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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



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


#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 argument.

Get Value

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

Return Values

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.

#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

See Also

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.