What does python do inside the gdb debugger?

Question

I was debugging a C++ program in the gdb debugger and tried to access the 5th element of vector which only contains 4 element. After trying it, this error was on the screen:

(gdb) list main
1   #include <memory>
2   #include <vector>
3   
4   int main(int argc, char *argv[]){
5   
6   
7       std::vector<int> v_num = {1, 3, 5, 67};
8       std::unique_ptr<std::vector<int>> p(&v_num);
9   
10  
(gdb) p v_num.at (5)
Python Exception <class 'IndexError'> Vector index "5" should not be >= 4.: 
Error while executing Python code.
(gdb)

I didn't expect to see a Python exception inside the the gdb. Can someone explain why I encountered such error? Does gdb uses python internally?

I guess it is Python Xmethods for std::vector sourceware.org/gdb/onlinedocs/gdb/Xmethods-In-Python.html. — ks1322
– ks1322, Commented Jul 18, 2022 at 17:49

ks1322 · Accepted Answer · 2022-07-19 21:11:14Z

4

Does gdb uses python internally?

Yes, it uses Python a lot to extend itself in many ways, see https://sourceware.org/gdb/onlinedocs/gdb/Python.html#Python.

What you discovered is called Python Xmethods, see https://sourceware.org/gdb/onlinedocs/gdb/Xmethods-In-Python.html. Xmethods are used as a replacement of inlined or optimized out method defined in C++ source code. libstdc++ has a number of xmethods for standard containers. You can see them with info xmethod command. For std::vector there are 6 xmethods defined on my box:

  libstdc++::vector
    size
    empty
    front
    back
    at
    operator[]

You can disable all xmethods with disable xmethod command. If you do it, GDB will unable to call at method:

(gdb) p v_num.at (5)
Cannot evaluate function -- may be inlined

answered Jul 19, 2022 at 21:11

ks1322

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3 Comments

Amir reza Riahi Over a year ago

Thanks for your answer. Does this apply only to libstdc++? If my own member classes become inline by decision of the compiler, there is no way to use xmethods to recover them in gdb?

ks1322 Over a year ago

You can use xmethods for your class the same way as you do it for libstdc++, but you have to write them yourself. For libstdc++ they are already written by libstdc++ developers.

phreed Over a year ago

When I create an 'xmethod' how do I activate it in GDB? I tried 'enable xmethod /home/me/xmethods MyClass'. I gave me an error, complaining about the locus not being a regexp. Any advice on how to specify the locus? I will open a new issue about this.

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