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Archives for July, 2012

Rochester Castle

July 29th, 2012

I had some surprisingly well-timed holiday booked this week so I decided to visit a nearby(ish) castle at Rochester. The last time I came here I was a small child and all I remember is being terrified by how tall and massive it was. Some shrinkage has occurred over the last twenty years. Still it’s a very pleasant castle and nicely preserved too. Worth visiting, although there’s not a whole lot else to do in Rochester.

Some photos I took (not my best):

Come back for more castle next week!


July 15th, 2012

Got this really cute hemp chalk bag. Climbing is all about the fashion.

Filed in musings - Comments closed

Automagic Debugging

July 8th, 2012

It’s kind of annoying to have to go back and run GDB after your program crashes. Here’s a signal handler I’ve been using which drops you into GDB straight away:

static void gdb_sighandler(int sig, siginfo_t *info)
   char exe[256];
   if (readlink("/proc/self/exe", exe, sizeof(exe)) < 0) {
   char pid[16];
   snprintf(pid, sizeof(pid), "%d", getpid());
   pid_t p = fork();
   if (p == 0) {
      execl("/usr/bin/gdb", "gdb", "-ex", "cont", 
            exe, pid, NULL);
   else if (p < 0) {
   else {
      // Allow a little time for GDB to start before 
      // dropping into the default signal handler
      signal(sig, SIG_DFL);

This probably breaks all sorts of rules on what you shouldn’t do in a signal handler but hey, it was going to crash anyway. Resetting the handler to SIG_DFL at the end is important or you’ll end up stuck in a loop when you try to quit GDB. Install it for all the unpleasant signals like this:

struct sigaction sa;
sa.sa_sigaction = (void*)gdb_sighandler;
sa.sa_flags = SA_RESTART | SA_SIGINFO;
sigaction(SIGSEGV, &sa, NULL);
sigaction(SIGUSR1, &sa, NULL);
sigaction(SIGFPE, &sa, NULL);
sigaction(SIGBUS, &sa, NULL);
sigaction(SIGILL, &sa, NULL);
sigaction(SIGABRT, &sa, NULL);

It’s probably worth checking whether GDB is already running before you do this. On Linux you can check this by trying to ptrace yourself: if it succeeds then a debugger isn’t attached.

static int is_debugger_running(void)
   pid_t pid = fork();
   if (pid == -1) {
   else if (pid == 0) {
      int ppid = getppid();
      if (ptrace(PTRACE_ATTACH, ppid, NULL, NULL) == 0) {
         waitpid(ppid, NULL, 0);
         ptrace(PTRACE_CONT, NULL, NULL);
         ptrace(PTRACE_DETACH, getppid(), NULL, NULL);
   else {
      int status;
      waitpid(pid, &status, 0);
      return WEXITSTATUS(status);

Red Flowers, Big Stone

July 7th, 2012

Title says it all: some red flowers in a field last weekend:

I don’t know what sort they are :-(. Also a big stone in a place appropriately called Ibstone:

Wikipedia thinks the stone is marking the Bucks/Oxfordshire border, which is a bit mundane.

Also I’ve decided that driving is among the most stressful and miserable experiences known to mankind. I have no idea how people can do this all day and actually enjoy it…

UPDATE: a reader informs me that the flowers are poppies.