assembly - Segfault with pop/push in x86/OSX -

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i struggling understand why x86 assembly code, compiles fine gcc 4.2.1 (llvm) on osx, gives segmentation fault when executable run: 

    .globl  _main _main:         push    %rbp         mov     %rsp, %rbp         mov     $1, %rbx         push    %rbx         lea     l_.str0(%rip), %rdi         mov     %rbx, %rsi         call    _printf         pop     %rbx         pop     %rbp         ret          .section        __text,__cstring,cstring_literals l_.str0:         .asciz  "%d \000"

i observed if pop %rbx line moved before call _printf, program works correctly. why should fail @ in original form?

this question answered in detail by: how print argv[0] in nasm? , x86 assembly on mac. it's gotcha when programming assembly on macosx.

to summarize:

  • this seg fault due stack misalignment.
  • this occurs on oses use system v calling convention (including macosx, excluding linux), insists stack pointer multiple of 16 prior making function call (e.g. printf).

a simple solution align stack pointer (ie, align multiple of 16 bytes per sys v requirement) prior call, , restore after call: 

.globl  _main _main:         push    %rbp         mov     %rsp, %rbp         mov     $1, %rbx         lea     l_.str0(%rip), %rdi         mov     %rbx, %rsi         push    %rbx      mov %rsp, %rax   ; save copy of stack pointer (sp)     , $-16, %rsp   ; align sp nearest multiple of 16.     sub $8, %rsp     ; pad sp 8 bytes when ...       push %rax        ; push saved sp (=8 bytes on 64-bit os),                       ; remain aligned 16 bytes (8+8 = 16).          call    _printf      pop %rax         ; retrieve saved sp     mov %rax, %rsp   ; restore sp using saved value.           pop     %rbx         pop     %rbp         ret          .section        __text,__cstring,cstring_literals l_.str0:         .asciz  "%d \000"

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