'ndk-gdb' Overview
IMPORTANT: IF YOU ARE DEBUGGING THREADED PROGRAMS, PLEASE READ THE
SECTION BELOW TITLED 'Thread Support'.
I. Usage:
---------
The Android NDK r4 introduced a helper shell script named 'ndk-gdb' to
easily launch a native debugging session for your NDK-generated machine code.
The script is located at the top-level directory of the NDK, and shall
be invoked from the command-line when in your application project
directory, or any of its sub-directories. For example:
cd $PROJECT
$NDK/ndk-gdb
Where $NDK points to your NDK installation path. You can also create an
alias or add $NDK to your PATH to avoid typing it every time.
IMPORTANT: Native debugging can only work if *all* these conditions are met:
1. Your application is built with the 'ndk-build' script:
Building with the legacy "make APP=<name>" method is not
supported by ndk-gdb.
2. Your application is debuggable:
In other words, your AndroidManifest.xml has an <application>
element that sets the android:debuggable attribute to "true"
3. You are running your application on Android 2.2 (or higher):
ndk-gdb will not work if you try to run your application on
previous versions of the system. That does not mean that your
application should target the Android 2.2. API level, just
that the debugging session should happen on a 2.2+ device or
emulator system image.
IMPORTANT IMPORTANT IMPORTANT !!
If you are using the ADT Eclipse plug-in to build your
application, make sure you're using version 0.9.7 or
later.
If you are using the 'ant' build tool, make sure that you
have the latest revision of the SDK Platform components.
The following minimal revisions are required:
Android 1.5 r4
Android 1.6 r3
Android 2.1 r2
Android 2.2 r1
These should be available through the SDK updater.
If these conditions are not met, the generated .apk will
not contain required support files and native debugging
will not be possible.
'ndk-gdb' handles many error conditions and will dump an informative error
message if it finds a problem. For example, it:
- checks that adb is in your path.
- checks that your application is declared debuggable in its manifest.
- checks that, on the device, the installed application with the same
package name is also debuggable.
By default, ndk-gdb will search for an already-running application process,
and will dump an error if it doesn't find one. You can however use the --start
or --launch=<name> option to automatically start your activity before the
debugging session.
When it successfully attaches to your application process, ndk-gdb will give
you a normal GDB prompt, after setting up the session to properly look for
your source files and symbol/debug versions of your generated native
libraries.
You can set breakpoints with 'b <location>' and resume execution with 'c'
(for 'continue'). See the GDB manual for a list of commands.
IMPORTANT: When quitting the GDB prompt, your debugged application process
will be stopped! This is a gdb limitation.
IMPORTANT: The GDB prompt will be preceded by a long list of error messages,
where gdb complains that it cannot find various system libraries
(e.g. libc.so, libstdc++.so, liblog.so, libcutils.so, etc...)
This is normal, because there are no symbol/debug versions of
these libraries corresponding to your target device on your
development machine. You can safely ignore these messages.
II. Options:
------------
To see a list of options, type 'ndk-gdb --help'. Notable ones are:
--verbose:
Print verbose information about the native debugging session setup.
Only needed to debug problems when you can't connect and that the
error messages printed by ndk-gdb are not enough.
--force:
By default, ndk-gdb aborts if it finds that another native debugging
session is running on the same device. Using --force will kill the
session, and replace it with a new one. Note that the debugged program
is *not* killed and will be stopped again.
--start:
By default, ndk-gdb will try to attach to an existing running instance
of your application on the target device. You can use --start to
explicitly launch your application before the debugging session.
NOTE: This launches the first launchable activity listed from your
application manifest. Use --launch=<name> to start another one.
See --launch-list to dump the list of such activities.
--launch=<name>:
This is similar to --start, except that it allows you to start a specific
activity from your application. This is only useful if your manifest
defines several launchable activities.
--launch-list:
Convenience option that prints the list of all launchable activity names
found in your application manifest. The first one will be used by --start
--project=<path>:
Specify application project directory. Useful if you want to launch
the script without cd-ing to the directory before that.
--port=<port>:
By default, ndk-gdb will use local TCP port 5039 to communicate with
the debugged application. By using a different port, it is possible
to natively debug programs running on different devices/emulators
connected to the same development machine.
--adb=<file>:
Specify the adb tool executable, in case it is not in your path.
-d, -e, -s <serial>:
These flags are similar to the ADB ones and allow you to handle the
case where you have several devices/emulators connected to your
development machine.
-d: Connect to a single physical device
-e: Connect to a single emulator device
-s <serial>: Connect to a specific device or emulator
where <serial> is the device's name as listed
by the "adb devices" command.
Alternatively, you can define the ADB_SERIAL environment variable
to list a specific device, without the need for a specific option.
--exec=<file>:
-x <file>:
After connecting to the debugged process, run the GDB initialization
commands found in <file>. This is useful if you want to do something
repeatedly, e.g. setting up a list of breakpoints then resuming
execution automatically.
III. Requirements:
------------------
At the moment 'ndk-gdb' requires a Unix shell to run. This means that
Cygwin is required to run it on Windows. We hope to get rid of this
limitation in a future NDK release.
The other NDK requirements apply: e.g. GNU Make 3.81 or higher.
IV. Thread Support:
-------------------
If your application runs on a platform older than Android 2.3, ndk-gdb will
not be able to debug native threads properly. Instead, the debugger will only
be able to put breakpoints on the main thread, completely ignoring the
execution of other ones.
The root of the problem is complex, but is essentially due to a very unfortunate
bug in the platform, which was only discovered lately.
The gdbserver binary that comes with this NDK has special code to detect this
condition at runtime and adapt its behaviour automatically (in other words,
you don't have anything special to do when building your code).
What this means in practical terms are:
- If you are on Android 2.3, or a prior platform release which has had the
platform bugfix back-ported to it, you will be able to debug native
threads automatically.
- If you are not, you will only be able to debug the main thread
(as in previous NDK releases). You will also see the following message
when launching ndk-gdb (just before the gdb prompt):
Thread debugging is unsupported on this Android platform!
If you place a breakpoint on a function executed on a non-main thread, the
program will exit with the following message in GDB:
Program terminated with signal SIGTRAP, Trace/breakpoint trap.
The program no longer exists.