Function pointers

I am trying to pass function pointers as struct members to a c api using ffi. Here is the function definitions and the struct in c

struct Session {
    PWLOG *pwLog;
    PWACCEPT *pwAccept;
    PWSEND *pwSend;
};

void * create_session(struct Session *session);

typedef int PWLOG(const char *type, const char *msg);

typedef const char * PWACCEPT(const char *keys, const char *replyStr, size_t replyStrSize);

typedef void PWSEND(const char *type, const char *msg);

Here is the struct defined in javascript using ref-struct with ffi.

const pwLogFunc = function(type, message) {

    console.log(type.readCString(), message.readCString());
};

const pwSendFunc = function (type, msg) {

   console.log(type.readCString(), msg.readCString());
};

const pwAcceptFunc= function (keys, replyStr, replyStrSize) {

   console.log(keys.readCString(), replyStr.readCString(), replyStrSize);
}


const Session = Struct({
    pwLog:  pwLogFunc,
    pwSend: pwSendFunc,
    pwAccept: pwAcceptFunc
});

const structPtr = Ref.refType(Session);    

const lib = Ffi.Library(dllPath, { 
    'create_session': ['void *', [structPtr]]
});

const session = new Session({
    pwLog: pwLogFunc,
    pwAccept: pwAcceptFunc,
    pwSend: pwSendFunc
});

const result = lib.create_session(session.ref());

Any help would be greatly appreciated as my knowledge of c is very limited!

[asm-jaime]

Perhaps the topic is already outdated, but here’s a example, how to call a function pointer inside a c-struct: data.h:

typedef struct data {
  int some;       // some variable
  void (*exec)(); // pointer to a function
} data;

void exec_data(data *v);

data.c:

#include "data.h"

void exec_data(data *v) {
  v->some = v->some >> 2;
}

wrapper.c, here we can allocate/free memory for the structs:

#include <stdint.h>
#include <stdlib.h>

#include "data.h"

#if defined(WIN32) || defined(_WIN32)
#define EXPORT __declspec(dllexport)
#else
#define EXPORT
#endif

EXPORT data *init_data(void){
  data *p_data = (data *)malloc(1*sizeof(data));
  p_data->some = 0;
  p_data->exec = exec_data;
  return p_data;
}

EXPORT void free_data(void *v){
  free(v);
}

after gcc -shared -fpic data.c wrapper.c -o data.so we can use it in data.js:

'use strict'

const ffi = require('ffi');
const ref = require('ref');
const struct = require('ref-struct');

const data = struct({
  'some': 'int',
});
data.defineProperty('exec', ffi.Function('void', [ref.refType(data)]));

const dataPtr = ref.refType(data);

const lib = ffi.Library('./data', {
  'init_data': [ dataPtr, [ ] ],
  'free_data': ['void', ['void *']]
})

const r_data = lib.init_data().deref();

r_data.some = 100;
console.log(`data.some: ${r_data.some}`);
r_data.exec(r_data.ref());  // <== call function pointer
console.log(`data.some: ${r_data.some}`);

lib.free_data(r_data.ref());

result:

data.some: 100
data.some: 25

I hope this example will be useful.

[TooTallNate]

If your pwCreate() function really does return the Session *, then you probably want to iteratively define the struct fields, so that you can access Session:

const Session = Struct();
Session.defineProperty('pwLog', ffi.Function(ref.refType(Session), ['string', 'string']),
...