Different disassembly for the same function?

Question

I have this code:

#include <stdio.h>
int main()
{
printf("a=%d; b=%d; c=%d", 1, 2, 3);
return 0;
};

I compile it with gcc -g 3arg.cpp. Disassemble with objdump -d -M intel a.out. In the output there is exactly one function named _init and it is disassembled like this:

00000000004003e0 <_init>:
  4003e0:       48 83 ec 08             sub    rsp,0x8
  4003e4:       48 8b 05 0d 0c 20 00    mov    rax,QWORD PTR [rip+0x200c0d]        # 600ff8 <_DYNAMIC+0x1d0>
  4003eb:       48 85 c0                test   rax,rax
  4003ee:       74 05                   je     4003f5 <_init+0x15>
  4003f0:       e8 3b 00 00 00          call   400430 <__gmon_start__@plt>
  4003f5:       48 83 c4 08             add    rsp,0x8
  4003f9:       c3                      ret

This is gdb output:

$ gdb -q a.out
Reading symbols from a.out...done.
(gdb) disassemble _init
Dump of assembler code for function _init:
   0x00000000004003e0 <+0>:  sub    $0x8,%rsp
   0x00000000004003e4 <+4>:  mov    0x200c0d(%rip),%rax        # 0x600ff8
   0x00000000004003eb <+11>: test   %rax,%rax
   0x00000000004003ee <+14>: je     0x4003f5 <_init+21>
   0x00000000004003f0 <+16>: callq  0x400430 <__gmon_start__@plt>
   0x00000000004003f5 <+21>: add    $0x8,%rsp
   0x00000000004003f9 <+25>: retq   
End of assembler dump.
(gdb) break _init
Breakpoint 1 at 0x4003e0
(gdb) run
Starting program: [redacted]/a.out 

Breakpoint 1, _init (argc=1, argv=0x7fffffffdf68, envp=0x7fffffffdf78) at ../csu/init-first.c:52
52  ../csu/init-first.c: No such file or directory.
(gdb) disassemble _init
Dump of assembler code for function _init:
=> 0x00007ffff7a36c20 <+0>:  push   %r13
   0x00007ffff7a36c22 <+2>:  push   %r12
   0x00007ffff7a36c24 <+4>:  mov    %edi,%r12d
   0x00007ffff7a36c27 <+7>:  push   %rbp
   0x00007ffff7a36c28 <+8>:  mov    %rsi,%rbp
   0x00007ffff7a36c2b <+11>: push   %rbx
[etc.]

Why is the last disassembly different? It's not even the same data disassembled in a different way (As seen when comparing the objdump output with output of gdb command x/10b $rip when the breakpoint has been hit). If it's a different _init function:

• how can I put a breakpoint at the one that's seen in the objdump output and the first disassembly in gdb?
• why did gdb say Breakpoint 1 was at 0x4003e0 and then stopped at a different function?

Answer 1

To arrive to your _init function just continue execution. This will stop once again in the intended place.

    gdb -q test.out
    Reading symbols from test.out...done.
    (gdb) b _init
    Breakpoint 1 at 0x4003e0
    (gdb) run
    Starting program: /home/[censored]/stk/test.out 

    Breakpoint 1, _init (argc=1, argv=0x7fffffffe008, envp=0x7fffffffe018) at ../csu/init-first.c:52
    52  ../csu/init-first.c: No such file or directory.
    (gdb) bt
    #0  _init (argc=1, argv=0x7fffffffe008, envp=0x7fffffffe018) at ../csu/init-first.c:52
    #1  0x00007ffff7dea0fd in call_init (l=0x7ffff7ff74c0, argc=argc@entry=1, argv=argv@entry=0x7fffffffe008, env=env@entry=0x7fffffffe018) at dl-init.c:64
    #2  0x00007ffff7dea223 in call_init (env=<optimized out>, argv=<optimized out>, argc=<optimized out>, l=<optimized out>) at dl-init.c:36
    #3  _dl_init (main_map=0x7ffff7ffe1c8, argc=1, argv=0x7fffffffe008, env=0x7fffffffe018) at dl-init.c:126
    #4  0x00007ffff7ddb30a in _dl_start_user () from /lib64/ld-linux-x86-64.so.2
    #5  0x0000000000000001 in ?? ()
    #6  0x00007fffffffe364 in ?? ()
    #7  0x0000000000000000 in ?? ()
    (gdb) c
    Continuing.

    Breakpoint 1, 0x00000000004003e0 in _init ()
    (gdb) bt
    #0  0x00000000004003e0 in _init ()
    #1  0x0000000000400593 in __libc_csu_init ()
    #2  0x00007ffff7a36e55 in __libc_start_main (main=0x40052d <main>, argc=1, argv=0x7fffffffe008, init=0x400560 <__libc_csu_init>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffdff8)
        at libc-start.c:246
    #3  0x0000000000400469 in _start ()
    (gdb) info shared
    From                To                  Syms Read   Shared Object Library
    0x00007ffff7ddaae0  0x00007ffff7df54e0  Yes         /lib64/ld-linux-x86-64.so.2
    0x00007ffff7a344a0  0x00007ffff7b79583  Yes         /lib/x86_64-linux-gnu/libc.so.6
    (gdb) x/10i $pc
=> 0x4003e0 <_init>:    sub    $0x8,%rsp
   0x4003e4 <_init+4>:  mov    0x200c0d(%rip),%rax        # 0x600ff8
   0x4003eb <_init+11>: test   %rax,%rax
   0x4003ee <_init+14>: je     0x4003f5 <_init+21>
   0x4003f0 <_init+16>: callq  0x400430 <__gmon_start__@plt>
   0x4003f5 <_init+21>: add    $0x8,%rsp
   0x4003f9 <_init+25>: retq 
(gdb) info breakpoints
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   <MULTIPLE>         
    breakpoint already hit 2 times
1.1                         y     0x00000000004003e0 <_init>
1.2                         y     0x00007ffff7a36c20 in _init at ../csu/init-first.c:52

According to information about shared libraries, it stopped first time in address area of libc.so, which looks like also has _init function, which caused gdb to set this breakpoint there too (see breakpoint definition at the end of the listing).

If you want to set a breakpoint in your specific address, you can use

 b *address

notation. If you want to set a breakpoint on a specific function from specific source file you can use something like

(gdb) break test.c:call1

The same thing is discussed a bit deeper here .

Good luck.