How to compile an Erlang driver?

Question

I am trying to better understand how Erlang drivers work, and I started with a simple example from a book, but when I went to compile the C file containing the native Erlang driver code I got the following compile error messages:

/tmp/ccQ0GroH.o:example1_lid.c:(.text+0xe): undefined reference to driver_alloc' /tmp/ccQ0GroH.o:example1_lid.c:(.text+0x2f): undefined reference todriver_free' /tmp/ccQ0GroH.o:example1_lid.c:(.text+0xb0): undefined reference to `driver_output'

Does anyone know why this might be happening and how I could fix it? The C file is posted below for reference.

Thanks.

/* example1_lid.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 = twice(arg);
    } else if (fn == 2) {
      res = sum(buff[1], buff[2]);
    }
    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
               data to the port */
    NULL,               /* F_PTR ready_input, 
                           called when input descriptor ready to read*/
    NULL,               /* F_PTR ready_output, 
                           called when output descriptor ready to write */
    "example1_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;
}

Answer 1

Your code implies you are trying to build a linked in driver. Such drivers should be compiled as shared libraries as documented. If you use gcc, you need to pass -shared and -fpic. The fact that you are getting an undefined reference error from the linker suggests you are not trying to build a shared library.

There are at least two additional issues here:

1. The example in your book is quite outdated. This is fine if you are using the same version of Erlang. If you are using the latest release, you should refer to the documentation. In particular, your ErlDrvEntry structure is too small as it now includes many other fields.

2. DRIVER_INIT macro must take an argument that shall match the name in driver_entry, as mentioned in the comment. Yet, this is not the case in the code: the name is "example1_drv" while the macro is invoked with example_drv.

Additionally, driver_free now (?) takes void* and the cast is superfluous.