NAME¶
Tcl_SetChannelError, Tcl_SetChannelErrorInterp, Tcl_GetChannelError,
Tcl_GetChannelErrorInterp - functions to create/intercept Tcl errors by
channel drivers.
SYNOPSIS¶
#include <tcl.h>
void
Tcl_SetChannelError(chan, msg)
void
Tcl_SetChannelErrorInterp(interp, msg)
void
Tcl_GetChannelError(chan, msgPtr)
void
Tcl_GetChannelErrorInterp(interp, msgPtr)
ARGUMENTS¶
- Tcl_Channel chan (in)
- Refers to the Tcl channel whose bypass area is accessed.
- Tcl_Interp* interp (in)
- Refers to the Tcl interpreter whose bypass area is accessed.
- Tcl_Obj* msg (in)
- Error message put into a bypass area. A list of return options and values,
followed by a string message. Both message and the option/value
information are optional.
- Tcl_Obj** msgPtr (out)
- Reference to a place where the message stored in the accessed bypass area
can be stored in.
DESCRIPTION¶
The current definition of a Tcl channel driver does not permit the direct return
of arbitrary error messages, except for the setting and retrieval of channel
options. All other functions are restricted to POSIX error codes.
The functions described here overcome this limitation. Channel drivers are
allowed to use
Tcl_SetChannelError and
Tcl_SetChannelErrorInterp
to place arbitrary error messages in
bypass areas defined for channels
and interpreters. And the generic I/O layer uses
Tcl_GetChannelError
and
Tcl_GetChannelErrorInterp to look for messages in the bypass areas
and arrange for their return as errors. The POSIX error codes set by a driver
are used now if and only if no messages are present.
Tcl_SetChannelError stores error information in the bypass area of the
specified channel. The number of references to the
msg value goes up by
one. Previously stored information will be discarded, by releasing the
reference held by the channel. The channel reference must not be NULL.
Tcl_SetChannelErrorInterp stores error information in the bypass area of
the specified interpreter. The number of references to the
msg value
goes up by one. Previously stored information will be discarded, by releasing
the reference held by the interpreter. The interpreter reference must not be
NULL.
Tcl_GetChannelError places either the error message held in the bypass
area of the specified channel into
msgPtr, or NULL; and resets the
bypass, that is, after an invocation all following invocations will return
NULL, until an intervening invocation of
Tcl_SetChannelError with a
non-NULL message. The
msgPtr must not be NULL. The reference count of
the message is not touched. The reference previously held by the channel is
now held by the caller of the function and it is its responsibility to release
that reference when it is done with the value.
Tcl_GetChannelErrorInterp places either the error message held in the
bypass area of the specified interpreter into
msgPtr, or NULL; and
resets the bypass, that is, after an invocation all following invocations will
return NULL, until an intervening invocation of
Tcl_SetChannelErrorInterp with a non-NULL message. The
msgPtr
must not be NULL. The reference count of the message is not touched. The
reference previously held by the interpreter is now held by the caller of the
function and it is its responsibility to release that reference when it is
done with the value.
Which functions of a channel driver are allowed to use which bypass function is
listed below, as is which functions of the public channel API may leave a
messages in the bypass areas.
- Tcl_DriverCloseProc
- May use Tcl_SetChannelErrorInterp, and only this function.
- Tcl_DriverInputProc
- May use Tcl_SetChannelError, and only this function.
- Tcl_DriverOutputProc
- May use Tcl_SetChannelError, and only this function.
- Tcl_DriverSeekProc
- May use Tcl_SetChannelError, and only this function.
- Tcl_DriverWideSeekProc
- May use Tcl_SetChannelError, and only this function.
- Tcl_DriverSetOptionProc
- Has already the ability to pass arbitrary error messages. Must not
use any of the new functions.
- Tcl_DriverGetOptionProc
- Has already the ability to pass arbitrary error messages. Must not
use any of the new functions.
- Tcl_DriverWatchProc
- Must not use any of the new functions. Is internally called and has
no ability to return any type of error whatsoever.
- Tcl_DriverBlockModeProc
- May use Tcl_SetChannelError, and only this function.
- Tcl_DriverGetHandleProc
- Must not use any of the new functions. It is only a low-level
function, and not used by Tcl commands.
- Tcl_DriverHandlerProc
- Must not use any of the new functions. Is internally called and has
no ability to return any type of error whatsoever.
Given the information above the following public functions of the Tcl C API are
affected by these changes; when these functions are called, the channel may
now contain a stored arbitrary error message requiring processing by the
caller.
Tcl_Flush Tcl_GetsObj Tcl_Gets
Tcl_ReadChars Tcl_ReadRaw Tcl_Read
Tcl_Seek Tcl_StackChannel Tcl_Tell
Tcl_WriteChars Tcl_WriteObj Tcl_WriteRaw
Tcl_Write
All other API functions are unchanged. In particular, the functions below leave
all their error information in the interpreter result.
Tcl_Close Tcl_UnstackChannel Tcl_UnregisterChannel
SEE ALSO¶
Tcl_Close(3tcl), Tcl_OpenFileChannel(3tcl), Tcl_SetErrno(3tcl)
KEYWORDS¶
channel driver, error messages, channel type