idiomatic JNA to handle C string allocated in native code

[Kyle Hayes]

I am wrapping native code that returns an allocated C string.   The string is allocated with malloc() inside the native code.  I want to convert the string to a Java string and then make sure that the C string is freed so that I do not leak memory.   The C function returns a simple char * value.

What is the most idiomatic way to do this in JNA?   Note that due to the C API, I need the Java code to take ownership of the C string and deleted it as soon as it is converted to a Java string.  The string is in ASCII, not even UTF8.

I am already wrapping another API call that returns a const string from the C code.  Ownership there is with the C library (static strings) so there is nothing to free.  The default translation works great in that case.   The C API call there returns const char*.

Thanks for any pointers.

Best,

Kyle

[Daniel Widdis]

Generally if the native API has a function that allocates memory, the native API also has a function that releases the memory.  I don’t think there’s any way (idiomatic or not) to release memory from Java via JNA if it was allocated inside the native code.

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[Matthias Bläsing]

Hi Kyle,

I would wrap such a case so that the wrapper returns a plain "com.sun.jna.Pointer". That can be used to acess the string and can be used to free it.

The relevant functions are:

• com.sun.jna.Pointer#getString to fetch the value
• com.sun.jna.Pointer#nativeValue to convert from Pointer to long memory address (this is static and is invoked with the Pointer as an argument)
• com.sun.jna.Native#free to free the memory

In the JNA codebase such an API would have an abstract companion class "<Interface>Util", that does the unwrapping.

Greetings

Matthias

[Kyle Hayes]

Hi Matthias,

See below...

Oh!  OK.  Can I just declare the native function as returning a Pointer?

static private native Pointer plc_tag_get_string(int tag_id, int string_offset);

And then I wrap it:

public String getString(int stringOffset) {

    // error handling omitted

    Pointer rawPointer = plc_tag_get_string(this.id, stringOffset);

    String result = rawPointer.getString();

    Native.free(rawPointer);

    return result;

}

Something like that?

Thanks so much.  I had not looked at raw pointer values like this.

Best,

Kyle

[Matthias Bläsing]

Hi Kyle,

Am Samstag, den 31.10.2020, 07:53 -0700 schrieb Kyle Hayes:
> > [Function returning char* that needs freeing]

>
> Oh!  OK.  Can I just declare the native function as returning a
> Pointer?
>
> static private native Pointer plc_tag_get_string(int tag_id, int
> string_offset);
>
> And then I wrap it:
>
> public String getString(int stringOffset) {
>     // error handling omitted
>     Pointer rawPointer = plc_tag_get_string(this.id, stringOffset);
>     String result = rawPointer.getString();
>     Native.free(rawPointer);
>     return result;
> }
>
> Something like that?
>
> Thanks so much.  I had not looked at raw pointer values like this.

that should work in principle. The free should be:

Native.free(Pointer.nativeValue(rawPointer))

Greetings

Matthias

[Kyle Hayes]

Thanks, Matthias.

I will try this out.

Best,

Kyle