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