1catch(n) Tcl Built-In Commands catch(n)
2______________________________________________________________________________
6
8 catch - Evaluate script and trap exceptional returns
9
11 catch script ?resultVarName? ?optionsVarName?
12______________________________________________________________________________
13
15 The catch command may be used to prevent errors from aborting command
16 interpretation. The catch command calls the Tcl interpreter recur‐
17 sively to execute script, and always returns without raising an error,
18 regardless of any errors that might occur while executing script.
19 If script raises an error, catch will return a non-zero integer value
21 corresponding to the exceptional return code returned by evaluation of
22 script. Tcl defines the normal return code from script evaluation to
23 be zero (0), or TCL_OK. Tcl also defines four exceptional return
24 codes: 1 (TCL_ERROR), 2 (TCL_RETURN), 3 (TCL_BREAK), and 4 (TCL_CON‐
25 TINUE). Errors during evaluation of a script are indicated by a return
26 code of TCL_ERROR. The other exceptional return codes are returned by
27 the return, break, and continue commands and in other special situa‐
28 tions as documented. Tcl packages can define new commands that return
29 other integer values as return codes as well, and scripts that make use
30 of the return -code command can also have return codes other than the
31 five defined by Tcl.
32 If the resultVarName argument is given, then the variable it names is
34 set to the result of the script evaluation. When the return code from
35 the script is 1 (TCL_ERROR), the value stored in resultVarName is an
36 error message. When the return code from the script is 0 (TCL_OK), the
37 value stored in resultVarName is the value returned from script.
38 If the optionsVarName argument is given, then the variable it names is
40 set to a dictionary of return options returned by evaluation of script.
41 Tcl specifies two entries that are always defined in the dictionary:
42 -code and -level. When the return code from evaluation of script is
43 not TCL_RETURN, the value of the -level entry will be 0, and the value
44 of the -code entry will be the same as the return code. Only when the
45 return code is TCL_RETURN will the values of the -level and -code
46 entries be something else, as further described in the documentation
47 for the return command.
48 When the return code from evaluation of script is TCL_ERROR, four addi‐
50 tional entries are defined in the dictionary of return options stored
51 in optionsVarName: -errorinfo, -errorcode, -errorline, and -errorstack. │
52 The value of the -errorinfo entry is a formatted stack trace containing
53 more information about the context in which the error happened. The
54 formatted stack trace is meant to be read by a person. The value of
55 the -errorcode entry is additional information about the error stored
56 as a list. The -errorcode value is meant to be further processed by
57 programs, and may not be particularly readable by people. The value of
58 the -errorline entry is an integer indicating which line of script was
59 being evaluated when the error occurred. The value of the -errorstack │
60 entry is an even-sized list made of token-parameter pairs accumulated │
61 while unwinding the stack. The token may be “CALL”, in which case the │
62 parameter is a list made of the proc name and arguments at the corre‐ │
63 sponding level; or it may be “UP”, in which case the parameter is the │
64 relative level (as in uplevel) of the previous CALL. The salient dif‐ │
65 ferences with respect to -errorinfo are that: │
66 [1] │
68 it is a machine-readable form that is amenable to processing │
69 with [foreach {tok prm} ...], │
70 [2] │
72 it contains the true (substituted) values passed to the func‐ │
73 tions, instead of the static text of the calling sites, and │
74 [3] │
76 it is coarser-grained, with only one element per stack frame │
77 (like procs; no separate elements for foreach constructs for │
78 example).
79 The values of the -errorinfo and -errorcode entries of the most recent
81 error are also available as values of the global variables ::errorInfo
82 and ::errorCode respectively. The value of the -errorstack entry sur‐ │
83 faces as info errorstack.
84 Tcl packages may provide commands that set other entries in the dictio‐
86 nary of return options, and the return command may be used by scripts
87 to set return options in addition to those defined above.
88
90 The catch command may be used in an if to branch based on the success
91 of a script.
92 if { [catch {open $someFile w} fid] } {
94 puts stderr "Could not open $someFile for writing\n$fid"
95 exit 1
96 }
97 There are more complex examples of catch usage in the documentation for
99 the return command.
100
102 break(n), continue(n), dict(n), error(n), errorCode(n), errorInfo(n),
103 info(n), return(n)
104
106 catch, error, exception
107Tcl 8.5 catch(n)