108 lines
2.9 KiB
C
108 lines
2.9 KiB
C
/* 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);
|
|
}
|