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Question

I have a (big) class and an offset into that class. How can I efficiently find the member defined at that offset?

Example:

struct Dummy {
    int a, b, c;
}

Given an offset of 4 and assuming sizeof(int) == 4 , I would like to get 'b'.

Obviously I wouldn't want do this at runtime, so I have been playing around with nm , objdump and gdb for a while now, but don't manage to do this.

Answer 1

Had the same problem earlier today,

The most suitable tool I can find is pahole . Usage example:

$ cat dummy.cpp
#include <string>

struct Dummy {
    int a, b, c;
    std::string d;
};
struct Dummy x; // gcc doesn't emit debug info for unused stuff
$ g++ -c dummy.cpp -ggdb3
$ pahole dummy.o
die__process_class: tag not supported (template_type_parameter)!
//trimmed structs __va_list_tag, tm and lconv
struct Dummy {
    int                        a;                    /*     0     4 */
    int                        b;                    /*     4     4 */
    int                        c;                    /*     8     4 */
    string                     d;                    /*    16     8 */
    //trimmed some constructors
};

Unfortunately, it doesn't print template contents, like std::string aka std::basic_string< char>.

I also found pstruct (easier confused than pahole, doesn't accept C++ at all), Clang -cc1 -fdump-record-layouts (it gave me a pile of pointers to Clang's address space, but no offsets) and MSVC -d1reportAllClassLayout (that flag made no difference when I tried).

Or the manual method:

Memset the struct to zero, set the relevant offsets to 0xFF, print struct in gdb (use set print pretty on so it doesn't put everything on a single huge line), grep the output for nonzeroes, and hope the STL pretty-printers don't get too confused.

Maybe I should make a GDB module to automate this, it's quite tedious no matter what I do...