty strings as val‐
208 ues if Tcl could not retrieve any relevant information. In
209 addition, extensions and applications may add additional values
210 to the array. The predefined elements are:
211 byteOrder
213 The native byte order of this machine: either littleEn‐
214 dian or bigEndian.
215 debug If this variable exists, then the interpreter was com‐
217 piled with and linked to a debug-enabled C run-time.
218 This variable will only exist on Windows, so extension
219 writers can specify which package to load depending on
220 the C run-time library that is in use. This is not an
221 indication that this core contains symbols.
222 machine
224 The instruction set executed by this machine, such as
225 intel, PPC, 68k, or sun4m. On UNIX machines, this is the
226 value returned by uname -m.
227 os The name of the operating system running on this machine,
229 such as Windows 95, Windows NT, or SunOS. On UNIX
230 machines, this is the value returned by uname -s. On
231 Windows 95 and Windows 98, the value returned will be
232 Windows 95 to provide better backwards compatibility to
233 Windows 95; to distinguish between the two, check the
234 osVersion.
235 osVersion
237 The version number for the operating system running on
238 this machine. On UNIX machines, this is the value
239 returned by uname -r. On Windows 95, the version will be
240 4.0; on Windows 98, the version will be 4.10.
241 platform
243 Either windows, or unix. This identifies the general
244 operating environment of the machine.
245 threaded
247 If this variable exists, then the interpreter was com‐
248 piled with threads enabled.
249 user This identifies the current user based on the login
251 information available on the platform. This comes from
252 the USER or LOGNAME environment variable on Unix, and the
253 value from GetUserName on Windows.
254 wordSize
256 This gives the size of the native-machine word in bytes
257 (strictly, it is same as the result of evaluating
258 sizeof(long) in C.)
259 pointerSize
261 This gives the size of the native-machine pointer in
262 bytes (strictly, it is same as the result of evaluating
263 sizeof(void*) in C.)
264 tcl_precision
266 This variable controls the number of digits to generate when
267 converting floating-point values to strings. It defaults to 0.
268 Applications should not change this value; it is provided for
269 compatibility with legacy code.
270 The default value of 0 is special, meaning that Tcl should con‐
272 vert numbers using as few digits as possible while still distin‐
273 guishing any floating point number from its nearest neighbours.
274 It differs from using an arbitrarily high value for tcl_preci‐
275 sion in that an inexact number like 1.4 will convert as 1.4
276 rather than 1.3999999999999999 even though the latter is nearer
277 to the exact value of the binary number.
278 If tcl_precision is not zero, then when Tcl converts a floating
280 point number, it creates a decimal representation of at most
281 tcl_precision significant digits; the result may be shorter if
282 the shorter result represents the original number exactly. If no
283 result of at most tcl_precision digits is an exact representa‐
284 tion of the original number, the one that is closest to the
285 original number is chosen. If the original number lies pre‐
286 cisely between two equally accurate decimal representations,
287 then the one with an even value for the least significant digit
288 is chosen; for instance, if tcl_precision is 3, then 0.3125 will
289 convert to 0.312, not 0.313, while 0.6875 will convert to 0.688,
290 not 0.687. Any string of trailing zeroes that remains is
291 trimmed.
292 a tcl_precision value of 17 digits is “perfect” for IEEE float‐
294 ing-point in that it allows double-precision values to be con‐
295 verted to strings and back to binary with no loss of informa‐
296 tion. For this reason, you will often see it as a value in
297 legacy code that must run on Tcl versions before 8.5. It is no
298 longer recommended; as noted above, a zero value is the pre‐
299 ferred method.
300 All interpreters in a thread share a single tcl_precision value:
302 changing it in one interpreter will affect all other inter‐
303 preters as well. Safe interpreters are not allowed to modify
304 the variable.
305 Valid values for tcl_precision range from 0 to 17.
307 tcl_rcFileName
309 This variable is used during initialization to indicate the name
310 of a user-specific startup file. If it is set by application-
311 specific initialization, then the Tcl startup code will check
312 for the existence of this file and source it if it exists. For
313 example, for wish the variable is set to ~/.wishrc for Unix and
314 ~/wishrc.tcl for Windows.
315 tcl_traceCompile
317 The value of this variable can be set to control how much trac‐
318 ing information is displayed during bytecode compilation. By
319 default, tcl_traceCompile is zero and no information is dis‐
320 played. Setting tcl_traceCompile to 1 generates a one-line sum‐
321 mary in stdout whenever a procedure or top-level command is com‐
322 piled. Setting it to 2 generates a detailed listing in stdout
323 of the bytecode instructions emitted during every compilation.
324 This variable is useful in tracking down suspected problems with
325 the Tcl compiler.
326 This variable and functionality only exist if TCL_COMPILE_DEBUG
328 was defined during Tcl's compilation.
329 tcl_traceExec
331 The value of this variable can be set to control how much trac‐
332 ing information is displayed during bytecode execution. By
333 default, tcl_traceExec is zero and no information is displayed.
334 Setting tcl_traceExec to 1 generates a one-line trace in stdout
335 on each call to a Tcl procedure. Setting it to 2 generates a
336 line of output whenever any Tcl command is invoked that contains
337 the name of the command and its arguments. Setting it to 3 pro‐
338 duces a detailed trace showing the result of executing each
339 bytecode instruction. Note that when tcl_traceExec is 2 or 3,
340 commands such as set and incr that have been entirely replaced
341 by a sequence of bytecode instructions are not shown. Setting
342 this variable is useful in tracking down suspected problems with
343 the bytecode compiler and interpreter.
344 This variable and functionality only exist if TCL_COMPILE_DEBUG
346 was defined during Tcl's compilation.
347 tcl_wordchars
349 The value of this variable is a regular expression that can be
350 set to control what are considered “word” characters, for
351 instances like selecting a word by double-clicking in text in
352 Tk. It is platform dependent. On Windows, it defaults to \S,
353 meaning anything but a Unicode space character. Otherwise it
354 defaults to \w, which is any Unicode word character (number,
355 letter, or underscore).
356 tcl_nonwordchars
358 The value of this variable is a regular expression that can be
359 set to control what are considered “non-word” characters, for
360 instances like selecting a word by double-clicking in text in
361 Tk. It is platform dependent. On Windows, it defaults to \s,
362 meaning any Unicode space character. Otherwise it defaults to
363 \W, which is anything but a Unicode word character (number, let‐
364 ter, or underscore).
365 tcl_version
367 When an interpreter is created Tcl initializes this variable to
368 hold the version number for this version of Tcl in the form x.y.
369 Changes to x represent major changes with probable incompatibil‐
370 ities and changes to y represent small enhancements and bug
371 fixes that retain backward compatibility. The value of this
372 variable is returned by the info tclversion command.
373
375 The following variables are only guaranteed to exist in tclsh and wish
376 executables; the Tcl library does not define them itself but many Tcl
377 environments do.
378 argc The number of arguments to tclsh or wish.
380 argv Tcl list of arguments to tclsh or wish.
382 argv0 The script that tclsh or wish started executing (if it was speci‐
384 fied) or otherwise the name by which tclsh or wish was invoked.
385 tcl_interactive
387 Contains 1 if tclsh or wish is running interactively (no script
388 was specified and standard input is a terminal-like device), 0
389 otherwise.
390 The wish executable additionally specifies the following global vari‐
392 able:
393 geometry
395 If set, contains the user-supplied geometry specification to use
396 for the main Tk window.
397
399 eval(n), tclsh(1), wish(1)
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402 arithmetic, bytecode, compiler, error, environment, POSIX, precision,
403 subprocess, variables
404Tcl 8.0 tclvars(n)