Advanced Programming in the UNIX® Environment

ptg10805159

Section 12.6 Thread-Specific Data 449

void

thread_init(void)

{

err = pthread_key_create(&key, destructor);

}

int

threadfunc(void *arg)

{

pthread_once(&init_done, thread_init);

..

.

}

Once a key is created, we can associate thread-specific data with the key by calling

pthread_setspecific.Wecan obtain the address of the thread-specific data with

pthread_getspecific.

#include <pthread.h>

void *pthread_getspecific(pthread_key_tkey);

Returns: thread-specific data value orNULLif no value

has been associated with the key

int pthread_setspecific(pthread_key_tkey,const void *value);

Returns: 0 if OK, error number on failure

If no thread-specific data has been associated with a key,pthread_getspecificwill

return a null pointer.Wecan use this return value to determine whether we need to call

pthread_setspecific.

Example

In Figure12.11, we showed a hypothetical implementation ofgetenv.Wecame up

with a new interface to provide the same functionality,but in a thread-safe way

(Figure12.12). But what would happen if we couldn’t modify our application programs

to use the new interface? In that case, we could use thread-specific data to maintain a

per-thread copy of the data buffer used to hold the return string. This is shown in

Figure12.13.

#include <limits.h>

#include <string.h>

#include <pthread.h>

#include <stdlib.h>

#define MAXSTRINGSZ 4096

static pthread_key_t key;

static pthread_once_t init_done = PTHREAD_ONCE_INIT;

pthread_mutex_t env_mutex = PTHREAD_MUTEX_INITIALIZER;