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debuggingassemblygdbbreakpoints

Breakpoint has Two Addresses?

I recently set a breakpoint where the initial address was :

(gdb) b viewscreen_movieplayerst::create(char, viewscreenst*)
Breakpoint 1 at 0x804beec

and the second (usual address) was :

   (gdb) run

   Breakpoint 1, 0xf7b46630 
   in viewscreen_movieplayerst::create(char,viewscreenst*)()
   from/path/libs/libgraphics.so

Is this because the executable is stripped? Or is the executable executed and the address changes after the execution?

Also, main is :

(gdb) b main 
Breakpoint 1 at 0x804bdec

Which seems pretty close to the address so I'll include it.

Edit:

What does the concept of relocation mean?

So most of the binary is composed of reloc table?

Solution

  • Perhaps you are seeing the Procedure Linkage Table (PLT) address from a shared library function on Unix?

    The compiler builds a table of 'trampoline' addresses that indirectly refer to the shared library dependencies. It is a complicated topic, there is a detailed article here

    Here is a simple example.

    lib.cpp

    #include <iostream>


int foo()
{
  return 0;
}

    junk.cpp

    #include<iostream>

int foo();
int main(int argc, char* argv[])
{
  int k = foo();
  return 0;
}

    Compile 

    /tmp$ g++ -fPIC -g -std=c++11 -shared -o libtest_library.so lib.cpp
/tmp$ g++  -g -std=c++11  -o tramp junk.cpp -L . -l test_library

    gdb tramp

    Reading symbols from tramp...done.
(gdb) p foo
$1 = (<text from jump slot in .got.plt, no debug info>) 0x4006d6 <foo()@plt+6>
(gdb) b foo
Breakpoint 1 at 0x4006d0
(gdb) d 1
(gdb) r
Starting program: /tmp/tramp 
lib:  0x602010
[Inferior 1 (process 12804) exited normally]
(gdb) b foo
Breakpoint 2 at 0x4006d0 (2 locations)
(gdb) info break
Num     Type           Disp Enb Address            What
2       breakpoint     keep y   <MULTIPLE>         
2.1                         y     0x00000000004006d0 <foo()@plt>
2.2                         y     0x00002aaaaacd0a99 in foo() at lib.cpp:6

    As you can see the symbol 'foo' has two addresses and two breakpoints. One is the PLT and the other the symbol location in the shared library.

    As the comments noted, template instantiations and compiler optimizations can also create multiple addresses for one symbol or source line.