How can I show gdb to show my variables properly?

Question

For example I have a simple substring class and a simple array. When I'm debugging it's a headache because I need many clicks just to get some kind of sensible information. Is there a way I can markup my source of have some kind of config that says when I print the variable a I actually mean

p *a.start@(a.end-a.start)

When I exam array I'd like it to show the variable in the style above (p *arr.array@arr.pos is a mess)

Current GDB output

$ gdb ./a.out 
(gdb) br a.cpp:38
Breakpoint 1 at 0x128f: file a.cpp, line 38.
(gdb) r
(gdb) p a
$1 = {start = 0x555555556004 "My test string that has two parts", end = 0x555555556012 " that has two parts"}
(gdb)

My Source compiled with g++ -g a.cpp

#include <cstdio>
struct MyString
{
    const char *start, *end;
    int size() { return end-start; }
};
template<class T>
struct MyArray
{
    int pos;
    T array[10];
    MyArray() : pos(0) {}
    void push(T val) {
        if (pos >= 10)
            return;
        array[pos++] = val;
    }
};

struct MoreComplex
{
    int val;
    MyString substring;
};

int main() {
    const char* str = "My test string that has two parts";
    MyString a{str, str+14};
    MyString b{str+20, str+27};
    MoreComplex c{5, b};
    MyArray<MyString> arr;
    arr.push(a);
    arr.push(b);

    MyArray<MoreComplex*> arr2;
    arr2.push(&c);
    puts("Breakpoint here");
    return 0;
}

You can write macros in gdb, related: [Print the whole linked list in gdb?](https://stackoverflow.com/questions/16480045/print-the-whole-linked-list-in-gdb) — Yksisarvinen, Oct 30 '20 at 23:07

score 2 · Accepted Answer · answered Oct 31 '20 at 00:37

If you have a python-enabled build of gdb, you can add custom pretty printers for your types.

You could try adding the following to your .gdbinit:

python

class MyStringPrinter:
    "Print a MyString"

    def __init__ (self, val):
        self.val = val

    def to_string (self):
        ptr = self.val['start']
        len = self.val['end'] - ptr
        return ptr.string (length = len)

    def display_hint (self):
        return 'string'

pp = gdb.printing.RegexpCollectionPrettyPrinter("mine")
pp.add_printer('MyString', '^MyString$', MyStringPrinter)
gdb.printing.register_pretty_printer(gdb.current_objfile(), pp)

end

This will give you this result when printing a MyString object:

(gdb) p a
$1 = "My test string"
(gdb) p b
$2 = "has two"
(gdb) p c
$3 = {val = 5, substring = "has two"}

Thanks. I tried doing the same for array but got stuck https://stackoverflow.com/q/64618273/12936536 — Eric Stotch, Oct 31 '20 at 04:16

How can I show gdb to show my variables properly?

1 Answers1

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