importing functions

[no one special]

Hi,

If I have an asm file with code like this

_KillFloppy:
push eax
mov dx, 0x3f2
mov al, 0
out dx, al
pop eax
ret

and I compile it like this

nasm -f elf floppy.asm

and I link in with a c file, how can I use this function? do I need an extern declaration or what?

thanks.

[dronkit]

"extern" declarations are for calling C functions from asm code. in "gas", every asm label is local (private) to the file the code is in. To define a symbol as an "exportable" one, you use the keyword ".globl", for example:

Code: Select all

hello:
   ret

Can't be called from outside the file, but:

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.globl hello
hello:
   ret

then from your C file you declare:

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extern void hello(void);

now you're ready to use the function.

[Whatever5k]

You even don't need the "extern declaration...Just have something like this:

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; this is for NASM
[global _hello]
_hello:
  ; do stuff
  ret

And in the C file:

Code: Select all

void hello(void);

void foo(void)
{
   hello();
}

[dronkit]

uising the "extern" keyword is a manners matter

[Pype.Clicker]

just be aware that it is not necessary to add a leading underscore to your asm function name when assembling an ELF file (but it is necessary if you're compiling a COFF/PE/COM/OBJ/whatever file from the dos/windows ABI)