Simon pointed out that gdb_printing_disassembler::m_in_comment can be used uninitialised by the Python disassembler API code. This issue was spotted when GDB was built with the undefined behaviour sanitizer, and causes the gdb.python/py-disasm.exp test to fail like this: (gdb) PASS: gdb.python/py-disasm.exp: global_disassembler=GlobalPreInfoDisassembler: python add_global_disassembler(GlobalPreInfoDisassembler) disassemble main Dump of assembler code for function main: 0x0000555555555119 <+0>: push %rbp 0x000055555555511a <+1>: mov %rsp,%rbp 0x000055555555511d <+4>: nop /home/user/src/binutils-gdb/gdb/disasm.h:144:12: runtime error: load of value 118, which is not a valid value for type 'bool' The problem is that in disasmpy_builtin_disassemble we create a new instance of gdbpy_disassembler, which is a sub-class of gdb_printing_disassembler, however, the m_in_comment field is never initialised. This commit fixes the issue by providing a default initialisation value for m_in_comment in disasm.h. As we only ever disassemble a single instruction in disasmpy_builtin_disassemble then we don't need to worry about reseting m_in_comment back to false after the single instruction has been disassembled. With this commit the above issue is resolved and gdb.python/py-disasm.exp now passes.