Setting breakpoint in GDB where the function returns

Question

I have a C++ function which has many return statements at various places. How to set a breakpoint at the return statement where the function actually returns ?

And what does "break" command without argument means?

Answer 1

Break without argument sets a breakpoint at the current line.

There is no way for a single breakpoint to catch all return paths. Either set a breakpoint at the caller immediately after it returns, or break at all return statements.

Since this is C++, I suppose you could create a local sentry object, and break on its destructor, though.

Answer 2

break without arguments stops execution at the next instruction in the currently selected stack frame. You select strack frames via the frame or up and down commands. If you want to debug the point where you are actually leaving the current function, select the next outer frame and break there.

Answer 3

You can use reverse debugging to find out where function actually returns. Finish executing current frame, do reverse-step and then you should stop at just returned statement.

(gdb) fin
(gdb) reverse-step

Answer 4

Contrary to answers so far, most compilers will create a single return assembly instruction, regardless of how many return statements are in the function (it is convenient for the compiler to do that, so there is only a single place to perform all the stack frame cleanup).

If you wanted to stop on that instruction, all you have to do is disas and look for retq (or whatever the return instruction for your processor is), and set a breakpoint on it. For example:

int foo(int x)
{
  switch(x) {
   case 1: return 2;
   case 2: return 3;
   default: return 42;
  }
}

int main()
{
  return foo(0);
}


(gdb) disas foo
Dump of assembler code for function foo:
   0x0000000000400448 <+0>: push   %rbp
   0x0000000000400449 <+1>: mov    %rsp,%rbp
   0x000000000040044c <+4>: mov    %edi,-0x4(%rbp)
   0x000000000040044f <+7>: mov    -0x4(%rbp),%eax
   0x0000000000400452 <+10>:    mov    %eax,-0xc(%rbp)
   0x0000000000400455 <+13>:    cmpl   $0x1,-0xc(%rbp)
   0x0000000000400459 <+17>:    je     0x400463 <foo+27>
   0x000000000040045b <+19>:    cmpl   $0x2,-0xc(%rbp)
   0x000000000040045f <+23>:    je     0x40046c <foo+36>
   0x0000000000400461 <+25>:    jmp    0x400475 <foo+45>
   0x0000000000400463 <+27>:    movl   $0x2,-0x8(%rbp)
   0x000000000040046a <+34>:    jmp    0x40047c <foo+52>
   0x000000000040046c <+36>:    movl   $0x3,-0x8(%rbp)
   0x0000000000400473 <+43>:    jmp    0x40047c <foo+52>
   0x0000000000400475 <+45>:    movl   $0x2a,-0x8(%rbp)
   0x000000000040047c <+52>:    mov    -0x8(%rbp),%eax
   0x000000000040047f <+55>:    leaveq 
   0x0000000000400480 <+56>:    retq   
End of assembler dump.
(gdb) b *0x0000000000400480
Breakpoint 1 at 0x400480
(gdb) r

Breakpoint 1, 0x0000000000400480 in foo ()
(gdb) p $rax
$1 = 42