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