Conference bulova::decw_jan-89_to_nov-90

2996.0. "Reserved Operand Fault w/ Compound Strings" by NITMOI::WITHERS (Another Hallmark Moment. -Al Bundy) Tue Jun 26 1990 17:13

Help.  I am having trouble with strings in DECwindows.  I happen upon a
Reserved Operand Fault.  The problem is it comes at strange intervals and
happens on different commands.  Any help would be appreciated.

What I am trying to do is take the text from a simple_text widget, append
a keyword onto it, and then Add/Remove the result from a list box.  This
is my first UI design attempt and so it could be an obvious mistake.  Please
help!

The code in question:  The fault has occured at varying times at either line
#1, #2, or #3.  It is most often seen in #1 and dies there consitently if a 
single character comes back from the simple_text widget (text_value).

.
.
.
#define S_TEXT " (Text)"
char *constant_string = S_TEXT;
.
.
.
activate_callback()
{
	static char *list_entry, *status, *text_value;
	.
	.
	.
	text_value = DwtSTextGetString (mysimpletextwidget);
	status = strcat (text_value,constant_string);
	list_entry = DwtLatin1String (text_value);			<--#1
	item_exists = DwtListBoxItemExists (mylistboxwidget,list_entry);<--#2
	if (item_exists == 0) {
		DwtListBoxAddItem (mylistboxwidget,list_entry,0);	<--#3
.
.
.
	
Agaian, any help/pointers/etc is *really* appreciated!

George 

    
    Here's one hint that's probably not affecting you yet.  If you do a
    DwtSTextGetString on a strcpy, and there's nothing to get, sometimes a
    segmentation fault will occur.  What you might want to do is :
    
    	if( strlen(DwtSTextGetString( whatever) ) >0)
    		assign DwtSTextGetString
    I would step through the debugger and see exactly what's coming out of
    each stmt.  Perhaps by the time it gets to DwtLatin1String, it's no
    longer a good string?
    
    Rachael
    

It looks like you're overwriting memory. DwtSTextGetString allocates only 
enough memory for the string it returns. Since you're concatenating more 
bytes onto that string, you will need to reallocate 'text_value' to hold it.

This would explain why it's seg faulting at various lines - depends upon what
is in the memory that being overwritten.

Pat

>    	if( strlen(DwtSTextGetString( whatever) ) >0)
>    		assign DwtSTextGetString
You don't want to do it this way... every call to DwtSTextGetString results
in malloc'd memory.  If you don't save the return value, the memory is lost.