SerialIP and the passing of Variables

I've been playing with SerialIP for the past few days and have been remarkably successful in connecting it with ubuntu. Lately I have been trying to pass my own data from the arduino and viewing it in the browser using the web server example.

I promise I will get the offending code up soon but my problem is the following: Any time I try to pass an integer to PSOCK_SEND, the arduino server either hangs or the browser times out and no page is displayed.

I have tried using sprintf() and a couple other means of converting an integer to char* but they all seem to bring the whole thing down.

Does anyone have similar problems?

Does anyone have similar problems?

Would it help if we did?

I promise I will get the offending code up soon

I promise to try to help, then.

I have tried using sprintf() and a couple other means of converting an integer to char* but they all seem to bring the whole thing down.

I know that it sounds trite, but, if you do it right, it will work. We need to see what you are doing that is not (quite) right.


I’m trying to pass temperature data through PSOCK_SEND_STR(&s->p, tempchar);

 * SerialIP Hello World example.
 * SerialIP is a TCP/IP stack that can be used over a serial port (a bit
 * like a dial-up Internet connection, but without the modem.)  It works with
 * stock Arduinos (no shields required.)  When attached to a PC supporting
 * SLIP, the Arduino can host network servers and access the Internet (if the
 * PC is configured to share its Internet connection of course!)
 * SerialIP uses the fine uIP stack by Adam Dunkels <>
 * For more information see the SerialIP page on the Arduino wiki:
 *   <>
 *      -----------------
 * This Hello World example sets up a server at on port 1000.
 * Telnet here to access the service.  The uIP stack will also respond to
 * pings to test if you have successfully established a SLIP connection to
 * the Arduino.
 * SLIP connection set up under Linux:
 *  # modprobe slip
 *  # slattach -L -s 115200 -p slip /dev/ttyUSB0     (see note below)
 *  # ifconfig sl0 dstaddr
 *  # ping
 *  # telnet 1000
 * Here is the address you will give to your PC, and it must be
 * unique on your LAN. is the IP you will give the Arduino.  It
 * must also be unique, and must match the address the Arduino is expecting
 * as set by the "myIP" variable below.
 * Note that slattach won't return so you'll need to run ifconfig from
 * another terminal.  You can press Ctrl+C to kill slattach and release the
 * serial port, e.g. to upload another sketch.
 * This example was based upon uIP hello-world by Adam Dunkels <>
 * Ported to the Arduino IDE by Adam Nielsen <>

#include <SerialIP.h>
#include <Stdio.h>

// The connection_data struct needs to be defined in an external file.
#include "HelloWorldData.h"

int temp;
char* tempchar;

void setup() {

  // Set up the speed of our serial link.

  // Tell SerialIP which serial port to use (some boards have more than one.)
  // Currently this is limited to HardwareSerial ports, until both it and 
  // SoftwareSerial inherit from a common base class.

  // We're going to be handling uIP events ourselves.  Perhaps one day a simpler
  // interface will be implemented for the Arduino IDE, but until then...  

  // Set the IP address we'll be using.  Make sure this doesn't conflict with
  // any IP addresses or subnets on your LAN or you won't be able to connect to
  // either the Arduino or your LAN...
  IP_ADDR myIP = {192,168,5,2};
  IP_ADDR subnet = {255,255,255,0};
  SerialIP.begin(myIP, subnet);

  // If you'll be making outgoing connections from the Arduino to the rest of
  // the world, you'll need a gateway set up.
  //IP_ADDR gwIP = {192,168,5,1};

  // Listen for incoming connections on TCP port 1000.  Each incoming
  // connection will result in the uip_callback() function being called.

void loop() {
  // Check the serial port and process any incoming data.
  temp = 32;
  dtostrf(temp, 2, 0, tempchar);
  // We can do other things in the loop, but be aware that the loop will
  // briefly pause while IP data is being processed.

void uip_callback(uip_tcp_appstate_t *s)
  if (uip_connected()) {

    // We want to store some data in our connections, so allocate some space
    // for it.  The connection_data struct is defined in a separate .h file,
    // due to the way the Arduino IDE works.  (typedefs come after function
    // definitions.)
    connection_data *d = (connection_data *)malloc(sizeof(connection_data));

    // Save it as SerialIP user data so we can get to it later.
    s->user = d;

    // The protosocket's read functions need a per-connection buffer to store
    // data they read.  We've got some space for this in our connection_data
    // structure, so we'll tell uIP to use that.
    PSOCK_INIT(&s->p, d->input_buffer, sizeof(d->input_buffer));


  // Call/resume our protosocket handler.
  handle_connection(s, (connection_data *)s->user);

  // If the connection has been closed, release the data we allocated earlier.
  if (uip_closed()) {
    if (s->user) free(s->user);
    s->user = NULL;

// This function is going to use uIP's protosockets to handle the connection.
// This means it must return int, because of the way the protosockets work.
// In a nutshell, when a PSOCK_* macro needs to wait for something, it will
// return from handle_connection so that other work can take place.  When the
// event is triggered, uip_callback() will call this function again and the
// switch() statement (see below) will take care of resuming execution where
// it left off.  It *looks* like this function runs from start to finish, but
// that's just an illusion to make it easier to code :-)
int handle_connection(uip_tcp_appstate_t *s, connection_data *d)
  // All protosockets must start with this macro.  Its internal implementation
  // is that of a switch() statement, so all code between PSOCK_BEGIN and
  // PSOCK_END is actually inside a switch block.  (This means for example,
  // that you can't declare variables in the middle of it!)

  // Send some text over the connection.
  PSOCK_SEND_STR(&s->p, tempchar);

  // Read some returned text into the input buffer we set in PSOCK_INIT.  Data
  // is read until a newline (\n) is received, or the input buffer gets filled
  // up.  (Which, at 16 chars by default, isn't hard!)
  PSOCK_READTO(&s->p, '\n');

  // Since any subsequent PSOCK_* functions would overwrite the buffer, we
  // need to make a copy of it first.  We can't use a local variable for this,
  // because any PSOCK_* macro may make the function return and resume later,
  // thus losing the data in any local variables.  This is why uip_callback
  // has allocated the connection_data structure for us to use instead.  (You
  // can add/remove other variables in this struct to store different data.
  // See the other file in this sketch, serialip_conn.h)
  strncpy(d->name, d->input_buffer, sizeof(d->name));
  // Note that this will misbehave when the input buffer overflows (i.e.
  // more than 16 characters are typed in) but hey, this is supposed to be
  // a simple example.  Fixing this problem will be left as an exercise for
  // the reader :-)

  // Send some more data over the connection.
  PSOCK_SEND_STR(&s->p, "Hello ");
  PSOCK_SEND_STR(&s->p, d->name);

  // Disconnect.

  // All protosockets must end with this macro.  It closes the switch().

My problem is at dtostrf(temp, 2, 0, tempchar);

I’ve even used sprintf(tempchar, “%d”, temp); with the same results, a server that hangs and will not ping upon connecting to the server.

If I do away with sprintf() or dtostrf() altogether and make tempchar = “45”; it passes through just fine!

What a mystery…

What a mystery...

There's no mystery. This code:

  dtostrf(temp, 2, 0, tempchar);

calls a function that has this signature:

char *  dtostrf (double __val, signed char __width, unsigned char __prec, char *__s)

It converts a double, temp, to a string with a width of 2 and a precision of 0 to a string, and stores that string in the memory pointed to be tempchar.

The only question is what memory does tempchar point to? Lets look.

char* tempchar;

By default, the compiler initializes all global variables, like tempchar, to appropriate values. In the case of a pointer, the appropriate value is NULL.

So, what memory does it point to? None. It's a NULL pointer, and dtostrf is smart enough to know to do nothing with a NULL pointer.

On the other hand,

  PSOCK_SEND_STR(&s->p, tempchar);

expects, apparently, for you to be smart enough to NOT pass it a NULL pointer.

Change tempchar's declaration to

char tempchar[24];

and try again.

The difference here is that tempchar is still usable wherever a pointer is needed, but, it is not NULL and does point to writable memory.

Thank you! I will try this ASAP; it looks promising. Thanks for your help.

I wishlisted the ethernet shield and had been looking for ways to connect an arduino without it for ages. I think SerialIP needs to be a little more popular.

Ultimately I'm working toward something like this: It looks much more fun than buying an ethernet shield.

It worked! Well done and thank you. The world is now mine :slight_smile: