Darryl Gove's blog

When threads go bad

When a thread hits an error in a multithreaded application, that error will take out the entire app. Here's some example code:

#include <pthread.h>
#include <stdio.h>

void *work(void * param)
{
  int*a;
  a=(int*)(1024*1024);
  (*a)++;
  printf("Child thread exit\n");
}

void main()
{
  pthread_t thread;
  pthread_create(&thread,0,work,0);
  pthread_join(thread,0);
  printf("Main thread exit\n");
}

Compiling and running this produces:

% cc -O -mt pthread_error.c
% ./a.out
Segmentation Fault (core dumped)

Not entirely unexpected, that. The app died without the main thread having the chance to clear up resources etc. This is probably not ideal. However, it is possible to write a signal handler to capture the segmentation fault, and terminate the child thread without causing the main thread to terminate. It's important to realise that there's probably little chance of actually recovering from the unspecified error, but this at least might give the app the chance to report the symptoms of its demise.

#include <pthread.h>
#include <stdio.h>
#include <signal.h>

void *work(void * param)
{
  int*a;
  a=(int*)(1024*1024);
  (*a)++;
  printf("Child thread exit\n");
}

void hsignal(int i)
{
  printf("Signal %i\n",i);
  pthread_exit(0);
}

void main()
{
  pthread_t thread;
  sigset(SIGSEGV,hsignal);
  pthread_create(&thread,0,work,0);
  pthread_join(thread,0);
  printf("Main thread exit\n");
}

Which produces the output:

% cc -O -mt pthread_error.c
% ./a.out
Signal 11
Main thread exit

Posted at 10:02AM Nov 23, 2009 by Darryl Gove in Sun  |  Comments[3]