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Re^2: XS/Inline::C and ellipsis syntax

by syphilis (Canon)
on Jun 29, 2008 at 01:32 UTC ( #694570=note: print w/ replies, xml ) Need Help??


in reply to Re: XS/Inline::C and ellipsis syntax
in thread XS/Inline::C and ellipsis syntax

You'll have to change the vatest method signature

I'm mainly interested in keeping the vatest() signature as is - though, admittedly, that probably wasn't very clear from my post.

I know how to work things with vatest(SV* format, ...), but was wondering if there's any way that perl can access vatest(char* format, ...) even if we impose limitations that, eg, all of the arguments passed to vatest are strings (PV's).

It's probably a dumb question. I know that in both XS and Inline::C perl, can access foo(char* str) and can also access the multi-arg form foo(char* str1, char* str2, char* str3), but to access a variable-length list of char*'s might stretch the friendship between perl and C a little too far.

Cheers,
Rob


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Re^3: XS/Inline::C and ellipsis syntax
by BrowserUk (Pope) on Jun 29, 2008 at 02:56 UTC

    The number of arguments to a varags C function is known at compile time. The number of arguments to a Perl function isn't known until runtime. That is, when you code:

    XS_func( char *templ, ... ) { ... vsprintf( templ, x, y, z ); }

    The C compiler knows it is passing 4 parameters to the varargs function printf and stacks them appropriately. But when you call the XS function from Perl, you can pass any number of arguments at runtime, so there is no way to write that call to vsprintf() with an appropriate number of arguments.

    About the best you could do is code the call to vsprintf() with an arbitrarily large number of arguments and yell if the Perl code passes you more:

    #! perl -slw use strict; #use Inline 'FORCE'; use Inline C => 'DATA', NAME => 'varargs', CLEAN_AFTER_BUILD => 0; myPrintf( '%s ' x @ARGV . "\n", @ARGV ); __DATA__ __C__ #include <windows.h> #include <stdio.h> #include <stdarg.h> void eprintf( const char *template, ... ) { va_list ap; va_start( ap, template ); vfprintf( win32_stderr(), template, ap ); va_end( ap ); } void myPrintf( char *templ, ... ) { inline_stack_vars; char* a[20]; int i; if( inline_stack_items > 20 ) croak( "myPrintf can handle at most 20 arguments." ); for( i = 0; i < inline_stack_items; i++ ) { a[ i ] = SvPVX( inline_stack_item( i ) ); } eprintf( a[ 0], a[ 1], a[ 2], a[ 3], a[ 4], a[ 5], a[ 6], a[ 7], a[ 8], + a[ 9], a[10], a[11], a[12], a[13], a[14], a[15], a[16], a[17], a[18], + a[19] ); inline_stack_reset; }

    That's crude. it should be checking that the SVs have a PV assigned and lots of other stuff, but it will pass whatever args (upto 20) you give on the command line and print them to stderr via C and eprintf().

    It could also inspect the contents of the template and attempt to extract the relevant IV/NV/PV, but then you'd have to do something trick with the declaration of a[].

    Another way to do this would be for the XS/Inline C function to push the (relevant parts of) the SVs from the Perl stack, onto the C stack. That is, it would have to emulate the C compilers prologue and epilogue code manually, before transferring control (assembler call instruction) to the varargs C function.

    (And it would have to be the relevant prologue code depending upon the calling convention (__stdcall, __cdecl or __fastcall) declared for that function.)

    This approach would require compiler dependant asm{} hacks.


    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
Re^3: XS/Inline::C and ellipsis syntax
by renodino (Curate) on Jun 29, 2008 at 04:50 UTC
    Hmmm. I believe I now understand your dilemna.

    Basically, you want to be able to dynamically create parameter lists at runtime in C the same as Perl.

    Alas, it can't be done. Well, at least not easily, and definitely not in a portable manner.

    Long ago, when Kernigan and Ritchie still had full heads of hair with nary a gray follicle, some C compilers supported the varargs' complementary ability to create variable length argument lists. (I vaguely recall using it occasionally on SysVR3 and MSDOS...aha, look what google found!).

    But somewhere in the past 2 decades, our friends on the various C standards committees seem to have mislaid that capability. Presumably because it was very dangerous, and because the way stackframes get built on different platforms varies significantly.

    So about the only way to do that sort of thing these days is to dive into the assembler and handcraft your own stackframes. Or, more likely, rewrite your programs to avoid the need for dynamically building argument lists. Which unfortunately usually means writing a lot more code.


    Perl Contrarian & SQL fanboy

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