Segmentation fault assembler

Question

When I run my program I have segmentation fault error. I don't understand why.

   .data
    str1: .string "hello"
    str2: .string "world"
.text
.globl _start
_start:
    pushl $str1
    call puts
    call strcall
    call finish

strcall:
    pushl $str2
    call puts
    ret
finish:
    movl $1, %eax
    movl $0, %ebx
    int $0x80

Any idea why is it happening like that?

Answer 1

The normal cdecl calling convention mandates that the caller remove the arguments it has placed on the stack. Since you don't do that in strcall, the pushl $str2 is still on the stack and ret will try to use that as the return address. Solution: insert addl $4, %esp before the ret.

Next time use a debugger to see where the problem is for yourself.

Also, if you intend to use C library functions, you should really use main as entry point and compile with gcc so you get the C library properly initialized. Similarly, you should not use the exit system call, you should just return from main or if abolsutely necessary, call exit from the C library.