Provided by: tcl9.0-doc_9.0.1+dfsg-1_all 
NAME
Tcl_GetReturnOptions, Tcl_SetReturnOptions, Tcl_AddErrorInfo, Tcl_AppendObjToErrorInfo,
Tcl_AddObjErrorInfo, Tcl_SetObjErrorCode, Tcl_SetErrorCode, Tcl_SetErrorLine, Tcl_GetErrorLine,
Tcl_PosixError, Tcl_LogCommandInfo - retrieve or record information about errors and other return options
SYNOPSIS
#include <tcl.h>
Tcl_Obj *
Tcl_GetReturnOptions(interp, code)
int
Tcl_SetReturnOptions(interp, options)
Tcl_AddErrorInfo(interp, message)
Tcl_AppendObjToErrorInfo(interp, objPtr)
Tcl_AddObjErrorInfo(interp, message, length)
Tcl_SetObjErrorCode(interp, errorObjPtr)
Tcl_SetErrorCode(interp, element, element, ... (char *)NULL)
int
Tcl_GetErrorLine(interp)
Tcl_SetErrorLine(interp, lineNum)
const char *
Tcl_PosixError(interp)
Tcl_LogCommandInfo(interp, script, command, commandLength)
ARGUMENTS
Tcl_Interp *interp (in) Interpreter in which to record information.
int code The code returned from script evaluation.
Tcl_Obj *options A dictionary of return options.
const char *message (in) For Tcl_AddErrorInfo, this is a conventional C string to append to
the -errorinfo return option. For Tcl_AddObjErrorInfo, this
points to the first byte of an array of length bytes containing a
string to append to the -errorinfo return option. This byte array
may contain embedded null bytes unless length is negative.
Tcl_Obj *objPtr (in) A message to be appended to the -errorinfo return option in the
form of a Tcl_Obj value.
Tcl_Size length (in) The number of bytes to copy from message when appending to the
-errorinfo return option. If negative, all bytes up to the first
null byte are used.
Tcl_Obj *errorObjPtr (in) The -errorcode return option will be set to this value.
const char *element (in) String to record as one element of the -errorcode return option.
Last element argument must be (char *)NULL.
int lineNum The line number of a script where an error occurred.
const char *script (in) Pointer to first character in script containing command (must be
<= command).
const char *command (in) Pointer to first character in the command that generated the
error; must point within the string given by script.
Tcl_Size commandLength (in) Number of bytes in command; a negative value means use all bytes
up to the first null byte.
________________________________________________________________________________________________________________
DESCRIPTION
The Tcl_SetReturnOptions and Tcl_GetReturnOptions routines expose the same capabilities as the return and
catch commands, respectively, in the form of a C interface.
Tcl_GetReturnOptions retrieves the dictionary of return options from an interpreter following a script
evaluation. Routines such as Tcl_Eval are called to evaluate a script in an interpreter. These routines
return an integer completion code. These routines also leave in the interpreter both a result and a
dictionary of return options generated by script evaluation. Just as Tcl_GetObjResult retrieves the
result, Tcl_GetReturnOptions retrieves the dictionary of return options. The integer completion code
should be passed as the code argument to Tcl_GetReturnOptions so that all required options will be
present in the dictionary. Specifically, a code value of TCL_ERROR will ensure that entries for the keys
-errorinfo, -errorcode, and -errorline will appear in the dictionary. Also, the entries for the keys
-code and -level will be adjusted if necessary to agree with the value of code. The (Tcl_Obj *) returned
by Tcl_GetReturnOptions points to an unshared Tcl_Obj with reference count of zero. The dictionary may
be written to, either adding, removing, or overwriting any entries in it, without the need to check for a
shared value. As with any Tcl_Obj with reference count of zero, it is up to the caller to arrange for
its disposal with Tcl_DecrRefCount or to a reference to it via Tcl_IncrRefCount (or one of the many
functions that call that, notably including Tcl_SetObjResult and Tcl_SetVar2Ex).
A typical usage for Tcl_GetReturnOptions is to retrieve the stack trace when script evaluation returns
TCL_ERROR, like so:
int code = Tcl_EvalEx(interp, script, -1, 0);
if (code == TCL_ERROR) {
Tcl_Obj *options = Tcl_GetReturnOptions(interp, code);
Tcl_Obj *key = Tcl_NewStringObj("-errorinfo", -1);
Tcl_Obj *stackTrace;
Tcl_IncrRefCount(key);
Tcl_DictObjGet(NULL, options, key, &stackTrace);
Tcl_DecrRefCount(key);
/* Do something with stackTrace */
Tcl_DecrRefCount(options);
}
Tcl_SetReturnOptions sets the return options of interp to be options. If options contains any invalid
value for any key, TCL_ERROR will be returned, and the interp result will be set to an appropriate error
message. Otherwise, a completion code in agreement with the -code and -level keys in options will be
returned.
As an example, Tcl's return command itself could be implemented in terms of Tcl_SetReturnOptions like so:
if ((objc % 2) == 0) { /* explicit result argument */
objc--;
Tcl_SetObjResult(interp, objv[objc]);
}
return Tcl_SetReturnOptions(interp, Tcl_NewListObj(objc-1, objv+1));
(It is not really implemented that way. Internal access privileges allow for a more efficient
alternative that meshes better with the bytecode compiler.)
Note that a newly created Tcl_Obj may be passed in as the options argument without the need to tend to
any reference counting. This is analogous to Tcl_SetObjResult.
While Tcl_SetReturnOptions provides a general interface to set any collection of return options, there
are a handful of return options that are very frequently used. Most notably the -errorinfo and
-errorcode return options should be set properly when the command procedure of a command returns
TCL_ERROR. The -errorline return option is also read by commands that evaluate scripts and wish to
supply detailed error location information in the stack trace text they append to the -errorinfo option.
Tcl provides several simpler interfaces to more directly set these return options.
The -errorinfo option holds a stack trace of the operations that were in progress when an error occurred,
and is intended to be human-readable. The -errorcode option holds a Tcl list of items that are intended
to be machine-readable. The first item in the -errorcode value identifies the class of error that
occurred (e.g., POSIX means an error occurred in a POSIX system call) and additional elements hold
additional pieces of information that depend on the class. See the manual entry on the errorCode
variable for details on the various formats for the -errorcode option used by Tcl's built-in commands.
The -errorinfo option value is gradually built up as an error unwinds through the nested operations.
Each time an error code is returned to Tcl_Eval, or any of the routines that performs script evaluation,
the procedure Tcl_AddErrorInfo is called to add additional text to the -errorinfo value describing the
command that was being executed when the error occurred. By the time the error has been passed all the
way back to the application, it will contain a complete trace of the activity in progress when the error
occurred.
It is sometimes useful to add additional information to the -errorinfo value beyond what can be supplied
automatically by the script evaluation routines. Tcl_AddErrorInfo may be used for this purpose: its
message argument is an additional string to be appended to the -errorinfo option. For example, when an
error arises during the source command, the procedure Tcl_AddErrorInfo is called to record the name of
the file being processed and the line number on which the error occurred. Likewise, when an error arises
during evaluation of a Tcl procedures, the procedure name and line number within the procedure are
recorded, and so on. The best time to call Tcl_AddErrorInfo is just after a script evaluation routine
has returned TCL_ERROR. The value of the -errorline return option (retrieved via a call to
Tcl_GetReturnOptions) often makes up a useful part of the message passed to Tcl_AddErrorInfo.
Tcl_AppendObjToErrorInfo is an alternative interface to the same functionality as Tcl_AddErrorInfo.
Tcl_AppendObjToErrorInfo is called when the string value to be appended to the -errorinfo option is
available as a Tcl_Obj instead of as a char array.
Tcl_AddObjErrorInfo is nearly identical to Tcl_AddErrorInfo, except that it has an additional length
argument. This allows the message string to contain embedded null bytes. This is essentially never a
good idea. If the message needs to contain the null character U+0000, Tcl's usual internal encoding
rules should be used to avoid the need for a null byte. If the Tcl_AddObjErrorInfo interface is used at
all, it should be with a negative length value.
The procedure Tcl_SetObjErrorCode is used to set the -errorcode return option to the list value
errorObjPtr built up by the caller. Tcl_SetObjErrorCode is typically invoked just before returning an
error. If an error is returned without calling Tcl_SetObjErrorCode or Tcl_SetErrorCode the Tcl
interpreter automatically sets the -errorcode return option to NONE.
The procedure Tcl_SetErrorCode is also used to set the -errorcode return option. However, it takes one or
more strings to record instead of a value. Otherwise, it is similar to Tcl_SetObjErrorCode in behavior.
The procedure Tcl_GetErrorLine is used to read the integer value of the -errorline return option without
the overhead of a full call to Tcl_GetReturnOptions. Likewise, Tcl_SetErrorLine sets the -errorline
return option value.
Tcl_PosixError sets the -errorcode variable after an error in a POSIX kernel call. It reads the value of
the errno C variable and calls Tcl_SetErrorCode to set the -errorcode return option in the POSIX format.
The caller must previously have called Tcl_SetErrno to set errno; this is necessary on some platforms
(e.g. Windows) where Tcl is linked into an application as a shared library, or when the error occurs in a
dynamically loaded extension. See the manual entry for Tcl_SetErrno for more information.
Tcl_PosixError returns a human-readable diagnostic message for the error (this is the same value that
will appear as the third element in the -errorcode value). It may be convenient to include this string
as part of the error message returned to the application in the interpreter's result.
Tcl_LogCommandInfo is invoked after an error occurs in an interpreter. It adds information about the
command that was being executed when the error occurred to the -errorinfo value, and the line number
stored internally in the interpreter is set.
In older releases of Tcl, there was no Tcl_GetReturnOptions routine. In its place, the global Tcl
variables errorInfo and errorCode were the only place to retrieve the error information. Much existing
code written for older Tcl releases still access this information via those global variables.
It is important to realize that while reading from those global variables remains a supported way to
access these return option values, it is important not to assume that writing to those global variables
will properly set the corresponding return options. It has long been emphasized in this manual page that
it is important to call the procedures described here rather than setting errorInfo or errorCode directly
with Tcl_ObjSetVar2.
If the procedure Tcl_ResetResult is called, it clears all of the state of the interpreter associated with
script evaluation, including the entire return options dictionary. In particular, the -errorinfo and
-errorcode options are reset. If an error had occurred, the Tcl_ResetResult call will clear the error
state to make it appear as if no error had occurred after all. The global variables errorInfo and
errorCode are not modified by Tcl_ResetResult so they continue to hold a record of information about the
most recent error seen in an interpreter.
REFERENCE COUNT MANAGEMENT
The result of Tcl_GetReturnOptions will have at least one reference to it from the Tcl interpreter. If
not using it immediately, you should use Tcl_IncrRefCount to add your own reference.
The options argument to Tcl_SetReturnOptions will have a reference added by the Tcl interpreter; it may
safely be called with a zero-reference value.
Tcl_AppendObjToErrorInfo only reads its objPtr argument; it does not modify its reference count at all.
The errorObjPtr argument to Tcl_SetObjErrorCode will have a reference added by the Tcl interpreter; it
may safely be called with a zero-reference value.
SEE ALSO
Tcl_DecrRefCount(3tcl), Tcl_IncrRefCount(3tcl), Tcl_Interp(3tcl), Tcl_ResetResult(3tcl),
Tcl_SetErrno(3tcl), errorCode(3tcl), errorInfo(3tcl)
KEYWORDS
error, value, value result, stack, trace, variable
Tcl 8.5 Tcl_AddErrorInfo(3tcl)