Android compilation calls C code, android compiles c code

Android compilation calls C code, android compiles c code

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Preface demand Source

In the past few days, it has no function to help others build a simple android client. The main function is to call the C code to set the Wi-Fi network of the mobile phone, which leads to technical difficulties, how to callC programAchieve what we need.

Solution

I have come up with two solutions for this:

Final Choice

Finally, I chose the second solution. The reason is that the second method is relatively simple and operable when I already have executable files. The first solution is to download android NDK, which is a collection of tools and provides dynamic libraries that help developers quickly develop C or C ++, it is very convenient to automatically package so and java applications into apk.

Technical Implementation Executable File

First, you need to get an executable file. Of course, the executable file you want is not as simple as you think, not directly in linux.GccThe C code is required here.Cross-CompilationObtain executable files that can be run on the android server. Here we will not go into details about how to cross-compile the C files. You can search for them online. In addition, NDK is also a good tool.

Resource Storage

The resource storage page here is a small pitfall. When we write a java program, if we want to open a file, we can directly enter the path, for example, if the file to be used is in the project directory, you can directly enter the file name to call it. However, the running environment here is an embedded device, not a PC, which involves a problem, how to store resources.

Here we will talk aboutAsset folderAndRes/raw folderSimilarities and Differences:

• Similarities
  • Files in both directories are packagedIntactWill not be compiled into binary.
• Differences
  • Files in res/raw will be mapped to R. java. You can directly use the resource ID when accessing the files. Files in the assets folder will not be mapped to R. java.
  • Res/raw cannot haveDirectory structureAnd create a folder under the assets Directory.

Resource Acquisition

Let's talk about it here.Res/rawThe following method is used to obtain the input stream for write operations.

1	InputStream is =getResources().openRawResource(R.id.filename);

The next step is how to readAssetsThe following method is also used to obtain the input stream for write operations.

123	AssetManager am = null; am = getAssets(); InputStream is = am.open("filename");

Note:It is said that Assert can only put a single file up1 MBut it is not really specific and has not been verified, if you encounter problems, you should consider this point of attention.

Although the read is successfulShellIf the script is executed, the file should be stored on the mobile phone. If the file is read, it cannot be found on the mobile phone. Therefore, we need an operation to store the file. Here I wrote a function for saving files, which combines methods for getting data from assets.

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public void copyDataToSD(String outFileName)throws IOException{InputStream myInputStream;OutputStream myOutputStream = new FileOutputStream(outFileName);myInputStream = this.getAssets().open("a.out");byte[] buffer = new byte[1024];int length = myInputStream.read(buffer);while (length > 0) {myOutputStream.write(buffer, 0, length);length = myInputStream.read(buffer);}myOutputStream.flush();myInputStream.close();myOutputStream.close();}

The imported outFileName that I defined is the defined file path and file name.

12	private static String EXE_PATH = "data/data/com.example.g3wifi/a.out";private static File exe_file;

Shell Command Execution

Now, everything is ready, and we have nothing to do with it. We need to execute the executable file, because android is based onLinuxTherefore, some basic commands are supported. to execute shell commands in android, follow the format below:

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Public void exeC (String cmd) throws IOException {Runtime runtime = Runtime. getRuntime (); Process process = runtime.exe c (cmd); // Process process = runtime.exe c (new String [] {"su", "reboot "}); // you can run two commands. // The result of executing the shell command is BufferedReader ie = new BufferedReader (new InputStreamReader (process. getErrorStream ()));}

How does android call c code?

Use JNI to compile c code into a. so file using NDK...

How to compile c source code in the android Environment

You need to compile this file into an executable file. I can use it in linux on the pc (use the makefile automatically generated by configure carved by the author to compile the file) and it is unavailable on the android platform, how to compile the android platform