Assembly multiple definition -

giving code

section .data                             msg     db      "hello, world!",0xa      len     equ     $ - msg                    section .text                                                    ;we must export entry point elf linker or      global _start         _start:           mov     eax,4             mov     ebx,1             mov     ecx,msg           mov     edx,len           int     0x80                mov     eax,1             xor     ebx,ebx          int     0x80 

when try run it, shows command

linux1[8]% nasm -f elf -l hello.lst hello.asm linux1[9]% ls hello.asm  hello.lst  hello.o linux1[10]% gcc -o hello hello.o hello.o: in function `_start': hello.asm:(.text+0x0): multiple definition of `_start' /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../lib64/crt1.o:(.text+0x0): first defined here hello.o: not read symbols: file in wrong format collect2: ld returned 1 exit status 

how fix multiple definition problem? define _start once, how comes out said multiple definition? thanks

you using gcc link , default add c libraries expect entry point main , contain _start invokes main. that's why have multiple definition.

if not need c library (and in code doesn't), still use gcc linking, try gcc -nostdlib -m32 -o hello hello.o.

the wrong format error due trying produce 64 bit executable 32 bit object file. adding -m32 fixes 32 bit executable (since code 32 bit). if intend create 64 bit program, use -f elf64 nasm , of course write 64 bit compatible code.