i have a rooted android phone, on which i run native c executables through the adb shell.
I am not using neither Java nor the Java NDK-apps, since i want to do low-level work with this app and no form of interaction with the user.
While everything runs fine, when i try to write a file (i.e. using std::ofstream of fprintf etc), the file is created but it is empty.
For Apps development i know you need to add a line in the Manifest.XML, so that you have write permissions.
Any suggestions for the native executable?
Related
I have C Linux based application and now I want to port it to Android.
I figured out, that I can extract the toolchain from Android NDK and build my application, but how to make the APK such that I can install it on the android devices without the need of root access.
In Linux, I used to install it using a bash script which used to put my application related files in different folders like /opt, /etc (files shared with other applications) and /var . How can we handle this in Android. Is there a folder similar to /etc in Android where I can put files that other applications can read.
Thanks
-M
First of all, you are lucky if your project compiles "as is" with NDK standalone toolchain. Often, bionic is not enough, and people need to tweak the build environment (from libpthread to full-blown buildroot alternate toolchain with static C runtime).
As for the shared files location, on Android it's named "external storage". Your app and other app may require special permissions to write and read to this location. Directory /opt does not exist here. You don't have write access to /etc, but files like /etc/hosts are available for read.
Regarding the APK. You are right, this is the ultimate way to distribute and install apps on Android. But you can, even without root, to locally install and run a command-line executable. Usually it's done with Developers Options turned on, and enabled USB debugging. Now you can open an adb shell, install and run your program. The trick is that external storage (see above) is marked as 'non-executable'. Therefore, you must find another place for your binary. Usually, /data/local/tmp will be a good choice.
Instead of adb, you can use a terminal emulator on the device.
If you choose to build an APK, you will probably prefer to convert your app to shared library that will perform actions for Java via JNI. But it is also possible to package your command-line binary as part of the APK and use Java Runtime.exec().
I downloaded the c4droid app for Android and am running commands through system();. I'm learning that somethings work while others don't. Most of the cool stuff don't work and this appears to be due to my user profile not being given the rights to execute such commands at the Linux OS level.
So tried another experiment. I got a special Gnu compiler for the arm processor and compiled a simple hello world app. Then I put on my phone and tried to execute it through the c4droid app like system("./myapp.bin"); . I got a permission denied message.
So I'm just trying to understand what I can do and what I can't do on my phone that paid good money for? Can I execute such a hello world app or not? Do I really need root access to execute an application I made? Is there a way to get my code to run by wrapping it in android/java code? Do I have to go through the Dalvikvm to get this to run?
I'm really looking for a way to do this without rooting it or downloading something like busybox and using su.
Many many different issues.
permission denied is one of the few error messages the primitive shell knows, and it's used for many other types of failures including not finding the requested command.
The toolbox suite is missing many typical unix commands. People sometimes install busybox as a more comprehensive supplement
Some of the things you may want to do will actually require permissions that the shell (or even more so, application user id) accounts do not have. Only on an emulator or engineering device does the adb shell run as root by default, though on a 'rooted' device you may be able to get to a root shell.
You do not need root access to run compiled code, however the code must have a suitable ABI, must have all it's library dependencies satisfied, and must be installed in a file with the executable flag, on a partition which is not mounted with non-executable flag. Some of the issues you face there are glibc (or other) vs Android's bionic libc. Executable scratch directories vary by android version, though the private storage area of an app is an option if the app sets the file to world readable and executable.
The usual (and only "recommended") means of running native code is to build it as a shared library linked against android's Bionic libc, and call into it from a dalvik/java app using JNI.
I don't know whether i make sense or not. But just wanted to now that can i run the c/c++ code in android without the need of writing JNI wrapper or using Native Activity way?
I mean, I am having a simple C++ Template Matching code which contains main function and I use to run it using g++ in terminal.
Now I want this Template Matching code to be run in android usng NDK. Is there any way? I have googled a lot but all they is to either use JNI wrapper or use SWIG which makes JNI wrapper, but can't actually get into any of them. I need more simpler solution.
If you have a rooted phone, you can compile and run programs as on any linux machine, from the adb shell.
However, that requires a bit of knowledge about Linux.
So, to start:
Root your phone (force it to give you admin privileges). That is easy for developer phones/boards, and a kill for regular phones bought at a shop downstreet.
compile your app for android and run it from adb as you would do from a linux shell.
Here is a more detailed answer on the matter
How to compile C into an executable binary file and run it in Android from Android Shell?
i have the following doubt:
i have read that android os is based on linux, and i imagine it may have the same structure that ubuntu (in terms of file configurations: /root, /dev, etc).
so, is it possible to run an application written in C in android? just as it is possible to do in ubuntu? if so, how can i do that?
also, how can i get access to the root files through an android application (written in java)? and how to get access to the behavior of the os (in terms of interruptions for example)?
thanks in advance for your answers.
gus
Basic answer: Running a C app on Android isn't supported, except through the Native Development Kit (NDK).
You can't get access to the root files except by rooting a phone.
You can get access to some OS behavior through the API. Basically, if it's not part of the API, you can't access it.
The Android OS is based on Linux, but it's an OS, not a windowing server like X or a desktop environment like Gnome or KDE.
You may run C and C++ code on android using NDK. You may use also QT framework. But code is runing in virtual machine named Davlik. Android have root acount , but it is default not available for user. Therefore, access to directory is dependend for chmod.
If you would like read about access to low level in android:
http://www.phrack.org/issues.html?issue=68&id=6
And about architecture this system:
https://developer.android.com/guide/basics/what-is-android.html
You can run programs using Runtime.exec. As an example, you can see Terminal IDE which runs many different programs including ssh, vim and bash. It's also open source so you can learn from the code. You will probably have to include the executable as a resource or asset and then copy to a local directory, grant execute permissions, then run with Runtime.exec. You still have limited user permissions as your app runs under a restricted account unless the device is rooted and you request root access.
an android smartphone/tablet works with an Arm cpu, not a x85. the architecture is different.
but you CAN run a C application in android if you cross compile it for arm linux. or you can use a c compiler inside android device. people ported c compiler to android. you can try C4DROID and in android market. but you can only run compiled program in system memory because of android permissions about sd card.
I want to run tcpdump on the NDK using Process.start() and want to capture this output to a buffer.Can anybody show me how i can do this?Also is the process created in the same Dalvik Vm as the JNI call or is it in a different VM?
Running native processes is not really a supported use of the ndk, but you wouldn't be the first person to do it.
Also note that tcpdump may need to be run as root or setuid root to be useful. And you do have a version of tcpdump compiled and linked appropriately for android?
I think that you actually want android.os.Exec.createSubprocess which is um, not public, rather than Process.start. There's a right way to do it using reflection, and a wrong way to do it by copying the Exec.java (which has declarations of the native methods) into your project. You can use a pipe to recover the results, or you can launch it through the shell and redirect the output to a file which you read from java.
See the source of an android terminal emulator with a local shell option for an example of how to do it.
(Actually, I should restate this - the right way to do it is to duplicate the native functionality of the non-public Exec methods in your own jni lib, so that you are immune to changes in that non-public part of the platform).