The Boston Diaries

Wednesday, Debtember 02, 2009

I told you handing errors was error prone

From
Mark Grosberg <XXXXXXXXXXXXXXXXX>
To
sean@conman.org
Subject
Blog post.
Date
Wed, 2 Dec 2009 15:59:55 -0500 (EST)
I find it even more amusing that you didn't get the error handling right in the create_socket() on your current blog post.
Notice that you leak the socket and/or memory in the error cases. I guess it really is hard to handle errors. ;-)
Sorry, I just had to take this cheap shot!
-MYG

Heh.

Yup, I blew it again for demonstration purposes.

The code I posted yesterday was actually pulled from a current project where the create_socket() is only called during initialization and if it fails, the program exits. Since I'm on a Unix system, the “lost” resources like memory and sockets are automatically reclaimed. Not all operating systems are nice like this.

There are a few ways to fix this. One, is to use a langauge that handles such details automatically with garbage collection, but I'm using C so that's not an option. The second one is to add cleanup code at each point we exit, but using that we end up with code that looks like:

  /* ... */

  if fcntl(listen->sock,F_GETFL,0) == -1)
  {
    perror("fcntl(GETFL)");
    close(listen->socket); 
    free(listen);
    return -1;
  }
   
  if (fcntl(listen->sock,F_SETFL,rc | O_NONBLOCK) == -1)
  {
    perror("fcntl(SETFL)");
    close(listen->socket); 
    free(listen);
    return -1;   
  }
   
  if (bind(listen->sock,paddr,saddr) == -1)
  {
    perror("bind()");
    close(listen->socket);
    free(listen);
    return -1;   
  }

  /* ... */

Lots of duplicated code and the more complex the routine, the more complex the cleanup and potential to leak memory (or other resources like files and network connections). The other option looks like:

  /* ... */

  if fcntl(listen->sock,F_GETFL,0) == -1)
  {
    perror("fcntl(GETFL)");
    goto create_socket_cleanup;
  }
  
  if (fcntl(listen->sock,F_SETFL,rc | O_NONBLOCK) == -1)
  {
    perror("fcntl(SETFL)");
    goto create_socket_cleanup;
  }

  if (bind(listen->sock,paddr,saddr) == -1)
  {
    perror("bind()");
    goto create_socket_cleanup;
  }

  /* rest of code */

  return listen->sock; /* everything is okay */

create_socket_cleanup:
  close(listen->sock);
create_socket_cleanup_mem:
  free(listen);
  return -1;
}

This uses the dreaded goto construct, but is one of the few places that it's deemed “okay” to use goto, for cleaning up errors. No code duplication, but you need to make sure you cleanup (or unwind, or whatever) in reverse order.

So yeah, error handling … maddening.

I still wish there was a better way …

This actually doesn't sound half bad …

It's no secret that we've been openly critical of the prices charged by automakers for built-in GPS navigation systems. Frankly, paying $2,000 or more for an in-dash system when you can buy stand-alone navigation units for as little as $100 is ridiculous. Even the newer, larger seven-inch screen units are now down to as little as $250, and even though they aren't tied in to a vehicles' wheel sensors, they tend to be plenty accurate. Now, however, there is a new option that is even cheaper – as in (sort of) free.

It's only "sort of" free because the Google maps turn-by-turn navigation app is built into the new Motorola Droid smartphone (see sister-site Engadget's full review of the Droid here) that recently became available from Verizon Wireless. In this case, you have to sign up for two years of mobile phone service, which includes a data plan. I've been a Verizon customer for a decade and just happened to be up for a biennial discounted phone upgrade. When the Droid appeared a few weeks ago, the plan to wait until the new year for a Palm Pre was discarded. We've now had the chance to play with the Droid and its new navigation software, so follow the jump to find out if it lives up to expectations.

Via Instapundit, Review: Google Maps turn-by-turn navigation on Android 2.0 — Autoblog

For Corsair, who loves his gadgets, and because he hates AT&T.

Personally, I wouldn't mind this. I think the combination of Google Maps and a GPS is wonderful, although I wouldn't use the turn-by-turn navigation (map view is fine by me, with spot checks with the street view to avoid potentially bad areas). I would also like a larger screen, but hey, you can't have everything.