Does JNI preserve errno?

Question

I'm writing some JNI code to wrap some C functions. Some of these C functions set errno is case of an error. My code would be simpler if I could move the exception handling from the JNI code to the Java code. My plan is to have to JNI function return null in case of an error and then in the Java code query errno and construct and throw and exception there. For this to work JNI would have to leave errno untouched.

Answer 1

If I were you, I'd have gone with the exception as soon as possible

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/errno.h>
#include "recipeNo019_ThrowException.h"

JNIEXPORT void JNICALL Java_recipeNo019_ThrowException_throwException
  (JNIEnv * env, jobject obj) {

  char exceptionBuffer[1024];

  if(fopen("/this_will_fail", "r") == NULL) {

    sprintf (exceptionBuffer, "%d:%s", errno, strerror(errno));
    (*env)->ThrowNew (env,
                    (*env)->FindClass (env, "java/lang/Exception"),
                    exceptionBuffer);
  }

}

and then, in Java code, I'd have decomposed the error code and error message.

Then, inside Java you can easily access both: errno and message.

public static native void throwException();

...
...

try {
  throwException();  
} catch(Exception ex) {
  ex.getMessage();
}

---

Update

Note that with JNI being involved into play, things may get complicated (especially with shared data - errno).

You can imagine one of the scenarios - like this:

public static class ErrnoGenerator implements Runnable {

  boolean shouldIFail = false;
  ThrowException exceptionGenerator = null;

  public ErrnoGenerator(
             ThrowException exceptionGenerator, 
             boolean shouldIFail) {
    this.exceptionGenerator = exceptionGenerator;
    this.shouldIFail = shouldIFail;
  }

  public void run() {
    try {
      if(shouldIFail) {
        exceptionGenerator.throwException( shouldIFail );
        Thread.sleep(1000);
        System.out.println(
            "true:    expected == 2: " 
          + exceptionGenerator.getErrno());
      } else {
        Thread.sleep(500);
        exceptionGenerator.throwException( shouldIFail );
        System.out.println(
            "false: no expectations: " 
          + exceptionGenerator.getErrno());
      }
    } catch (InterruptedException e) {
    }
  }
}

public static void main(String[] args) {
  ThrowException thx = new ThrowException();

  Thread t1 = new Thread(new ErrnoGenerator(thx, true));
  Thread t2 = new Thread(new ErrnoGenerator(thx, false));
  t1.start();
  t2.start();
  while(t1.isAlive() || t2.isAlive());
}

However, even without Java based threads calling JNI, you can enter race condition as well

// This will set errno to "2"
thx.throwException( true );
try {
  Path file = Paths.get("/tmp/path-to-file");
  byte[] buf = "hello".getBytes();
  Files.write(file, buf);
} catch(Exception ex) {
}

// it's gone - errno contains something completely
// different to what you have expected to be found
System.out.println(
    "true:    expected == 2: " 
  + thx.getErrno());