Obtain the native value of errno from Java in macOS

Question

I'm porting my JNA-based library to "pure" Java using the Foreign Function and Memory API (JEP 424) in JDK 19.

I've successfully implemented the sysctl() function and am fetching values using:

public static MethodHandle sysctl = Linker.nativeLinker().downcallHandle(
        SYSTEM_LIBRARY.lookup("sysctl").orElseThrow(), FunctionDescriptor.of(ValueLayout.JAVA_INT,
                ValueLayout.ADDRESS, ValueLayout.JAVA_INT, ValueLayout.ADDRESS, ValueLayout.ADDRESS,
                ValueLayout.ADDRESS, ValueLayout.JAVA_LONG));

In the process of debugging before getting this working, I was occasionally getting a -1 return value:

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

In JNA, this is easily obtainable using Native.getLastError(). This answer shows how I could write code in JNI to fetch it, but the whole point of JEP 424 is to remove the need for JNA or JNI.

The Windows API has a helpful GetLastError function that can be mapped, as demonstrated in this test in the JDK

However, I was unable to find any native function on macOS that I could map to return errno. The closest I have found is perror() which prints the error to STDERR:

public static MethodHandle perror = Linker.nativeLinker()
        .downcallHandle(SYSTEM_LIBRARY.lookup("perror").orElseThrow(),
                FunctionDescriptor.of(ValueLayout.ADDRESS, ValueLayout.ADDRESS));

While output to STDERR was helpful in the near term, I'd like to use this value for exception handling/error messages.

This seems a common enough use case that there should be a method in the JDK for it, but I can't find it.

Any ideas other than temporarily redirecting STDERR into a string I can parse?

score 3 · Accepted Answer · answered Sep 25 '22 at 13:38

I don't have access to a mac to test this, but if this source is accurate: https://opensource.apple.com/source/xnu/xnu-4570.41.2/bsd/sys/errno.h.auto.html

Then errno is a macro defined like so:

__BEGIN_DECLS
extern int * __error(void);
#define errno (*__error())
__END_DECLS

It's a call to a function that returns an int* that points to the errno value.

So, to access it from Java this should work:

static final MethodHandle __error = Linker.nativeLinker()
    .downcallHandle(SYSTEM_LIBRARY.lookup("__error").orElseThrow(),
            FunctionDescriptor.of(ValueLayout.ADDRESS));

static int errno() {
    try {
        MemoryAddress addr = (MemoryAddress) __error.invokeExact();
        return addr.get(ValueLayout.JAVA_INT, 0);
    } catch (Throwable t) {
        throw new RuntimeException(t);
    }
}

Be aware that at the moment (JDK 19) this is not a 100% reliable way to access errno, since the VM might block somewhere and override errno with its own system calls before the value can be read.

We've been discussing ways to make it more reliable to access errno and other thread-locals that the VM can overwrite. At the same time, that should also make it much easier to access the value.

This works! Bonus points for the code suggestion for the wrapper method! — Daniel Widdis, Sep 25 '22 at 20:17

score 0 · Answer 2 · answered Sep 25 '22 at 10:05

Ah, the function you looked for is called strerror, which can map error codes(errno) to strings.

Based on Apple Documentation. Error messages are set in the errno variable:

When a system call detects an error, it returns an integer value indicat-ing indicating ing failure (usually -1) and sets the variable errno accordingly. <This allows interpretation of the failure on receiving a -1 and to take action accordingly.> Successful calls never set errno; once set, it remains until another error occurs. It should only be examined after an error.

#include <string.h> //header for strerror
#include <stdio.h>
#include <sys/errno.h> //Header for error enumerations.

int main(int argc, const char * argv[]) {

  //taking "EPERM" as an errno example
  int eperm = EPERM;

  char * errStr = strerror(eperm);

  printf("%s\n", errStr);//prints 'Operation not permitted'

  return 0;
}

So try strerror to convert errno to error message string

The problem with strerror is that it requires having errno to start with! — Daniel Widdis, Sep 25 '22 at 14:58

Obtain the native value of errno from Java in macOS

2 Answers2