Tcl_LinkVar

[nater]

Also, I'm attempting to link a C++ string to a TCL variable, and can't seem to do it. With the following code, I get strange behavior, such as the string being set to the *name* of the TCL variable. I assume I'm being an idiot and confusing the pointers or something, but I can't get it to work, and I'm starting to wonder if there might be a bug with LinkVar..
Now feel free to tell me why I'm an idiot. =)

...
char *statusString;
statusString = Tcl_Alloc(6);

Tcl_LinkVar(interp, "status", statusString, TCL_LINK_STRING);

statusString = "WHY??";
...

 

[AviaTraining]

I believe that this line is your problem:

statusString = "WHY??";

You just overwrote your pointer, so statusString is now pointing to some statically allocated memory instead of the memory allocated by Tcl_Alloc(). And as the Tcl_LinkVar() documentation says when linking to string values:

"The C variable is of type char *. If its value is not null then it must be a pointer to a string allocated with Tcl_Alloc."

So, you'll want to do something like this instead:

strcpy(statusString,"WHY??");

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Avia Training and Consulting

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[nater]

Thanks.. I actually realized that stupidity before I saw your post.. but it uncovered a larger problem. When I remove the attempt to assign a value to the string there, the program compiles fine, but then LinkVar tries to read an invalid memory location. Apparently in order to get around this you have to create a pointer to the original string handle, like so:

char statusString[6] = "WHY??";
char *statusStringPtr = statusString;

Tcl_LinkVar(interp, "status", (char *)&statusStringPtr, TCL_LINK_STRING);

It was probably designed like this to avoid the original problem I was having. This way if you want to modify the string, you can deallocate the original (statusString[6]), and point statusStringPtr to a new string. Of course, as you pointed out, you can also just use strcpy.
In any event, the linking does not seem to work without this double pointer deal..

 

[nater]

Ack.. my bad. That last one.. worked, but I didn't really understand why. It should have been more like this. The "sizeof(char)" part is unnecessary, as you know it's 1 byte, but it makes it a bit easier to follow.. (At least, if you're me.) Basically what the linkvar function wants is a pointer to the string pointer.

Just in case anybody's actually reading this. =)

char *statusString = Tcl_Alloc(100 * sizeof(char));
char **statusStringPtr = (char **)Tcl_Alloc(sizeof(char *));
*statusStringPtr = statusString;

strcpy (statusString, "Start string"

Tcl_LinkVar(interp, "end", endFlg, TCL_LINK_INT);
Tcl_LinkVar(interp, "status", (char *)statusStringPtr, TCL_LINK_STRING);