In Tcl releases 8.5.0 through 8.5.7, the public header file `tcl.h`
contains the typedef:

===
typedef unsigned long mp_digit;
===

Starting in Tcl 8.5.8, this is changed to:

===
typedef unsigned int mp_digit;
===

The purpose of this page is to discuss what incompatibilities
this may introduce for C code that uses the Tcl public interface,
and how to manage them.

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First, note that on many (32-bit) systems, `long` and `int` are
just two names for the same thing.  If your C code is only ever
compiled on such systems, the change is not really a change at all,
and you have no compatibility issues to worry about.

For C code that does get compiled for L64 systems, read on...

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