catch(n) Tcl Built-In Commands catch(n)
NAME
catch - Evaluate script and trap exceptional returns
SYNOPSIS
catch script ?resultVarName? ?optionsVarName?
DESCRIPTION
The catch command may be used to prevent errors from aborting command
interpretation. The catch command calls the Tcl interpreter recur-
sively to execute script, and always returns without raising an error,
regardless of any errors that might occur while executing script.
If script raises an error, catch will return a non-zero integer value
corresponding to the exceptional return code returned by evaluation of
script. Tcl defines the normal return code from script evaluation to
be zero (0), or TCL_OK. Tcl also defines four exceptional return
codes: 1 (TCL_ERROR), 2 (TCL_RETURN), 3 (TCL_BREAK), and 4 (TCL_CON-
TINUE). Errors during evaluation of a script are indicated by a return
code of TCL_ERROR. The other exceptional return codes are returned by
the return, break, and continue commands and in other special situa-
tions as documented. Tcl packages can define new commands that return
other integer values as return codes as well, and scripts that make use
of the return -code command can also have return codes other than the
five defined by Tcl.
If the resultVarName argument is given, then the variable it names is
set to the result of the script evaluation. When the return code from
the script is 1 (TCL_ERROR), the value stored in resultVarName is an
error message. When the return code from the script is 0 (TCL_OK), the
value stored in resultVarName is the value returned from script.
If the optionsVarName argument is given, then the variable it names is
set to a dictionary of return options returned by evaluation of script.
Tcl specifies two entries that are always defined in the dictionary:
-code and -level. When the return code from evaluation of script is
not TCL_RETURN, the value of the -level entry will be 0, and the value
of the -code entry will be the same as the return code. Only when the
return code is TCL_RETURN will the values of the -level and -code
entries be something else, as further described in the documentation
for the return command.
When the return code from evaluation of script is TCL_ERROR, four addi-
tional entries are defined in the dictionary of return options stored
in optionsVarName: -errorinfo, -errorcode, -errorline, and -errorstack.
The value of the -errorinfo entry is a formatted stack trace containing
more information about the context in which the error happened. The
formatted stack trace is meant to be read by a person. The value of
the -errorcode entry is additional information about the error stored
as a list. The -errorcode value is meant to be further processed by
programs, and may not be particularly readable by people. The value of
the -errorline entry is an integer indicating which line of script was
being evaluated when the error occurred. The value of the -errorstack
entry is an even-sized list made of token-parameter pairs accumulated
while unwinding the stack. The token may be in which case the parameter
is a list made of the proc name and arguments at the corresponding
level; or it may be in which case the parameter is the relative level
(as in uplevel) of the previous CALL. The salient differences with
respect to -errorinfo are that:
[1] it is a machine-readable form that is amenable to processing
with [foreach {tok prm} ...],
[2] it contains the true (substituted) values passed to the func-
tions, instead of the static text of the calling sites, and
[3] it is coarser-grained, with only one element per stack frame
(like procs; no separate elements for foreach constructs for
example).
The values of the -errorinfo and -errorcode entries of the most recent
error are also available as values of the global variables ::errorInfo
and ::errorCode respectively. The value of the -errorstack entry sur-
faces as info errorstack.
Tcl packages may provide commands that set other entries in the dictio-
nary of return options, and the return command may be used by scripts
to set return options in addition to those defined above.
EXAMPLES
The catch command may be used in an if to branch based on the success
of a script.
if { [catch {open $someFile w} fid] } {
puts stderr "Could not open $someFile for writing\n$fid"
exit 1 }
There are more complex examples of catch usage in the documentation for
the return command.
SEE ALSO
break(n), continue(n), dict(n), error(n), errorCode(n), errorInfo(n),
info(n), return(n)
KEYWORDS
catch, error, exception
Tcl 8.5 catch(n)
tcl 8.6.1 - Generated Mon Sep 30 16:44:49 CDT 2013