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