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/* 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);
}
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