1 files changed, 107 insertions, 0 deletions
diff --git a/linuxthreads/Examples/ex4.c b/linuxthreads/Examples/ex4.c
new file mode 100644
index 0000000000..83bc54c913
--- /dev/null
+++ b/linuxthreads/Examples/ex4.c
@@ -0,0 +1,107 @@
+/* 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 */
+
+char * str_accumulate(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 */
+
+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, "first");
+  pthread_create(&th2, NULL, process, "second");
+  res = str_accumulate("initial thread");
+  printf("Thread %lx: \"%s\"\n", pthread_self(), res);
+  pthread_join(th1, NULL);
+  pthread_join(th2, NULL);
+  pthread_exit(NULL);
+}