[SOLVED] Calling an asm function from C

[gerard4143]

Hi,

I have a question about calling an asm function from C....It doesn't work unless I create an asm variable to hold the value of the function in....Why?

Here's the code that doesn't work...

asmfile.s - version one

.section .data
	mydata: .ascii "this is the message!\n"
	.equ mylen, . - mydata

.section .bss

.section .text
	.global printit
printit:
	pushq	%rbp
	movq	%rsp, %rbp

	movq	$1, %rax
	movq	$1, %rdi
	movq	$mydata, %rsi
	movq	$mylen, %rdx
	syscall

	movq	%rbp, %rsp
	popq	%rbp
	ret

testit.c - version one

#include <stdio.h>
#include <stdlib.h>

typedef void (*pfunc)(void);

extern pfunc printit;

int main(int argc, char**argv)
{
	printit();
	exit(EXIT_SUCCESS);
}

Now when I compile version one with
as asmfile.s -o asmfile.o
gcc testit.c asmfile.o -Wall -ansi -pedantic -o testit

and run ./testit I get a Segmentation fault


And here's the code that works....Why is it behaving this way?

asmfile.s - version two

.section .data
	mydata: .ascii "this is the message!\n"
	.equ mylen, . - mydata
	
	myprintit: .quad printit

.section .bss

.section .text
	.global myprintit
printit:
	pushq	%rbp
	movq	%rsp, %rbp

	movq	$1, %rax
	movq	$1, %rdi
	movq	$mydata, %rsi
	movq	$mylen, %rdx
	syscall

	movq	%rbp, %rsp
	popq	%rbp
	ret

testit.c - version two

#include <stdio.h>
#include <stdlib.h>

typedef void (*pfunc)(void);

extern pfunc myprintit;

int main(int argc, char**argv)
{
	myprintit();
	exit(EXIT_SUCCESS);
}

I compile with the same lines:

as asmfile.s -o asmfile.o
gcc testit.c asmfile.o -Wall -ansi -pedantic -o testit

and execute...and "this is the message!"

Does anyone know why the first one doesn't work but the second does?

Also my compiler version - gcc version 4.5.0 20100520 (prerelease) (GCC)

[darkrose0510]

You're declaring the function (prototype) as a pointer, it shouldn't be.

typedef void (*pfunc)(void);

try this:

typedef void pfunc(void);

[gerard4143]

You're declaring the function (prototype) as a pointer, it shouldn't be.

typedef void (*pfunc)(void);

try this:

typedef void pfunc(void);

Thanks darkrose0510 that was it...