The following routines in Tcl's public interface, tcl.h, make use of the mp_int type:
Tcl's public header, tcl.h also includes these declarations:
#ifndef MP_INT_DECLARED typedef struct mp_int mp_int; #define MP_INT_DECLARED #endif #ifndef MP_DIGIT_DECLARED typedef unsigned long mp_digit; #define MP_DIGIT_DECLARED #endif
which allows the compiler to make sense of the function declarations.
Callers of those functions must be able to allocate an mp_int struct so they can pass its address in. In order to do this a definition of the mp_int struct has to be in scope, but Tcl's public header tcl.h does not provide one.
Q1: What header file are callers of these routines expected to #include to get a suitable mp_int definition in scope?
Q2: If the answer is something other than a corrected tcl.h, how are we to verify that the mp_digit definition active wherever that definition comes from is the same as that in tcl.h? ( Failure to make them consistent leads to a binary incompatibility).
Several of Tcl's own source code files need to call these routines. They have the same problem. Their solution is #include "tommath.h". When Tcl sources are built, the -I compiler options are set so that this refers to the file tcl/generic/tommath.h which is only a wrapper pulling in tcl/generic/tclTomMath.h which is a patched version of the original header of libtommath, tcl/libtommath/tommath.h. That's the Tcl internals answer to Q1.
Each Tcl source file using the mp_int struct, follows the pattern of #include "tcl.h" before #include "tommath.h". Also the file tcl/generic/tclTomMath.h has been patched so that when MP_DIGIT_DECLARED is defined, any declarations of mp_digit are skipped. This combination is the Tcl internals answer to Q2.
The file tcl/generic/tclTomMath.h is patched so that if you include it without first including tcl.h (and no other special MP_* directive is active), the controlling mp_digit definition is:
typedef unsigned int mp_digit
Because of the Tcl internals answer to Q2 though, the tcl.h declaration of mp_digit controls and the mp_int struct in use throughout Tcl's implementation is one built on an array of unsigned long digits. This is most unfortunate on L64 systems, because the rest of tcl/generic/tclTomMath.h forces a configuration where only 28 bits of each digit are used to store bits of the bignums. This doubles the memory weight of bignums on L64 systems for no gain at all.
Q3. Can we change tcl.h so that mp_digit is unsigned int instead of unsigned long ? Can we do it in a patch release?
This would be no change on L32 systems. On L64 systems it would be a huge improvement in memory efficiency.
It's a binary incompatibility, which is normally off limits in a patch release, but I think this is really a (very late) bug fix in the new interface routines of Tcl 8.5. I think it's worth doing, but we should find out if/how it will cause problems to any existing users of the interface.
Q4 Why is it that the `mp_int and mp_digit declarations in tcl.h` ...
more to come...
AMG: See my comments on tcl::tommath. Also see [L1 ] for a relevant bug report.