Run C library from Node.js

Question

I'm completely new to C, but have a small program (interfacing with hardware on RaspberryPi) that I'd like to be able to run from Node.js. From what I can make out in the Node.js docs, I can run a C++ program by exporting the program as a NODE_MODULE http://nodejs.org/api/addons.html

I've been trying to figure out the differences between C and C++, but am unsure if I can just export the code I want to run as a C++ file (maybe by changing the file extension to .cc?) Or if there is another way to use the C code in node.js.

Also, I don't understand if I need to 'build' the C file, or if I can provide node.js with the .c file extension.

I do not want to run the C code using the Node's child process, though I know that is possible. I would much prefer to export the C code as a module, as the Node.js documents describe.

Here's the code I'm looking to run in node.js

    //  How to access GPIO registers from C-code on the Raspberry-Pi
//  Example program
//  15-January-2012
//  Dom and Gert
//


// Access from ARM Running Linux

#define BCM2708_PERI_BASE        0x20000000
#define GPIO_BASE                (BCM2708_PERI_BASE + 0x200000) /* GPIO controller */

// WOULD I INCLUDE NODE.js HERE?? ##define BUILDING_NODE_EXTENSION
#include <node.h>

using namespace v8;

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <dirent.h>
#include <fcntl.h>
#include <assert.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <bcm2835.h>
#include <unistd.h>

#define MAXTIMINGS 100

//#define DEBUG

#define DHT11 11
#define DHT22 22
#define AM2302 22

int readDHT(int type, int pin);

int main(int argc, char **argv)
{
  if (!bcm2835_init())
        return 1;

  if (argc != 3) {
    printf("usage: %s [11|22|2302] GPIOpin#\n", argv[0]);
    printf("example: %s 2302 4 - Read from an AM2302 connected to GPIO #4\n", argv[0]);
    return 2;
  }
  int type = 0;
  if (strcmp(argv[1], "11") == 0) type = DHT11;
  if (strcmp(argv[1], "22") == 0) type = DHT22;
  if (strcmp(argv[1], "2302") == 0) type = AM2302;
  if (type == 0) {
    printf("Select 11, 22, 2302 as type!\n");
    return 3;
  }

  int dhtpin = atoi(argv[2]);

  if (dhtpin <= 0) {
    printf("Please select a valid GPIO pin #\n");
    return 3;
  }


  printf("Using pin #%d\n", dhtpin);
  readDHT(type, dhtpin);
  return 0;

} // main


int bits[250], data[100];
int bitidx = 0;

int readDHT(int type, int pin) {
  int counter = 0;
  int laststate = HIGH;
  int j=0;

  // Set GPIO pin to output
  bcm2835_gpio_fsel(pin, BCM2835_GPIO_FSEL_OUTP);

  bcm2835_gpio_write(pin, HIGH);
  usleep(500000);  // 500 ms
  bcm2835_gpio_write(pin, LOW);
  usleep(20000);

  bcm2835_gpio_fsel(pin, BCM2835_GPIO_FSEL_INPT);

  data[0] = data[1] = data[2] = data[3] = data[4] = 0;

  // wait for pin to drop?
  while (bcm2835_gpio_lev(pin) == 1) {
    usleep(1);
  }

  // read data!
  for (int i=0; i< MAXTIMINGS; i++) {
    counter = 0;
    while ( bcm2835_gpio_lev(pin) == laststate) {
    counter++;
    //nanosleep(1);     // overclocking might change this?
        if (counter == 1000)
      break;
    }
    laststate = bcm2835_gpio_lev(pin);
    if (counter == 1000) break;
    bits[bitidx++] = counter;

    if ((i>3) && (i%2 == 0)) {
      // shove each bit into the storage bytes
      data[j/8] <<= 1;
      if (counter > 200)
        data[j/8] |= 1;
      j++;
    }
  }


#ifdef DEBUG
  for (int i=3; i<bitidx; i+=2) {
    printf("bit %d: %d\n", i-3, bits[i]);
    printf("bit %d: %d (%d)\n", i-2, bits[i+1], bits[i+1] > 200);
  }
#endif

  printf("Data (%d): 0x%x 0x%x 0x%x 0x%x 0x%x\n", j, data[0], data[1], data[2], data[3], data[4]);

  if ((j >= 39) &&
      (data[4] == ((data[0] + data[1] + data[2] + data[3]) & 0xFF)) ) {
     // yay!
     if (type == DHT11)
    printf("Temp = %d *C, Hum = %d \%\n", data[2], data[0]);
     if (type == DHT22) {
    float f, h;
    h = data[0] * 256 + data[1];
    h /= 10;

    f = (data[2] & 0x7F)* 256 + data[3];
        f /= 10.0;
        if (data[2] & 0x80)  f *= -1;
    printf("Temp =  %.1f *C, Hum = %.1f \%\n", f, h);
    }
    return 1;
  }

  return 0;
}

Answer 1

If you're completely new to C, it might actually be less of a headache to do this all from javascript-land instead, using one of several modules on npm for interacting with GPIO (on the Pi). One such module is the onoff module.