maybe some Inline::C users could take from here and consider opening a related enhancement request

This is the bit I don't quite get.
Precisely what, IYO, should that "enhancement request" be seeking ?
At the moment, all I've got is that "undef, when passed from perl to unsigned char * C argument, should be NULL". (That problem, at least, is solved.)
What if we're passing something other than "undef" ? Should that be T_PV or T_PTR ? ... or something else ?

What happens when the C function unsigned char * foo() returns a NULL to perl ? Should that come back as undef ?
With T_PV it returns undef; with T_PTR it returns the IV zero. But this behaviour can be manipulated to whatever we want in ExtUtils/typemap (or user-provided typemap).

Just give me a clear spec, I'll write a patch to ExtUtils/typemap that enacts that spec,and, if it passes review here, I'll see if I can get perl porters to accept it.
That's where the change would best be made.

If the proposed change is unacceptable to them, then we can look at making the change in Inline::C by use of a customized typemap. (No guarantees that it will be accepted there, either ... we'll just have to wait and see.)
But I first need to see a clear spec of the requirement, telling me exactly what needs to be changed.

Cheers,
Rob

UPDATE: If the only thing we want to do is to ensure that "undef" is passed as NULL to a char * (either signed or unsigned) then I think we need to change the "INPUT" setting in ExtUtils/typemap for T_PV from:

    $var = ($type)SvPV_nolen($arg)
[download]

to

    if (SvOK($arg)) 
       $var = ($type)SvPV_nolen($arg);
    else
       $var = INT2PTR($type,SvIV($arg))
[download]

We can also achieve the same effect by creating a file named "typemap" that contains:

INPUT

T_PV
    if (SvOK($arg)) 
       $var = ($type)SvPV_nolen($arg);
    else
       $var = INT2PTR($type,SvIV($arg))
[download]

That file needs to be placed in a location where it will automatically be recognized as a typemap.

For example, place that typemap file in the same directory as this little Inline:C script:

use strict;
use warnings;
use Devel::Peek;

use Inline C => Config =>
  BUILD_NOISY => 1, # verbose build
  FORCE_BUILD => 1, # re-build whenever the script is run
  CLEAN_AFTER_BUILD => 0, # don't clean up the build directory
;

use Inline C =><<'EOC';

unsigned char * foo(unsigned char *name) {
  if(name) printf("name is: %s\n", name);
  else printf("NULL input (undef) was detected\n");
  return(name);
}

EOC

my $x = foo(undef);
Dump($x);

my $y = foo("hello world");
Dump($y);

my $z = foo('');
Dump $z;
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Then cd to that directory, run the script and tell me if it's doing something undesirable.

Of course unsigned char* is not the only thing that maps to T_PV - char*, const char*, caddr_t, wchar_t* and Time_t* all map to T_PV, and will therefore all be affected by that typemap.
But, if need be, we could always create a new and distinct type for those that need to use this revised setting.

To run that script with the settings specified by ExtUtils/typemap, just rename "typemap" to something else, and it will be ignored.