RE: [Tclplugin-core] TCl problem

[Sharma Anshoo-A. <an...@mo...>]

Don,

I should have mentioned this in my first mail itself, we are using Tcl_LinkVar for linking the C variables to TCL.

The code looks something like what is given below,

#define MAX_MSG_LEN 0

app_init_proc() /* The Callback function passed in Tcl_Main */
{
   /* Some initialization... */

    p_tcl_c_str = Tcl_Alloc(MAX_MSG_LEN); /* p_tcl_c_str being declared on global scope as (char *) */
    Tcl_LinkVar(p_interp, "p_tcl_c_str", (char *)&p_tcl_c_str, TCL_LINK_STRING);

   /* Some more intialization... */
}

   /* Later at some point in the TCL Script */
   set p_tcl_c_str "Hello World"
   /* The above statement releases the Memory allocated to p_tcl_c_str in app_init_proc. And reallocates it just enough memory to hold the given string. */

   /* At still later point in C Code */
   memset(p_tcl_c_str, 0, MAX_MSG_LEN);
   /* Results in a crash since the size of the string is no longer MAX_MSG_LEN now.

Any thoughts on how to circumvent this problem?

We have been able to solve this by reallocating the memory in C code everytime we want to assign the string. Something like, 
   p_tcl_c_str = Tcl_Realloc(p_tcl_c_str, MAX_MSG_LEN);

Is a more elegant solution available?

Best Regards
Anshoo.


-----Original Message-----
From: Donald G Porter [mailto:dg...@em...] 
Sent: Wednesday, July 02, 2003 1:27 AM
To: je...@Ac...; tcl...@li...; Sharma Anshoo-A19642
Cc: Karpoor Lakshmi Sanjay-A17738
Subject: Re: [Tclplugin-core] TCl problem



> During the Initialization we request TCL to allocate a pointer 
> "p_tcl_c_str" (say 1024 bytes) using the
Tcl_Alloc call. When we later write to this variable from a TCL Script, TCL reallocates this variable 
memory which is equal to length of the string that is being written. For Example if we do a "set 
p_tcl_c_str Hello_World", It will reallocate this variable only 12 bytes and release the previous chunk. 
As, in the C code we are not aware of this change we write to the C variable expecting that it would be 
able to support the write upto 1024 bytes and hence the process crashes.

It sounds like you are attempting to program based on a mistaken assumption that a C variable of a particular name is somehow tied to a Tcl variable (a Tcl_Var) of that same name.  No such automatic connection exists.

If you need access to the same variable at both Tcl and C levels, you should be working with Tcl_Var's all the time and use the procedures like Tcl_SetVar() and Tcl_GetVar() to read and write them.

Another option that might be suitable for certain situations is to create a "link" between a C variable and a Tcl variable using the Tcl_LinkVar() family of routines.

If these pointers do not answer your problem, then try again with some specific code examples of what you are attempting to do.

| Don Porter          Mathematical and Computational Sciences Division |
| don...@ni...             Information Technology Laboratory |
| http://math.nist.gov/~DPorter/                                  NIST |
|______________________________________________________________________|