In TCL 8.4, does the TclLib function Tcl_SetHashValue permit value to be NULL given #define NULL 0?

Question

I've read the documentation for Tcl_SetHashValue [1] but I have a question. Does the TclLib function Tcl_SetHashValue permit value to be NULL given #define NULL 0?

In other words, given

#define NULL 0

does a Tcl_SetHashValue invocation like the following have a defined behavior?

Tcl_SetHashValue( someHashEntry, NULL );

If the behavior is defined, what is the behavior? For example, if this code is executed

Tcl_HashTable myhash;
char *key_arg = 0;
Tcl_HashEntry *entry_arg;
ClientData val_arg;
Tcl_HashEntry *pEntry ;
char *pValue ;

Tcl_InitHashTable( &myhash, TCL_STRING_KEYS);

// store data

key_arg = "someKey";
entry_arg = Tcl_CreateHashEntry( &myhash, key_arg, &isEntryNew );
val_arg = NULL ;
Tcl_SetHashValue( entry_arg, val_arg );

// retrieve data

pEntry = Tcl_FindHashEntry( pArgs, criteria );
if (pEntry)
{
    pValue = Tcl_GetHashValue( pEntry ) ;
}

what would pEntry and pValue contain after the above code is executed?

1 - http://www.tcl.tk/man/tcl8.4/TclLib/Hash.htm

Answer 1

The value set by Tcl_SetHashValue (which is a macro, for historical reasons) is cast by the macro to be of type ClientData. That in turn is a typedef for void * (except on pre-C89 systems (!) where it is a char *) and has the promise that Tcl will not poke its grubby fingers inside it; a subsequent Tcl_GetHashValue will return exactly the value that you provided.

In your sample code, pEntry will have a pointer to a valid structure or a NULL if the criteria were not matched. If pEntry is non-NULL, pValue will have the value placed in. Which might well be NULL here; NULL is a perfectly fine thing to put in, and it's up to your code to understand what that might possibly mean.

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Later versions of Tcl are the same, but might have tinkered with exactly what the definitions are. And for sure we dropped pre-C89 support. (In fact, it turned out we did so for building Tcl itself in 8.4 by accident and not one user complained. We're conservative, but not that conservative!)