$3

`c
#include "stdlib/napi/argv_dataview_cast.h"
`

#### STDLIB_NAPI_ARGV_DATAVIEW_CAST( env, X, type, argv, index )

Macro for converting an add-on callback argument corresponding to a DataView to a pointer having a specified data type.

`c
struct complex128 {
double re;
double im;
};

static void fcn( const struct complex128 X, struct complex128 Y ) {
Y->re = X->re + Y->re;
Y->im = X->im + Y->im;
}

// ...

static napi_value addon( napi_env env, napi_callback_info info ) {
// Retrieve add-on callback arguments:
STDLIB_NAPI_ARGV( env, info, argv, argc, 2 );

// Convert the first argument to a C type:
STDLIB_NAPI_ARGV_DATAVIEW_CAST( env, X, struct complex128, argv, 0 );

// Convert the second argument to a C type:
STDLIB_NAPI_ARGV_DATAVIEW_CAST( env, Y, struct complex128, argv, 1 );

// ...

fcn( X, Y );
}
`

The macro expects the following arguments:

- env: environment under which the callback is invoked.
- X: output variable name.
- type: output variable type.
- argv: name of the variable containing add-on callback arguments.
- index: argument index.