Tcl_AddErrorInfo(3) Tcl (7.5) Tcl_AddErrorInfo(3)
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NAME
Tcl_AddObjErrorInfo, Tcl_AddErrorInfo, Tcl_SetErrorCode,
Tcl_PosixError - record information about errors
SYNOPSIS
#include <tcl.h>
Tcl_AddObjErrorInfo(interp, message, length)
Tcl_AddErrorInfo(interp, message)
Tcl_SetObjErrorCode(interp, errorObjPtr)
Tcl_SetErrorCode(interp, element, element, ... (char *) NULL)
char *
Tcl_PosixError(interp)
ARGUMENTS
Tcl_Interp *interp (in) Interpreter in which to
record information.
char *message (in) For Tcl_AddObjErrorInfo,
this points to the first
byte of an array of bytes
containing a string to
record in the errorInfo
variable. This byte array
may contain embedded null
bytes unless length is
negative. For
Tcl_AddErrorInfo, this is
a conventional C string to
record in the errorInfo
variable.
int length (in) The number of bytes to
copy from message when
setting the errorInfo
variable. If negative,
all bytes up to the first
null byte are used.
Tcl_Obj *errorObjPtr(in) This variable errorCode
will be set to this value.
char *element (in) String to record as one
element of errorCode
variable. Last element
argument must be NULL.
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Tcl_AddErrorInfo(3) Tcl (7.5) Tcl_AddErrorInfo(3)
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DESCRIPTION
These procedures are used to manipulate two Tcl global
variables that hold information about errors. The variable
errorInfo holds a stack trace of the operations that were in
progress when an error occurred, and is intended to be
human-readable. The variable errorCode holds a list of
items that are intended to be machine-readable. The first
item in errorCode identifies the class of error that
occurred (e.g. POSIX means an error occurred in a POSIX
system call) and additional elements in errorCode hold
additional pieces of information that depend on the class.
See the Tcl overview manual entry for details on the various
formats for errorCode.
The errorInfo variable is gradually built up as an error
unwinds through the nested operations. Each time an error
code is returned to Tcl_EvalObj (or Tcl_Eval, which calls
Tcl_EvalObj) it calls the procedure Tcl_AddObjErrorInfo to
add additional text to errorInfo 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
errorInfo beyond what can be supplied automatically by
Tcl_EvalObj. Tcl_AddObjErrorInfo may be used for this
purpose: its message and length arguments describe an
additional string to be appended to errorInfo. For example,
the source command calls Tcl_AddObjErrorInfo to record the
name of the file being processed and the line number on
which the error occurred; for Tcl procedures, the procedure
name and line number within the procedure are recorded, and
so on. The best time to call Tcl_AddObjErrorInfo is just
after Tcl_EvalObj has returned TCL_ERROR. In calling
Tcl_AddObjErrorInfo, you may find it useful to use the
errorLine field of the interpreter (see the Tcl_Interp
manual entry for details).
Tcl_AddErrorInfo resembles Tcl_AddObjErrorInfo but differs
in initializing errorInfo from the string value of the
interpreter's result if the error is just starting to be
logged. It does not use the result as a Tcl object so any
embedded null characters in the result will cause
information to be lost. It also takes a conventional C
string in message instead of Tcl_AddObjErrorInfo's counted
string.
The procedure Tcl_SetObjErrorCode is used to set the
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Tcl_AddErrorInfo(3) Tcl (7.5) Tcl_AddErrorInfo(3)
errorCode variable. errorObjPtr contains a list object built
up by the caller. errorCode is set to this value.
Tcl_SetObjErrorCode is typically invoked just before
returning an error in an object command. If an error is
returned without calling Tcl_SetObjErrorCode or
Tcl_SetErrorCode the Tcl interpreter automatically sets
errorCode to NONE.
The procedure Tcl_SetErrorCode is also used to set the
errorCode variable. However, it takes one or more strings to
record instead of an object. Otherwise, it is similar to
Tcl_SetObjErrorCode in behavior.
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 errorCode 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 errorCode). It may be convenient to
include this string as part of the error message returned to
the application in the interpreter's result.
It is important to call the procedures described here rather
than setting errorInfo or errorCode directly with
Tcl_ObjSetVar2. The reason for this is that the Tcl
interpreter keeps information about whether these procedures
have been called. For example, the first time
Tcl_AddObjErrorInfo is called for an error, it clears the
existing value of errorInfo and adds the error message in
the interpreter's result to the variable before appending
message; in subsequent calls, it just appends the new
message. When Tcl_SetErrorCode is called, it sets a flag
indicating that errorCode has been set; this allows the Tcl
interpreter to set errorCode to NONE if it receives an error
return when Tcl_SetErrorCode hasn't been called.
If the procedure Tcl_ResetResult is called, it clears all of
the state associated with errorInfo and errorCode (but it
doesn't actually modify the variables). If an error had
occurred, this will clear the error state to make it appear
as if no error had occurred after all.
SEE ALSO
Tcl_DecrRefCount, Tcl_IncrRefCount, Tcl_Interp,
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Tcl_AddErrorInfo(3) Tcl (7.5) Tcl_AddErrorInfo(3)
Tcl_ResetResult, Tcl_SetErrno
KEYWORDS
error, object, object result, stack, trace, variable
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