Debugging with GDB: Cygwin Native

Debugging with GDB: Cygwin Native

21.1.4 Features for Debugging MS Windows PE Executables

GDB supports native debugging of MS Windows programs, including DLLs with and without symbolic debugging information.

MS-Windows programs that call SetConsoleMode to switch off the special meaning of the ‘Ctrl-C’ keystroke cannot be interrupted by typing C-c. For this reason, GDB on MS-Windows supports C-BREAK as an alternative interrupt key sequence, which can be used to interrupt the debuggee even if it ignores C-c.

There are various additional Cygwin-specific commands, described in this section. Working with DLLs that have no debugging symbols is described in Non-debug DLL Symbols.

info w32

This is a prefix of MS Windows-specific commands which print information about the target system and important OS structures.

info w32 selector

This command displays information returned by the Win32 API GetThreadSelectorEntry function. It takes an optional argument that is evaluated to a long value to give the information about this given selector. Without argument, this command displays information about the six segment registers.

info w32 thread-information-block

This command displays thread specific information stored in the Thread Information Block (readable on the X86 CPU family using $fs selector for 32-bit programs and $gs for 64-bit programs).

signal-event id

This command signals an event with user-provided id. Used to resume crashing process when attached to it using MS-Windows JIT debugging (AeDebug).

To use it, create or edit the following keys in HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug and/or HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\AeDebug (for x86_64 versions):

- Debugger (REG_SZ) — a command to launch the debugger. Suggested command is: fully-qualified-path-to-gdb.exe -ex "attach %ld" -ex "signal-event %ld" -ex "continue".
The first %ld will be replaced by the process ID of the crashing process, the second %ld will be replaced by the ID of the event that blocks the crashing process, waiting for GDB to attach.
- Auto (REG_SZ) — either 1 or 0. 1 will make the system run debugger specified by the Debugger key automatically, 0 will cause a dialog box with “OK” and “Cancel” buttons to appear, which allows the user to either terminate the crashing process (OK) or debug it (Cancel).

set cygwin-exceptions mode

If mode is on, GDB will break on exceptions that happen inside the Cygwin DLL. If mode is off, GDB will delay recognition of exceptions, and may ignore some exceptions which seem to be caused by internal Cygwin DLL “bookkeeping”. This option is meant primarily for debugging the Cygwin DLL itself; the default value is off to avoid annoying GDB users with false SIGSEGV signals.

show cygwin-exceptions

Displays whether GDB will break on exceptions that happen inside the Cygwin DLL itself.

set new-console mode

If mode is on the debuggee will be started in a new console on next start. If mode is off, the debuggee will be started in the same console as the debugger.

show new-console

Displays whether a new console is used when the debuggee is started.

set new-group mode

This boolean value controls whether the debuggee should start a new group or stay in the same group as the debugger. This affects the way the Windows OS handles ‘Ctrl-C’.

show new-group

Displays current value of new-group boolean.

set debugevents

This boolean value adds debug output concerning kernel events related to the debuggee seen by the debugger. This includes events that signal thread and process creation and exit, DLL loading and unloading, console interrupts, and debugging messages produced by the Windows OutputDebugString API call.

set debugexec

This boolean value adds debug output concerning execute events (such as resume thread) seen by the debugger.

set debugexceptions

This boolean value adds debug output concerning exceptions in the debuggee seen by the debugger.

set debugmemory

This boolean value adds debug output concerning debuggee memory reads and writes by the debugger.

set shell

This boolean values specifies whether the debuggee is called via a shell or directly (default value is on).

show shell

Displays if the debuggee will be started with a shell.