Problems with calling an assembly function from C

Question

(Running MingW on 64-bit Windows 7 and the GCC on Kubuntu)

This may possibly be just a MingW problem, but it's failed on at least one Kubuntu installation as well, so I'm doubtful.

I have a short, simple C program, which is supposed to call an assembly function. I compile the assembler using nasm and the c program using MingW's implementation of the gcc. The two are linked together with a makefile - bog-simple. And yet, linkage fails on the claim the claim that the external function is an 'undefined reference'

Relevant part of the makefile:

assign0: ass0.o main.o gcc -v -m32 -g -Wall -o assign0 ass0.o main.o main.o: main.c gcc -g -c -Wall -m32 -o main.o main.c ass0.o: ass0.s nasm -g -f elf -w+all -o ass0.o ass0.s

The beginning of the assembly file:

section .data ; data section, read-write an: DD 0 ; this is a temporary var section .text ; our code is always in the .text section global do_str ; makes the function appear in global scope extern printf do_str: ; functions are defined as labels [Just Code]

And the c file's declaration:

extern int do_str(char* a);

This has worked on at least one Kubuntu installation, failed on another, and failed on MingW. Does anyone have an idea?

Sometimes an _ gets prepended to symbol names. What's the exact error message you get? — Carl Norum
– Carl Norum, Commented Mar 21, 2012 at 17:45
possible duplicate of Adding leading underscores to assembly symbols with GCC on Win32? — Carl Norum
– Carl Norum, Commented Mar 21, 2012 at 17:46
Have you tried swapping {ass0.o main.o} on the linker command line? — wildplasser
– wildplasser, Commented Mar 21, 2012 at 17:46
You want to add the _ on the assembly side, not the C side, if that's in fact the problem. There can't be an 'undefined reference' error related to a source code line - it's a link error. Please copy and paste the full text here. — Carl Norum
– Carl Norum, Commented Mar 21, 2012 at 17:50

wallyk · Accepted Answer · 2012-03-21 18:16:02Z

... the claim that the external function is an 'undefined reference'

LOL! Linkers do not "claim" falsehoods. You will not convince it to change its mind by insisting that you are correct or it is wrong. Accept what the tools tell you to be the truth without delay. This is key to rapidly identifying the problem.

Almost every C compiler, including those you are using, generates global symbols with an underscore prefix to minimize name collisions with assembly language symbols. For example, change your code to

extern _printf ... call _printf

and error messages about printf being undefined will go away. If you do get an undefined reference to _printf, it is because the linker is not accessing the C runtime library. The link command can be challenging to get correct. Usually doing so is not very educational, so crib from a working project, or look for an example. This is way that IDEs are very helpful.

As for the C code calling the assembly function, it is usually easiest to write the assembly function using C's conventions:

 global _do_str _do_str:

Alternatively, you could declare the function to use the Pascal calling convention:

extern int pascal do_str ( whatever parameters are needed); ... retval = do_str ("hello world");

The Pascal calling convention is substantially different from C's: it does not prepend a leading underscore to the symbol, the caller is responsible for removing the parameters after return, and the parameters are in a different order, possibly with some parameter data types being passed in registers rather than on the stack. See the compiler references for all the details.

ShinTakezou · Accepted Answer · 2012-03-21 18:00:56Z

C compilers may call the actual "function" differently, e.g. _do_str instead of do_str. Name mangling not happening always could depends on the system (and of course on the compiler). Try calling the asm function _do_str. Using proper attributes (in gcc) could also fix the problem. Also read this.

Collectives™ on Stack Overflow

Problems with calling an assembly function from C

2 Answers 2

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Linked

Related