Disassembly a C code [closed]

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How is it possbile to disassembly a C Code? I already read a few Questions here (stackoverflow). But if you want to disassembly you need a Machine code so how do this with nasm ? So if I create for ex. an Hello World in C how can do this ?

Answer 1

Nasm is a bad idea. There are a few options. IDA pro has given me some success, but if you really know your assembly, you can nm for symbols, then hexdump the code from there and manually make assembly out of it. There really isn't just a way to use nasm to produce recompilable code though.

otool (or objdump) will produce assembly.

If you need some examples: here:

#include <stdio.h>
main(argc, argv)
int argc; char * * argv;
{
    printf("Hello, World\n");
}

nm output:

hydrogen:tmp phyrrus9$ nm a.out
0000000100000000 T __mh_execute_header
0000000100000f40 T _main
                 U _printf
                 U dyld_stub_binder

otool output:

hydrogen:tmp phyrrus9$ otool -tv a.out
a.out:
(__TEXT,__text) section
_main:
0000000100000f40    pushq   %rbp
0000000100000f41    movq    %rsp, %rbp
0000000100000f44    subq    $0x10, %rsp
0000000100000f48    leaq    0x37(%rip), %rdi         ; this is our string
0000000100000f4f    movb    $0x0, %al
0000000100000f51    callq   0x100000f66              ; call printf
0000000100000f56    movl    $0x0, %ecx
0000000100000f5b    movl    %eax, 0xfffffffffffffffc(%rbp)
0000000100000f5e    movl    %ecx, %eax
0000000100000f60    addq    $0x10, %rsp
0000000100000f64    popq    %rbp
0000000100000f65    ret

hexdump output not shown.

Actual assembly:

hydrogen:tmp phyrrus9$ cat tmp.s
.section    __TEXT,__text,regular,pure_instructions
.globl  _main
.align  4, 0x90
_main:                                  ## @main
.cfi_startproc
## BB#0:
pushq   %rbp
Ltmp2:
.cfi_def_cfa_offset 16
Ltmp3:
.cfi_offset %rbp, -16
movq    %rsp, %rbp
Ltmp4:
.cfi_def_cfa_register %rbp
subq    $16, %rsp
leaq    L_.str(%rip), %rdi
movb    $0, %al
callq   _printf
movl    $0, %ecx
movl    %eax, -4(%rbp)          ## 4-byte Spill
movl    %ecx, %eax
addq    $16, %rsp
popq    %rbp
ret
.cfi_endproc

.section    __TEXT,__cstring,cstring_literals
L_.str:                                 ## @.str
.asciz   "Hello, world!\n"


.subsections_via_symbols

Hope this helps you get a grasp.