[node-ffi] Using function pointers - callback

[Christian Guimaraes]

Hi!

I'm trying to wrap a C library using node-ffi and I have some doubts regarding function pointers.

The C library has a following callback function:

// typedef void (*logfunc_t) (context_t *context, loglevel_t loglevel, const char *file, unsigned int line, const char *function, const char *message, void *userdata);

var logFunctionPtr = ffi.Function( ref.refType(ref.types.void), [ contextTPtr, "int", "string", "int", "string", "string", ref.refType(ref.types.void) ]);

And below the FFI definition that I did:

var myLib = ffi.Library("mylib", {

  // dc_status_t -> type int

  // dc_status_t context_set_logfunc (context_t *context, logfunc_t logfunc, void *userdata);

  "context_set_logfunc": [ "int", [ contextTPtr, logFunctionPtr, ref.refType(ref.types.void) ] ]

});

1. Is it the code above ok?

2. How should I pass the callback (as parameter, in red) to the function call? I'm trying this:

myLib.context_set_logfunc(context.ref(), ref.alloc(logFunctionPtr), null);

I'm having trouble trying to find some node-ffi examples where/how a callback function is defined and used?

Thanks.

--

Christian Guimarães

[Nathan Rajlich]

Currently the best place for examples of ffi.Function usage would be in the test files: https://github.com/node-ffi/node-ffi/blob/master/test/function.js#L31-L42

As for your case:

 1) I believe that you need to just use "void" rather than ref.refType(ref.types.void) for the logFunctionPtr return value, since it looks like the C declaration is defined that way.

 2) When invoking context_set_logfunc(), you pass the JS function directly where the `logfunc` argument goes. So probably something like:

myLib.context_set_logfunc(context.ref(), function(context, loglevel, file, line, fn, message, userData) {

  // do something with the arguments!

}, null);

Hope that helps. Cheers!

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[Christian Guimaraes]

Hey Nathan! 

This has helped me a lot!

Thanks.

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