CODEWITHSUNDEEP
cgccassemblynasmld
uNiverselEgacy
uNiverselEgacy

Reputation: 357

How to call c functions that call c standard library in nasm?

First I want to clarify that I know this question might have been answered hundreds of times. However after hours of Google search I simply couldn't find anything that's exactly what I want. Also even though I've been writing c programs for quite a while, I'm kind of new to nasm and ld. So I would really appreciate it if I can get a simple answer without having to read a whole nasm/ld tutorial or the complete manual.

What I want to do is:
say I have a function written in c that calls some function in the c standard library:

/* foo.c */
#include <stdio.h>
void foo(int i)
{
    printf("%d\n", i);
}

I want to call this function in nasm so I tried this:

; main.asm
global _start
extern foo

section .text
_start:
    push 1234567
    call foo
    add esp, 4

    mov eax, 1
    xor ebx, ebx
    int 80h

Then I tried to compile them and run: 

[user ~/Documents/asm/callc]#make all
nasm main.asm -felf
gcc -c foo.c -o foo.o -m32
ld -o main main.o foo.o -melf_i386 -lc
[user ~/Documents/asm/callc]#ls
foo.c  foo.o  main  main.asm  main.o  Makefile
[user ~/Documents/asm/callc]#./main 
bash: ./main: No such file or directory
[user ~/Documents/asm/callc]#bash main
main: main: cannot execute binary file

I didn't get any errors but apparently I couldn't run the executable output file.

If the c function doesn't call any library functions then the code above can be compiled and it will run without any problems. I also figured out a way to call library functions directly in nasm and use gcc to produce the final executable file. But none of them is exactly what I want.

EDIT:
1. I'm running 64-bit Ubuntu but I'm trying to write 32-bit programs so I used flags like -m32 and -melf_i386.
2. Output of file *:

[user ~/Documents/asm/sof]#file *
foo.c:     C source, ASCII text
foo.c~:    empty 
foo.o:     ELF 32-bit LSB  relocatable, Intel 80386, version 1 (SYSV), not stripped
main:      ELF 32-bit LSB  executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), not stripped
main.asm:  C source, ASCII text
main.asm~: empty 
main.o:    ELF 32-bit LSB  relocatable, Intel 80386, version 1 (SYSV), not stripped
Makefile:  makefile script, ASCII text
Makefile~: makefile script, ASCII text

3. I really have no idea of how to tell ld to include the c standard library. I found something like -lglibc or -lc in some other posts. -lgibc doesn't work and -lc seems to be able to get rid of all errors and I probably thought it worked at first but maybe that's the problem since it probably doesn't link the correct library.

UPDATE
Adding -I/lib32/ld-linux.so.2 to the ld command solved my problem.
Below are commands to compile/assemble/link and run the program: 

nasm main.asm -felf
gcc -c foo.c -o foo.o -m32
ld -o main main.o foo.o -melf_i386 -lc -I/lib32/ld-linux.so.2
./main

Upvotes: 3

Views: 5306

Answers (1)

user1157391
user1157391

Reputation:

The C library provides code using the _start interface that starts the C runtime, calls main(), and shuts the runtime down. Hence if you intend to use the C library in your program you must not use the _start interface but provide a main() function.

This is the correct way to do it:

; main.asm
global main
extern foo

section .text
main:
    push 1234567
    call foo
    add esp, 4

    xor eax, eax
    ret

Build with:

nasm -f elf32 -o main.o main.asm
gcc -m32 -o foo.o -c foo.c
gcc -m32 -o main main.o foo.o

Two remarks:

  • main() returns, instead of doing an exit system call, to allow the C runtime shutdown code to run.
  • gcc is used for linking. Internally gcc invokes ld with the appropriate parameters to link with the C library. These are platform specific and subject to change. Hence, don't use ld for this.

Upvotes: 4

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