linuxthreads/Examples/ex4.c

   1 /* Making a library function that uses static variables thread-safe.
   2    Illustrates: thread-specific data, pthread_once(). */
   3
   4 #include <stddef.h>
   5 #include <stdio.h>
   6 #include <stdlib.h>
   7 #include <string.h>
   8 #include <pthread.h>
   9
  10 /* This is a typical example of a library function that uses
  11    static variables to accumulate results between calls.
  12    Here, it just returns the concatenation of all string arguments
  13    that were given to it. */
  14
  15 #if 0
  16
  17 char *
  18 str_accumulate (char *s)
  19 {
  20   static char accu[1024] = { 0 };
  21   strcat (accu, s);
  22   return accu;
  23 }
  24
  25 #endif
  26
  27 /* Of course, this cannot be used in a multi-threaded program
  28    because all threads store "accu" at the same location.
  29    So, we'll use thread-specific data to have a different "accu"
  30    for each thread. */
  31
  32 /* Key identifying the thread-specific data */
  33 static pthread_key_t str_key;
  34 /* "Once" variable ensuring that the key for str_alloc will be allocated
  35    exactly once. */
  36 static pthread_once_t str_alloc_key_once = PTHREAD_ONCE_INIT;
  37
  38 /* Forward functions */
  39 static void str_alloc_key (void);
  40 static void str_alloc_destroy_accu (void *accu);
  41
  42 /* Thread-safe version of str_accumulate */
  43
  44 static char *
  45 str_accumulate (const char *s)
  46 {
  47   char *accu;
  48
  49   /* Make sure the key is allocated */
  50   pthread_once (&str_alloc_key_once, str_alloc_key);
  51   /* Get the thread-specific data associated with the key */
  52   accu = (char *) pthread_getspecific (str_key);
  53   /* It's initially NULL, meaning that we must allocate the buffer first. */
  54   if (accu == NULL)
  55     {
  56       accu = malloc (1024);
  57       if (accu == NULL)
  58         return NULL;
  59       accu[0] = 0;
  60       /* Store the buffer pointer in the thread-specific data. */
  61       pthread_setspecific (str_key, (void *) accu);
  62       printf ("Thread %lx: allocating buffer at %p\n", pthread_self (), accu);
  63     }
  64   /* Now we can use accu just as in the non thread-safe code. */
  65   strcat (accu, s);
  66   return accu;
  67 }
  68
  69 /* Function to allocate the key for str_alloc thread-specific data. */
  70
  71 static void
  72 str_alloc_key (void)
  73 {
  74   pthread_key_create (&str_key, str_alloc_destroy_accu);
  75   printf ("Thread %lx: allocated key %d\n", pthread_self (), str_key);
  76 }
  77
  78 /* Function to free the buffer when the thread exits. */
  79 /* Called only when the thread-specific data is not NULL. */
  80
  81 static void
  82 str_alloc_destroy_accu (void *accu)
  83 {
  84   printf ("Thread %lx: freeing buffer at %p\n", pthread_self (), accu);
  85   free (accu);
  86 }
  87
  88 /* Test program */
  89
  90 static void *
  91 process (void *arg)
  92 {
  93   char *res;
  94   res = str_accumulate ("Result of ");
  95   res = str_accumulate ((char *) arg);
  96   res = str_accumulate (" thread");
  97   printf ("Thread %lx: \"%s\"\n", pthread_self (), res);
  98   return NULL;
  99 }
 100
 101 int
 102 main (int argc, char **argv)
 103 {
 104   char *res;
 105   pthread_t th1, th2;
 106
 107   res = str_accumulate ("Result of ");
 108   pthread_create (&th1, NULL, process, (void *) "first");
 109   pthread_create (&th2, NULL, process, (void *) "second");
 110   res = str_accumulate ("initial thread");
 111   printf ("Thread %lx: \"%s\"\n", pthread_self (), res);
 112   pthread_join (th1, NULL);
 113   pthread_join (th2, NULL);
 114   return 0;
 115 }