CODEWITHSUNDEEP
cassemblycompilationlinkernasm
user1886376
user1886376

Reputation:

mixing c and assembly. On the terminal nothing printed

I wrote here is a simple code:

 printf.asm

 [EXTERN main]
 section .text
 global _start     
 _start:
       call main
       add esp, 8
       mov eax, 1
       xor ebx, ebx
       int 80h

and main.c

#include <stdio.h>

 int main()
 {
     char* str = "print from C :\\)";
     printf("%s", str);
 }

I compile code like this:

nasm -g -f elf printf.asm -o printf.o

gcc -c -o main.o main.c

ld -o printf printf.o main.o -lc -I/lib/ld-linux.so.2

And run:

./printf

On the terminal nothing printed. Why ?

when i doing linking with following command ld -Ttext 0x1000 -o printf printf.o main.o -lc -I/lib/ld-linux.so.2 , it displays "Killed" string. How to solve this problem ?

Code successfully earned just added a newline character in the printf function: printf("%s\n", str);. Thanks all, the problem is solved.

Upvotes: 0

Views: 183

Answers (3)

rkhb
rkhb

Reputation: 14409

Mixed source with NASM-entrypoint works in this way (32-bit on my 64-bit-Linux):

printf.asm:

global _start
extern main
extern fflush

section .text
 _start:
    call main

    push 0                      ; fflush (stdout)
    call fflush
    add esp, 4

    mov ebx,0                   ; exit code, 0=normal
    mov eax,1                   ; exit command to kernel
    int 0x80                    ; interrupt 80 hex, call kernel

main.c

#include <stdio.h>

extern int main (void)
{
    char* str = "print from C :\\)\n";
    printf("%s", str);
    return 0;
}

build:

nasm -felf32 -oprintf.o printf.asm
gcc -c -m32 -omain.o main.c
ld -I/lib32/ld-linux.so.2 -lc -m elf_i386 -o printf printf.o main.o

I guess you can manage the changes for 32-bit-Linux :-)

Upvotes: 0

alireza_fn
alireza_fn

Reputation: 944

You are writing _start yourself which is a libc startup function .this is the reason you cannot link the code correctly.also you should not touch _start otherwise you will break libc. For running a code before main you may use ''attribute ((constructor))'' (it is a gcc feature and it's not available in other compilers).

Upvotes: 1

ntremble
ntremble

Reputation: 71

You are attempting to call main() without first executing the C startup code. You should not do that. Basically, the C startup initialises the stack and variable storage before jumping to main().

You can call assembly language code from main() as this allows the startup to do its thing first.

Upvotes: 2

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