example_drv.c

#include <stdio.h>
#include "erl_driver.h"

typedef struct {
    ErlDrvPort port;
} example_data;

static ErlDrvData example_drv_start(ErlDrvPort port, char *buff) {
    example_data* d = (example_data*)driver_alloc(sizeof(example_data));
    d->port = port;
    return (ErlDrvData)d;
}

static void example_drv_stop(ErlDrvData handle) {
    driver_free((char*)handle);
}

static void example_drv_output(ErlDrvData handle, char *buff, int bufflen) {
    example_data* d = (example_data*)handle;
    char fn = buff[0], arg = buff[1], res;
    if (fn == 1) {
      res = foo(arg);
    } else if (fn == 2) {
      res = bar(arg);
    }
    driver_output(d->port, &res, 1);
}

ErlDrvEntry example_driver_entry = {
    NULL,			/* F_PTR init, N/A */
    example_drv_start,		/* L_PTR start, called when port is opened */
    example_drv_stop,		/* F_PTR stop, called when port is closed */
    example_drv_output,		/* F_PTR output, called when erlang has sent */
    NULL,			/* F_PTR ready_input, called when input descriptor ready */
    NULL,			/* F_PTR ready_output, called when output descriptor ready */
    "example_drv",		/* char *driver_name, the argument to open_port */
    NULL,			/* F_PTR finish, called when unloaded */
    NULL,			/* F_PTR control, port_command callback */
    NULL,			/* F_PTR timeout, reserved */
    NULL			/* F_PTR outputv, reserved */
};

DRIVER_INIT(example_drv) /* must match name in driver_entry */
{
    return &example_driver_entry;
}