pthread_key_create, pthread_key_create_once_np - create thread-specific
data key


cc -mt [ flag... ] file... -lpthread [ library... ]
#include <pthread.h>

int pthread_key_create(pthread_key_t *key,
void (*destructor)(void*));

int pthread_key_create_once_np(pthread_key_t *key,
void (*destructor)(void*));


The pthread_key_create() function creates a thread-specific data key
visible to all threads in the process. Key values provided by
pthread_key_create() are opaque objects used to locate thread-specific
data. Although the same key value may be used by different threads, the
values bound to the key by pthread_setspecific() are maintained on a per-
thread basis and persist for the life of the calling thread.

Upon key creation, the value NULL is associated with the new key in all
active threads. Upon thread creation, the value NULL is associated with
all defined keys in the new thread.

An optional destructor function may be associated with each key value. At
thread exit, if a key value has a non-NULL destructor pointer, and the
thread has a non-NULL value associated with that key, the function
pointed to is called with the current associated value as its sole
argument. Destructors can be called in any order.

If, after all the destructors have been called for all keys with
non-NULL values, there are still some keys with non-NULL values, the
process will be repeated. If, after at least
PTHREAD_DESTRUCTOR_ITERATIONS iterations of destructor calls for
outstanding non-NULL values, there are still some keys with non-NULL
values, the process is continued, even though this might result in an
infinite loop.

An exiting thread runs with all signals blocked. All thread termination
functions, including thread-specific data destructor functions, are
called with all signals blocked.

The pthread_key_create_once_np() function is identical to the
pthread_key_create() function except that the key referred to by *key
must be statically initialized with the value PTHREAD_ONCE_KEY_NP before
calling pthread_key_create_once_np(), and the key is created exactly
once. This function call is equivalent to using pthread_once(3C) to call
a onetime initialization function that calls pthread_key_create() to
create the data key.


If successful, the pthread_key_create() and pthread_key_create_once_np()
functions store the newly created key value at *key and return 0.
Otherwise, an error number is returned to indicate the error.


The pthread_key_create() and pthread_key_create_once_np() functions will
fail if:

The system lacked the necessary resources to create another
thread-specific data key, or the system-imposed limit on the
total number of keys per process PTHREAD_KEYS_MAX has been

Insufficient memory exists to create the key.

The pthread_key_create() and pthread_key_create_once_np() functions will
not return an error value of EINTR.


Example 1: Call thread-specific data in the function from more than one

thread without special initialization.

In the following example, the thread-specific data in the function can be
called from more than one thread without special initialization. For each
argument passed to the executable, a thread is created and privately
bound to the string-value of that argument.

/* cc -mt thisfile.c */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>

static void *thread_function(void *);
static void show_tsd(void);
static void cleanup(void*);

#define MAX_THREADS 20

static pthread_key_t tsd_key = PTHREAD_ONCE_KEY_NP;

main(int argc, char *argv[])
pthread_t tid[MAX_THREADS];
int num_threads;
int i;

if ((num_threads = argc - 1) > MAX_THREADS)
num_threads = MAX_THREADS;
for (i = 0; i < num_threads; i++)
pthread_create(&tid[i], NULL, thread_function, argv[i+1]);
for (i = 0; i < num_threads; i++)
pthread_join(tid[i], NULL);
return (0);

static void *
thread_function(void *arg)
char *data;

pthread_key_create_once_np(&tsd_key, cleanup);
data = malloc(strlen(arg) + 1);
strcpy(data, arg);
pthread_setspecific(tsd_key, data);
return (NULL);

static void
void *tsd = pthread_getspecific(tsd_key);

printf("tsd for %d = %s\n", pthread_self(), (char *)tsd);

/* application-specific clean-up function */
static void
cleanup(void *tsd)
printf("freeing tsd for %d = %s\n", pthread_self(), (char *)tsd);


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Committed. |
|MT-Level | MT-Safe |
|Standard | See below. |

For pthread_key_create(), see standards(7).


pthread_getspecific(3C), pthread_key_delete(3C), pthread_once(3C),
pthread_setspecific(3C), attributes(7), standards(7)

illumos November 2, 2007 PTHREAD_KEY_CREATE(3C)