Call external function?

[rym...@gmail.com]

Hello,

I'm new to AsmJit and have been wondering something.

Is it possible to call an external C(not C++) function in AsmJit? Because I need to print text to the screen, and I'm on Windows, so kernel interrupts don't work.

Thanks in advance!

[Bartosz Wójcik]

Hello Rymg19,

Friday, October 25, 2013, 4:37:20 PM, you wrote:

> Is it possible to call an external C(not C++) function in AsmJit?
> Because I need to print text to the screen, and I'm on Windows, so kernel interrupts don't work.

Sure thing, build up an entire PE header, add imports section and off
you go :)

Bartosz Wójcik | reda...@secnews.pl | www.secnews.pl

[Ryan Gonzalez]

...how would I go about doing that???

FYI, I'm using the Assembler interface, not the Compiler interface.

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[Bartosz Wójcik]

Hello Ryan,

Friday, October 25, 2013, 9:19:18 PM, you wrote:

> ...how would I go about doing that???

I would start with PE COFF library specification from MSDN.

Build up MZ header, then PE header, add sections, add import section,
then when your import section is ready and you know it's pointers
(WinAPI pointers) you can add direct calls to them in your non
relocable AsmJit code.

It's hard, but doable.

Bartosz Wójcik | reda...@secnews.pl | www.secnews.pl

[Ryan Gonzalez]

Woah...all I want to do is call putchar and getchar!!! Isn't there a...err..."simpler" way?

Bartosz Wójcik | reda...@secnews.pl | www.secnews.pl

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[Sean Dodd]

If you use the compiler interface, it's simple to do with "call".  Here's an example to call external function with two doubles as inputs and a double return type:

        GPVar address(c.newGP(VARIABLE_TYPE_INT64));

        c.mov( address, (sysint_t)(void*)&someExternalFn );

        // Assume these were populated somehow, probably c.movq(GPVar, XMMVar)

        GPVar param0( c.newGP(VARIABLE_TYPE_INT64) );
        GPVar param1( c.newGP(VARIABLE_TYPE_INT64) );
        GPVar output( c.newGP(VARIABLE_TYPE_INT64) );

        ECall* ctx = c.call( address );
        ctx->setPrototype(CALL_CONV_DEFAULT, FunctionBuilder2<double, double, double>());
        ctx->setArgument(0, param0);
        ctx->setArgument(1, param1);
        ctx->setReturn(output);

Hope that helps a bit.

-Sean

[Ryan Gonzalez]

Okay...I'll try using the compiler interface. Darn it...

[Petr Kobalíček]

Hi,

it's also possible with Assembler, but you have to know the calling convention of the function you want to call. I mean saving registers, preparing the stack and function arguments.

For example if you want to call a function having one parameter in 32-bit mode you would do something like:

Assembler a;

...

(save regs)

a.push(something)

a.call(ptr); // Assuming STDCALL that will advance the stack pointer back.

(restore regs)

...

Hope that helps.

Best,
Petr

[Ryan]

Uhh...what I really needed to know is how to put the function into the variable ptr. I've worked with NASM several times, so I know about calling conventions.

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[Tomislav Zubcic]

No need to complicate things, it's as simple as A.call((void *) getchar);

[Tomislav Zubcic]

Seems like my reply was not sent, sorry if I'm sending again now. Anyway, it's as simple as A.call((void *) getchar); - no need to complicate things. :)

On Friday, October 25, 2013 4:37:20 PM UTC+2, rym...@gmail.com wrote:

[Ryan Gonzalez]

Perfect! Thank you!

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