GDB - how to get into stepping mode from the very beginning

Question

Typically, to enter a stepping mode from the very beginning of a C++ program execution, one uses the break main command in GDB. But that breaks the program only at the entrance to the main() function.

How can I break a program on a very first user-written operation (say, a constructor of a statically defined class instance)?

For example, if I had the following code, how could I break at A() without using the break 5 command?

#include <iostream>

struct A {
    A() {
        std::cout << "A()" << std::endl;
    }
};

static A a;

int main() {
    return 0;
}

In reality, I debug a very large code written by someone else. The code has a lot of static class instances scattered across different source files. That is not feasible to manually set breakpoints on each of the constructors.

Answer 1

You can define a break point at the constructor.

(gdb) break 'A::A()'
Breakpoint 1 at 0x8048724: file x.cc, line 4.
(gdb) run
Starting program: /.../a.out

Breakpoint 1, A::A (this=0x804a0ce <a>) at x.cc:4
4            std::cout << __func__ << std::endl;
(gdb) bt
#0  A::A (this=0x804a0ce <a>) at x.cc:4
#1  0x08048700 in __static_initialization_and_destruction_0 (__initialize_p=1,
    __priority=65535) at x.cc:8
#2  0x0804871c in _GLOBAL__sub_I_main () at x.cc:10
#3  0x080487a2 in __libc_csu_init ()
#4  0xb7d44a1a in __libc_start_main (main=0x80486ad <main()>, argc=1,
    argv=0xbffff184, init=0x8048750 <__libc_csu_init>,
    fini=0x80487c0 <__libc_csu_fini>, rtld_fini=0xb7fed180 <_dl_fini>,
    stack_end=0xbffff17c) at libc-start.c:246
#5  0x080485d1 in _start ()
(gdb)

Note the use of single quotes to indicate that the identifier is C++ mangled. Also note that the stack trace shows that main() has not been called yet.

From the stack trace, there are a number of choices to set a breakpoint that will be before any global constructor gets called. One such breakpoint is on _start.

(gdb) break _start
Breakpoint 1 at 0x80485b0
(gdb) run
Starting program: /.../a.out

Breakpoint 1, 0x080485b0 in _start ()
(gdb) bt
#0  0x080485b0 in _start ()
(gdb)