1 files changed, 115 insertions, 0 deletions
diff --git a/linuxthreads/Examples/ex4.c b/linuxthreads/Examples/ex4.c
new file mode 100644
index 0000000000..5c8b929e22
--- /dev/null
+++ b/linuxthreads/Examples/ex4.c
@@ -0,0 +1,115 @@
+/* Making a library function that uses static variables thread-safe.
+   Illustrates: thread-specific data, pthread_once(). */
+
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <pthread.h>
+
+/* This is a typical example of a library function that uses
+   static variables to accumulate results between calls.
+   Here, it just returns the concatenation of all string arguments
+   that were given to it. */
+
+#if 0
+
+char *
+str_accumulate (char *s)
+{
+  static char accu[1024] = { 0 };
+  strcat (accu, s);
+  return accu;
+}
+
+#endif
+
+/* Of course, this cannot be used in a multi-threaded program
+   because all threads store "accu" at the same location.
+   So, we'll use thread-specific data to have a different "accu"
+   for each thread. */
+
+/* Key identifying the thread-specific data */
+static pthread_key_t str_key;
+/* "Once" variable ensuring that the key for str_alloc will be allocated
+   exactly once. */
+static pthread_once_t str_alloc_key_once = PTHREAD_ONCE_INIT;
+
+/* Forward functions */
+static void str_alloc_key (void);
+static void str_alloc_destroy_accu (void *accu);
+
+/* Thread-safe version of str_accumulate */
+
+static char *
+str_accumulate (const char *s)
+{
+  char *accu;
+
+  /* Make sure the key is allocated */
+  pthread_once (&str_alloc_key_once, str_alloc_key);
+  /* Get the thread-specific data associated with the key */
+  accu = (char *) pthread_getspecific (str_key);
+  /* It's initially NULL, meaning that we must allocate the buffer first. */
+  if (accu == NULL)
+    {
+      accu = malloc (1024);
+      if (accu == NULL)
+	return NULL;
+      accu[0] = 0;
+      /* Store the buffer pointer in the thread-specific data. */
+      pthread_setspecific (str_key, (void *) accu);
+      printf ("Thread %lx: allocating buffer at %p\n", pthread_self (), accu);
+    }
+  /* Now we can use accu just as in the non thread-safe code. */
+  strcat (accu, s);
+  return accu;
+}
+
+/* Function to allocate the key for str_alloc thread-specific data. */
+
+static void
+str_alloc_key (void)
+{
+  pthread_key_create (&str_key, str_alloc_destroy_accu);
+  printf ("Thread %lx: allocated key %d\n", pthread_self (), str_key);
+}
+
+/* Function to free the buffer when the thread exits. */
+/* Called only when the thread-specific data is not NULL. */
+
+static void
+str_alloc_destroy_accu (void *accu)
+{
+  printf ("Thread %lx: freeing buffer at %p\n", pthread_self (), accu);
+  free (accu);
+}
+
+/* Test program */
+
+static void *
+process (void *arg)
+{
+  char *res;
+  res = str_accumulate ("Result of ");
+  res = str_accumulate ((char *) arg);
+  res = str_accumulate (" thread");
+  printf ("Thread %lx: \"%s\"\n", pthread_self (), res);
+  return NULL;
+}
+
+int
+main (int argc, char **argv)
+{
+  char *res;
+  pthread_t th1, th2;
+
+  res = str_accumulate ("Result of ");
+  pthread_create (&th1, NULL, process, (void *) "first");
+  pthread_create (&th2, NULL, process, (void *) "second");
+  res = str_accumulate ("initial thread");
+  printf ("Thread %lx: \"%s\"\n", pthread_self (), res);
+  pthread_join (th1, NULL);
+  pthread_join (th2, NULL);
+  return 0;
+}