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I have taken Code Aurora's FM Radio code and merged with my Android Gingerbread codebase.

The FM app framework tries to access the fm radio device ( /dev/radio ) using JNI which is implemented in a file by name android_hardware_fm.cpp . There is a function in this file which tries to acquire a file descriptor to the device node using open() in the read/write mode. However, the call fails with error code -13 : Permission denied.

I also made a small C executable which tries to open the /dev/radio file ( in RDWR mode), prints its fd and closes it. It runs from /system/bin in the target system and displays a valid fd.

Btw, the JNI implementation is part of the android core library. It is located in frameworks/base/core/jni and is compiled as part of libandroid_runtime.so

Any ideas/solutions? Thanks in advance.

trans1st0r
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  • 4 years later I'm having a really similar issue: http://stackoverflow.com/questions/37486349 I even ran my executable as the same UID as the app and it works, and of course did everything in the answer below. Did you ever find a solution? – jkoreska May 27 '16 at 15:58
  • @jkoreska i think i used a workaround. see my comment in the answer below. – trans1st0r May 27 '16 at 19:43

1 Answers1

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Clearly you donot have permissions to open the device from user space. In the second case when you are running the executable from terminal, you are having permissions probably because you have done su before running the executable.

For your problem here, two things can be done.

1) Change the permissions of the node from terimnal.

Steps involved:

  • Open the terminal (adb shell)

  • Do su(In order to do this your device must be rooted)

  • Do chmod 777 /dev/radio in the terminal

Once this is done, your radio node is having proper permissions for the user to read and write. So you can now do open() call and it will work.

2) Programmatically you can achieve this (assuming your device is rooted and su is running on your device) by calling the below function - changePerm(). This is a small function I have written which will change the permissions of the device nodes or rather any system file that does not have user access. Once you have permissions, you can open it from user space. open() call will work properly after this.

void changePerm()
{
    Process chperm;
    try {
        chperm=Runtime.getRuntime().exec("su");


   DataOutputStream os = 
          new DataOutputStream(chperm.getOutputStream());
        os.writeBytes("chmod 777 /dev/radio\n");
        os.flush();

        os.writeBytes("exit\n");
        os.flush();

          chperm.waitFor();

} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
} catch (InterruptedException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}
}

I have tested this for other nodes. So it should also work for radio aswell. Let me know in case yo are facing any difficulty. Thanks

Sandeep
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  • One using adb shell has root permission my default. What I did after you suggested is put the command "chmod 777 /dev/radio0" in init.rc ...This however is a temporary fix. – trans1st0r May 08 '12 at 10:32
  • for the changes in init.rc to take effect you need to recompile the android and falash it – Sandeep May 08 '12 at 11:36
  • Oh in this case if you have root permissions by default, su might not be present on your device. Then only 1st method (without su is applicable) – Sandeep May 08 '12 at 11:45