/me just answers the prototype question ...

XS essentially boils down to two parts. The real C code that is littered with macros, which you are now writing, and a part that is preprocessed when converting XS files to C. The latter part might look like this:

MODULE = TEST        PACKAGE = main

long
mktime (sec, min, hr, day, mon, yr)
        int     sec
        int     min
        int     hr
        int     day
        int     mon
        int     yr
    PROTOTYPE: $$$$$$
    CODE:
        struct tm       ltm;

        ltm.tm_sec      = sec;
        ltm.tm_min      = min;
        ltm.tm_hour     = hr;
        ltm.tm_mday     = day;
        ltm.tm_mon      = mon;
        ltm.tm_year     = yr;
        ltm.tm_wday     = 0;
        ltm.tm_yday     = 0;
        ltm.tm_isdst    = 0;

        RETVAL = mktime (&ltm);
    OUTPUT:
        RETVAL
[download]

Which will be translated to something like

XS(XS_main_mktime); /* prototype to pass -Wmissing-prototypes */
XS(XS_main_mktime)
{
#ifdef dVAR
    dVAR; dXSARGS;
#else
    dXSARGS;
#endif
    if (items != 6)
       croak_xs_usage(cv,  "sec, min, hr, day, mon, yr");
    {
        int     sec = (int)SvIV(ST(0));
        int     min = (int)SvIV(ST(1));
        int     hr = (int)SvIV(ST(2));
        int     day = (int)SvIV(ST(3));
        int     mon = (int)SvIV(ST(4));
        int     yr = (int)SvIV(ST(5));
        long    RETVAL;
        dXSTARG;
#line 122 "TEST.xs"
        struct tm       ltm;

        ltm.tm_sec      = sec;
        ltm.tm_min      = min;
        ltm.tm_hour     = hr;
        ltm.tm_mday     = day;
        ltm.tm_mon      = mon;
        ltm.tm_year     = yr;
        ltm.tm_wday     = 0;
        ltm.tm_yday     = 0;
        ltm.tm_isdst    = 0;

        RETVAL = mktime (&ltm);
#line 239 "TEST.c"
        XSprePUSH; PUSHi((IV)RETVAL);
    }
    XSRETURN(1);
}

:
:

#ifdef __cplusplus
extern "C"
#endif
XS(boot_TEST); /* prototype to pass -Wmissing-prototypes */
XS(boot_TEST)
{
#ifdef dVAR
    dVAR; dXSARGS;
#else
    dXSARGS;
#endif
#if (PERL_REVISION == 5 && PERL_VERSION < 9)
    char* file = __FILE__;
#else
    const char* file = __FILE__;
#endif

    PERL_UNUSED_VAR(cv); /* -W */
    PERL_UNUSED_VAR(items); /* -W */
    XS_VERSION_BOOTCHECK ;

        (void)newXSproto_portable("main::mktime", XS_main_mktime, file
+, "$$$$$$");
#if (PERL_REVISION == 5 && PERL_VERSION >= 9)
  if (PL_unitcheckav)
       call_list(PL_scopestack_ix, PL_unitcheckav);
#endif
    XSRETURN_YES;
}
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The code to call to set the prototype is newXSproto_portable (), but I don't know if you can be in time to do so in Inline::C.