Subversion Repositories or1k

/* 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(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 */

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);

  exit(0);

}