This code was originally shipped alongside gdb by Fedora. It has since been extended by a number of contributors. It inherits the GPLv3 licence from the original Fedora python code.
Put the two python scripts somewhere handy, then copy gdbinit to ~/.gdbinit (or add the python scripts to your existing gdbinit) after editing the python include path in your ~/.gdbinit to add the directory where you installed the python scripts.
Then invoke gdb on your compiled binary and check the offsets of your C++ or C classes and structs like so:
$ gdb test-pahole
GNU gdb (Debian 10.1-1.7) 10.1.90.20210103-git
...
Reading symbols from test-pahole...
(gdb) pahole C
/* 16 */ struct C {
/* 0 4 */ int i
/* 4 1 */ char j
/* XXX 3 */ !! __24_bit_padding__
/* 8 8 */ void * l
}
(gdb) pahole testStruct
/* 152 */ struct testStruct {
/* 0 2 */ short a
/* 2 1 */ char b
/* XXX 5 */ !! __40_bit_padding__
/* 8 144 */ testStruct::more_struct more
} testStruct
When specifying walk right after the type name, pahole will walk your object recursively and reveal the content of nested structs and unions:
(gdb) pahole testStruct walk
/* 152 */ struct testStruct {
/* 0 2 */ short a
/* 2 1 */ char b
/* XXX 5 */ !! __40_bit_padding__
/* 8 144 */ struct testStruct::more_struct {
/* 0 128 */ union testStruct::more_struct::union_x {
/* 0 128 */ char [128] c
/* 0 2 */ struct testStruct::more_struct::union_x::s_y {
/* 0 1 */ char v
/* 1 0 */ char g1_1:7
/* XXX 0 */ !! __1_bit_padding__
} y
/* XXX 126 */ !! __1008_bit_padding__
} x
/* 128 8 */ long e
/* 136 0 */ char s1_1:1
/* 136 0 */ char s1_2:1
/* 136 0 */ char s2:2
/* XXX 7 */ !! __60_bit_padding__
} more
} testStruct
The script should work on both gdb using Python 2 (gdb versions <7.8 IIRC) and Python 3.