THR_KEYCREATE(3C) Standard C Library Functions THR_KEYCREATE(3C)


thr_keycreate, thr_keycreate_once, thr_setspecific, thr_getspecific -
thread-specific data functions


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

int thr_keycreate(thread_key_t *keyp,
void (*destructor)(void *));

int thr_keycreate_once(thread_key_t *keyp,
void (*destructor)(void *));

int thr_setspecific(thread_key_t key, void *value);

int thr_getspecific(thread_key_t key, void **valuep);


Create Key

In general, thread key creation allocates a key that locates data
specific to each thread in the process. The key is global to all threads
in the process, which allows each thread to bind a value to the key once
the key has been created. The key independently maintains specific values
for each binding thread. The thr_keycreate() function allocates a global
key namespace, pointed to by keyp, that is visible to all threads in the
process. Each thread is initially bound to a private element of this key,
which allows access to its thread-specific data.

Upon key creation, a new key is assigned the value NULL for all active
threads. Additionally, upon thread creation, all previously created keys
in the new thread are assigned the value NULL.

Optionally, a destructor function destructor can be associated with each
key. Upon thread exit, if a key has a non-null destructor function and
the thread has a non-null value associated with that key, the destructor
function is called with the current associated value. If more than one
destructor exists for a thread when it exits, the order of destructor
calls is unspecified.

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 thr_keycreate_once() function is identical to the thr_keycreate()
function except that the key pointed to by keyp must be statically
initialized with the value THR_ONCE_KEY before calling
thr_keycreate_once() and the key will be created exactly once. This is
equivalent to using pthread_once() to call a onetime initialization
function that calls thr_keycreate() to create the data key.

Set Value

Once a key has been created, each thread can bind a new value to the key
using thr_setspecific(). The values are unique to the binding thread and
are individually maintained. These values continue for the life of the
calling thread.

Proper synchronization of key storage and access must be ensured by the
caller. The value argument to thr_setspecific() is generally a pointer to
a block of dynamically allocated memory reserved by the calling thread
for its own use. See EXAMPLES below.

At thread exit, the destructor function, which is associated at time of
creation, is called and it uses the specific key value as its sole

Get Value

thr_getspecific() stores the current value bound to key for the calling
thread into the location pointed to by valuep.


If successful, thr_keycreate(), thr_keycreate_once(), thr_setspecific()
and thr_getspecific() return 0. Otherwise, an error number is returned to
indicate the error.


If the following conditions occur, thr_keycreate() and
thr_keycreate_once() return the corresponding error number:

The system lacked the necessary resources to create another
thread-specific data key.

Insufficient memory exists to create the key.

If the following conditions occur, thr_setspecific() returns the
corresponding error number:

Insufficient memory exists to associate the value with the key.

The thr_setspecific() function returns the corresponding error number:

The key value is invalid.


Example 1: Call the thread-specific data from more than one thread without

special initialization.

In this example, the thread-specific data in this 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 <thread.h>

void *thread_specific_data(void *);
void cleanup(void*);
#define MAX_ARGC 20
thread_t tid[MAX_ARGC];
int num_threads;

main(int argc, char *argv[]) {
int i;
num_threads = argc - 1;
for (i = 0; i < num_threads; i++)
thr_create(NULL, 0, thread_specific_data, argv[i+1], 0, &tid[i]);
for (i = 0; i < num_threads; i++)
thr_join(tid[i], NULL, NULL);
return (0);
} /* end main */

void *
thread_specific_data(void *arg) {
static thread_key_t key = THR_ONCE_KEY;
char *private_data = arg;
void *tsd = NULL;
void *data;

thr_keycreate_once(&key, cleanup);
thr_getspecific(key, &tsd);
if (tsd == NULL) {
data = malloc(strlen(private_data) + 1);
strcpy(data, private_data);
thr_setspecific(key, data);
thr_getspecific(key, &tsd);
printf("tsd for %d = %s\n", thr_self(), (char *)tsd);
thr_getspecific(key, &tsd);
printf("tsd for %d remains %s\n", thr_self(), (char *)tsd);
return (NULL);
} /* end thread_specific_data */

cleanup(void *v) {
/* application-specific clean-up function */


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

|Interface Stability | Committed |
|MT-Level | MT-Safe |


pthread_once(3C), thr_exit(3C), attributes(7), standards(7)


The thr_getspecific() and thr_setspecific() functions can be called
either explicitly or implicitly from a thread-specific data destructor
function. Calling thr_setspecific() from a destructor can result in lost
storage or infinite loops.

November 2, 2007 THR_KEYCREATE(3C)