How to set a 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

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 

Answer 2

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) record (gdb) fin (gdb) reverse-step