Volume 7A: XView Programming Manual

NULL);

break;

/* ... */

}

As you can see, unless you are implementing the internals of an XView package,

xv_error() may be of limited use. However, this example demonstrates how

xv_error() can be called internally by XView.

24.4.1.1 Error severity

In the above example, the reason to call xv_error() is not a serious one, at least not one

that should terminate the program. By default, calling xv_error() is a recoverable error,

so in order to specify a non-recoverable error, the call to xv_error() should pass the attri-

bute, ERROR_SEVERITY, and the value ERROR_NON_RECOVERABLE. Unrecoverable errors

generally terminate using exit() with a non-zero exit status. However, abort() may be

used instead to generate a core image used for debugging.

In any event, if the error handler is called, it should print a warning message and either con-

tinue or exit accordingly. If you install your own error routine, the choice is yours. Example

24-1 in the next section, shows how the error severity can be evaluated and acted upon

accordingly.

24.4.2 Revisiting the Error Handler

Advanced usage of the ERROR package provides us with the ability to scan the attribute list in

search of the causes of the error. With this information, you can print out more useful error

messages or display them in a manner other than printing to stderr.

Recall that there are two parameters passed to the function:

error_proc(object, avlist)

Xv_object object;

Attr_avlist avlist;

object is the object in which the xv_* call failed to operate. You can get the type of ob-

ject by calling:

Xv_pkg *pkg = (Xv_pkg *)xv_get(object, XV_TYPE);

If the error itself pertains to the object, then getting the type of the object may generate

another error. You should not attempt to get the package until you test the error code in the

Attr_avlist to be sure the error is not with the object parameter. If the error is not due

to the object itself, the package returned by xv_get() indicates to which XView package

the object belongs. This value matches the same argument as the second parameter to

xv_create(). For example, the package returned may be

MENU, CANVAS, PANEL_

BUTTON

, SERVER, etc.

572 XView Programming Manual

The Attr_avlist may be scanned for the ERROR attributes. Example 24-1 shows how

this is done. This is a very simplistic example and is for demonstration purposes only. A

more complete example may be found in the function xv_error_format() in the XView

source code.

Example 24-1. Example error parsing function

int

my_error_handler(object, avlist)

Xv_object object;

Attr_avlist avlist;

{

Attr_avlist attrs;

Error_layer layer;

Error_severity severity = ERROR_RECOVERABLE;

int n = 0;

char strs[64][7];

for (attrs = avlist; *attrs && n < 5; attrs = attr_next(attrs)) {

switch ((int) attrs[0]) {

case ERROR_BAD_ATTR:

sprintf(strs[n++], "bad attribute %s",

attr_name(attr[1]));

break;

case ERROR_BAD_VALUE:

sprintf(strs[n++],

"bad value (0x%x) for attribute %s", attrs[1],

attr_name(attrs[2]));

break;

case ERROR_INVALID_OBJECT:

sprintf(strs[n++], "invalid object (%s)",

(char *) attrs[1]);

break;

case ERROR_STRING:

sprintf(strs[n++], "%s", (char *) attrs[1]);

break;

case ERROR_PKG:

sprintf(strs[n++], "Package: %s",

((Xv_pkg *)attrs[1])->name);

break;

case ERROR_SEVERITY:

severity = attrs[1];

}

}

strcpy(strs[n++], "Core dump?");

strs[n] = 0;

if (notice_prompt(base_frame, (Event *)NULL,

NOTICE_MESSAGE_STRINGS_ARRAY_PTR, strs,

NOTICE_BUTTON_YES, "Yes",

NOTICE_BUTTON_NO, "No",

NULL) == NOTICE_YES)

abort();

if (severity == ERROR_NON_RECOVERABLE)

exit(1);

return XV_OK;

}

Error Recovery

Error Recovery 573

This error handling routine sets a set of error messages in an array of buffers. A notice is

used to display the messages and prompt the user to generate a core image of the program for

debugging. Selecting “Yes” causes abort() to be called. The program exits if the severity

is non-recoverable, and continues otherwise.

One particular note of interest is the use of the routine attr_name(). This is a hypotheti-

cal routine that you would have to write to convert the actual enumerated attribute-values

into strings that make sense to read. If you write your own XView package with new attri-

butes, you will have to write a routine equivalent to attr_name(). The routine should

return a static char * describing the attribute. If the attribute has no corresponding string

(e.g., it is an unknown attribute), it should return a string indicating the integer or hexade-

cimal value.

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