JNA calls are failing without any exception

Question

We are using JNA calls for running ImageMagick commands.

Initially every call is successful and we did not find any issues with these, later after some days we found some of the ImageMagick commands are failing. Now, every command is failing.

We don't find any exceptions in the server logs.

May I know what changes required to run these commands successfully.

public class runMagick {

    private interface CLibrary extends Library {
        CLibrary INSTANCE = (CLibrary) Native.loadLibrary((Platform.isWindows() ? "msvcrt" : "c"), CLibrary.class);
        int system(String cmd);
    }

    public void runCmd() {
        String strCommand = "convert test.jpg test.gif";
        CLibrary.INSTANCE.system(strCommand);
    }
}

OS: Linux JAVA Version: 1.6.0_34

Two thing we observed recently:

Every JNA calls are working fine after a server restart Found more number of java threads created and running. We used the following command to get the number of threads running

ps -e -T | grep java | wc -l

773

After server restart the count on the above command returning a less number.

ps -e -T | grep java | wc -l

153

Is the JNA calls will create separate java threads?

Is the number of java threads causing the failures for JNA calls?

Answer 1

You aren't getting any exceptions in Java because Java isn't seeing any problems. It's passing off the code to C, and getting control back after the C code runs, either successfully or not.

Notice that the system() call returns an int. This value has meaning:

The value returned is -1 on error, and the return status of the command otherwise.

So you should capture this return value, e.g.,

int ret = CLibrary.INSTANCE.system(strCommand);

Then you can test the value of ret. If the value is -1, it indicates a problem with running the shell command (system() calls /bin/sh -c so you would look there.) Otherwise, the return value will be the result of your command, in this case the convert command. Here is a list of possible results.

ret should equal 0 on success, and otherwise, you can look up the value of ret in the above link and get a specific reason why it didn't work.