linuxthreads/Examples/ex3.c

   1 /* Multi-thread searching.
   2    Illustrates: thread cancellation, cleanup handlers. */
   3
   4 #include <errno.h>
   5 #include <stdio.h>
   6 #include <unistd.h>
   7 #include <stdlib.h>
   8 #include <sys/types.h>
   9 #include <pthread.h>
  10
  11 /* Defines the number of searching threads */
  12 #define NUM_THREADS 5
  13
  14 /* Function prototypes */
  15 void *search(void *);
  16 void print_it(void *);
  17
  18 /* Global variables */
  19 pthread_t threads[NUM_THREADS];
  20 pthread_mutex_t lock;
  21 int tries;
  22 volatile int started;
  23
  24 int main(int argc, char ** argv)
  25 {
  26   int i;
  27   int pid;
  28
  29   /* create a number to search for */
  30   pid = getpid();
  31   printf("Searching for the number = %d...\n", pid);
  32
  33   /* Initialize the mutex lock */
  34   pthread_mutex_init(&lock, NULL);
  35
  36   /* Create the searching threads */
  37   for (started=0; started<NUM_THREADS; started++)
  38     pthread_create(&threads[started], NULL, search, (void *)pid);
  39
  40   /* Wait for (join) all the searching threads */
  41   for (i=0; i<NUM_THREADS; i++)
  42     pthread_join(threads[i], NULL);
  43
  44   printf("It took %d tries to find the number.\n", tries);
  45
  46   /* Exit the program */
  47   return 0;
  48 }
  49
  50 /* This is the cleanup function that is called
  51    when the threads are cancelled */
  52
  53 void print_it(void *arg)
  54 {
  55   int *try = (int *) arg;
  56   pthread_t tid;
  57
  58   /* Get the calling thread's ID */
  59   tid = pthread_self();
  60
  61   /* Print where the thread was in its search when it was cancelled */
  62   printf("Thread %lx was canceled on its %d try.\n", tid, *try);
  63 }
  64
  65 /* This is the search routine that is executed in each thread */
  66
  67 void *search(void *arg)
  68 {
  69   int num = (int) arg;
  70   int i, j, ntries;
  71   pthread_t tid;
  72
  73   /* get the calling thread ID */
  74   tid = pthread_self();
  75
  76   /* use the thread ID to set the seed for the random number generator */
  77   /* Since srand and rand are not thread-safe, serialize with lock */
  78
  79   /* Try to lock the mutex lock --
  80      if locked, check to see if the thread has been cancelled
  81      if not locked then continue */
  82   while (pthread_mutex_trylock(&lock) == EBUSY)
  83     pthread_testcancel();
  84
  85   srand((int)tid);
  86   i = rand() & 0xFFFFFF;
  87   pthread_mutex_unlock(&lock);
  88   ntries = 0;
  89
  90   /* Set the cancellation parameters --
  91      - Enable thread cancellation
  92      - Defer the action of the cancellation */
  93
  94   pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
  95   pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, NULL);
  96
  97   while (started < NUM_THREADS)
  98     sched_yield ();
  99
 100   /* Push the cleanup routine (print_it) onto the thread
 101      cleanup stack.  This routine will be called when the
 102      thread is cancelled.  Also note that the pthread_cleanup_push
 103      call must have a matching pthread_cleanup_pop call.  The
 104      push and pop calls MUST be at the same lexical level
 105      within the code */
 106
 107   /* Pass address of `ntries' since the current value of `ntries' is not
 108      the one we want to use in the cleanup function */
 109
 110   pthread_cleanup_push(print_it, (void *)&ntries);
 111
 112   /* Loop forever */
 113   while (1) {
 114     i = (i + 1) & 0xFFFFFF;
 115     ntries++;
 116
 117     /* Does the random number match the target number? */
 118     if (num == i) {
 119       /* Try to lock the mutex lock --
 120          if locked, check to see if the thread has been cancelled
 121          if not locked then continue */
 122       while (pthread_mutex_trylock(&lock) == EBUSY)
 123         pthread_testcancel();
 124
 125       /* Set the global variable for the number of tries */
 126       tries = ntries;
 127       printf("Thread %lx found the number!\n", tid);
 128
 129       /* Cancel all the other threads */
 130       for (j=0; j<NUM_THREADS; j++)
 131         if (threads[j] != tid) pthread_cancel(threads[j]);
 132
 133       /* Break out of the while loop */
 134       break;
 135     }
 136
 137     /* Every 100 tries check to see if the thread has been cancelled. */
 138     if (ntries % 100 == 0) {
 139       pthread_testcancel();
 140     }
 141   }
 142
 143   /* The only way we can get here is when the thread breaks out
 144      of the while loop.  In this case the thread that makes it here
 145      has found the number we are looking for and does not need to run
 146      the thread cleanup function.  This is why the pthread_cleanup_pop
 147      function is called with a 0 argument; this will pop the cleanup
 148      function off the stack without executing it */
 149
 150   pthread_cleanup_pop(0);
 151   return((void *)0);
 152 }