Call C++ dll with struct containing char array from Node.js

Question

I'm using the Node.js ffi addon to call a C++ DLL.

The problem I'm having is with the struct I'm supplying - it contains a char array - I don't believe I'm setting this up correctly. As a result I am unable to access the contents.

Routine's definition from C++ header file:

int GetSysConfig(MyConfig * config);

The MyConfig struct is defined in C++ as follows:

typedef struct{
    int attribute;
    char path[256];
}MyConfig;

My corresponding Node.js struct definition:

var ffi = require('ffi');
var ref = require('ref');
var StructType = require('ref-struct');
var ArrayType = require('ref-array');

// This seems to be the problematic part?
var charArray = ArrayType('char');
charArray.length = 256;

var MyConfig = StructType({
    'attribute' : 'int',
    'path' : charArray
})

Note: Below here is where I call the DLL from Node.js - I don't think there's a problem here although I could be wrong.

// Create a pointer to the config - we know we expect to supply this to the C++ routine.
var myConfigPtr  = ref.refType(MyConfig);

var lib = ffi.Library('my.dll', {
 "GetSysConfig": ["int", [myConfigPtr]]
});

var myConfigObj = new MyConfig();

lib.GetSysConfig.async(myConfigObj.ref(), function(err, res) {
    console.log("attribute: " + myConfigObj.attribute);
    // This is always empty [] - when it shouldn't be.
    console.log("path: " + JSON.Stringify(myConfigObj.path));
});

Does anyone know where I'm going wrong with this?

Davie Brown · Accepted Answer · 2016-06-27 08:56:55Z

4

For structs containing arrays: These should be defined with their size specified as a parameter to ArrayType.

For example:

ArrayType('char', 256)

Therefore the fix for my problem is the following:

var MyConfig = StructType({
    'attribute' : 'int',
    'path' : ArrayType('char', 256)
})

edited Jun 27, 2016 at 8:56

answered Jun 15, 2016 at 16:04

Davie Brown

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