Improper Synchronization

Description

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following function attempts to acquire a lock in order to perform operations on a shared resource.

void f(pthread_mutex_t *mutex) {

  pthread_mutex_lock(mutex);

  /* access shared resource */


  pthread_mutex_unlock(mutex);

}

However, the code does not check the value returned by pthread_mutex_lock() for errors. If pthread_mutex_lock() cannot acquire the mutex for any reason, the function may introduce a race condition into the program and result in undefined behavior.

In order to avoid data races, correctly written programs must check the result of thread synchronization functions and appropriately handle all errors, either by attempting to recover from them or reporting them to higher levels.

int f(pthread_mutex_t *mutex) {

  int result;

  result = pthread_mutex_lock(mutex);
  if (0 != result)
    return result;


  /* access shared resource */


  return pthread_mutex_unlock(mutex);

}

Example Two

The following code intends to fork a process, then have both the parent and child processes print a single line.

static void print (char * string) {

  char * word;
  int counter;
  for (word = string; counter = *word++; ) {

    putc(counter, stdout);
    fflush(stdout);
    /* Make timing window a little larger... */

    sleep(1);

  }

}

int main(void) {

  pid_t pid;

  pid = fork();
  if (pid == -1) {
    exit(-2);
  }
  else if (pid == 0) {
    print("child\n");
  }
  else {
    print("PARENT\n");
  }
  exit(0);

}

One might expect the code to print out something like:

However, because the parent and child are executing concurrently, and stdout is flushed each time a character is printed, the output might be mixed together, such as:

See Also