Using C struct and pointers

In this example, we show how to use C structs and pointers with js-ctypes.

Declaring a js-ctypes struct matching a C struct

If we have a C structure like this:

struct st_t
{
  void   *self;
  char   *str;
  size_t  buff_size;
  int     i;
  float   f;
  char    c;
};

we can use it in JavaScript by writing something like this:

var st_t = new ctypes.StructType("st_t",
                      [ { "self": ctypes.PointerType(ctypes.void_t) },
                        { "str": ctypes.PointerType(ctypes.char) },
                        { "buff_size": ctypes.size_t },
                        { "i": ctypes.int },
                        { "f": ctypes.float },
                        { "c": ctypes.char } ]);

Here we are using the StructType() factory method of the ctypes object to create a CType object that represents the C struct named st_t.  The first parameter of this method is the name of the type, which corresponds to the name of the C struct.  The second parameter is an array of field descriptors.  Each field descriptor contains the name and field type of the corresponding field of the C struct.  The call to StructType() returns a CType object, and we then apply the new operator to it to create a specific instance of this newly defined type - a JavaScript representation of the C struct.

Using C strings with js-ctypes

A pointer to char in JavaScript is declared as follows:

var str = ctypes.PointerType(ctypes.char);

Now imagine you call a C function that returns a C string and you want to modify the contents of this string. In the js-ctypes context, you can only operate on buffers of known capacity. Therefore, the size of the buffer must be specified. Additionally, the js-ctypes pointer must be casted to reflect the size of the buffer:

var ptr = ctypes.cast( str, ctypes.ArrayType( ctypes.char, buff_size ).ptr );

Here buff_size is of type ctypes.size_t.

Once we have a ctypes char pointer that points to a buffer of known size, we modiify the contents of the memory block as follows:

ptr.contents = String("Hello world from JavaScript!!!");

String() adds the '\0' character.

References

This example is based on code found here.