Access to C struct function pointer by C++ class

I want to write an event handler that runs a callback function when a bzip2 (v1.0.6) block is closed.

Starting with the bz_stream struct in bzlib.h, I add a function pointer to it as a member:

typedef struct {
    /* all the usual bz_stream member variables... */
    void (*block_close_callback)();
} bz_stream;

In my C++ header (foo.h), I declare a callback method signature for my_callback, and attempt to set block_close_callback to this method when initializing a bz_stream * instance in the constructor:

#ifndef FOO_H
#define FOO_H

#include <cstdio>
#include <cstdlib>
#include "bzlib.h"

namespace foo {
    class Foo {
        private:
            bz_stream* _bz_stream_ptr;
        public:
            Foo();
            ~Foo();
            bz_stream* get_bz_stream_ptr(void);
            void set_bz_stream_ptr(bz_stream** ptr);
            void free_bz_stream_ptr(void);
            static void my_callback(void);
    };
    bz_stream* Foo::get_bz_stream_ptr(void) { return _bz_stream_ptr; }
    void Foo::set_bz_stream_ptr(bz_stream** ptr) { _bz_stream_ptr = *ptr; }
    void Foo::free_bz_stream_ptr(void) { free(_bz_stream_ptr); _bz_stream_ptr = NULL; }

    Foo::Foo() {
        bz_stream* bz_stream_ptr = NULL;
        bz_stream_ptr = static_cast<bz_stream *>( malloc(sizeof(bz_stream)) );
        if (!bz_stream_ptr) {
            fprintf(stderr, "Error: Could not allocate space for bz stream\n");
            exit(ENOMEM);
        }
        bz_stream_ptr->block_close_callback = my_callback;
        set_bz_stream_ptr(&bz_stream_ptr);
    }

    Foo::~Foo() {
        if (get_bz_stream_ptr()) {
            free_bz_stream_ptr();
        }
    }
}

#endif

In the definition (foo.cpp), I define the callback function void foo::Foo::my_callback(void):

#include "foo.h"

int main(int argc, char** argv) {
    return EXIT_SUCCESS;
}

void foo::Foo::my_callback(void) {
    fprintf(stderr, "Callback called\n");
}

When I try to compile the C++ class, I get the following error:

error: no member named 'block_close_callback' in 'bz_stream'

What am I doing wrong in my modification to bzlib.h, such that the C++ class is unable to access this struct member?