pthread_key_create()--Create Thread Local Storage Key

 #include <pthread.h>
 int pthread_key_create(pthread_key_t *key, void (*destructor)(void *));  

The pthread_key_create() function creates a thread local storage key for the process and associates the destructor function with that key. After a key is created, that key can be used to set and get per-thread data pointer. When pthread_key_create() completes, the value associated with the newly created key is NULL.

When a thread terminates, if both the value and the destructor associated with a thread local storage key are not NULL, the destructor function is called. The stored pointer associated with the key is set to NULL before the call to the destructor funciton. The parameter passed to the destructor function when it is called is the value of the pointer before it was set to NULL that is associated with that key in the thread that is terminating.

After calling the destructors, if there are still non NULL values in the thread associated with the keys, the process is repeated. After PTHREAD_DESTRUCTOR_ITERATIONS attempts to destroy the thread local storage, no further attempts are made for that thread local storage value/key combination.

Do not call pthread_exit() from a destructor function.

A destructor function is not called as a result of the application calling pthread_key_delete().

Authorities and Locks

None.

Parameters

key

(Output) The address of the variable to contain the thread local storage key

destructor

(Input) The address of the function to act as a destructor for this thread local storage key

Return Value

0

pthread_key_create() was successful.

value

pthread_key_create() was not successful. value is set to indicate the error condition.

Error Conditions

If pthread_key_create() was not successful, the error condition returned usually indicates one of the following errors. Under some conditions, the value returned could indicate an error other than those listed here.

[EINVAL]

The value specified for the argument is not correct.

[EAGAIN]

The system did not have enough resources, or the maximum of PTHREAD_KEYS_MAX would have been exceeded.

[ENOMEM]

Not enough memory to create the key.

Related Information

• The <pthread.h> header file. See Header files for Pthread functions.
  
  
• pthread_getspecific()--Get Thread Local Storage Value by Key
  
  
• pthread_key_delete()--Delete Thread Local Storage Key
  
  
• pthread_setspecific()--Set Thread Local Storage by Key

Example

See Code disclaimer information for information pertaining to code examples.

#define _MULTI_THREADED
#include <pthread.h>
#include <sched.h>
#include <stdio.h>
#include "check.h"

pthread_key_t        tlsKey = 0;

void globalDestructor(void *value)
{
  printf("In the data destructor\n");
  free(value);
  pthread_setspecific(tlsKey, NULL);
}


int main(int argc, char **argv)
{
  int                   rc=0;
  int                   i=0;

  printf("Enter Testcase - %s\n", argv[0]);

  printf("Create a thread local storage key\n");
  rc = pthread_key_create(&tlsKey, globalDestructor);
  checkResults("pthread_key_create()\n", rc);
  /* The key can now be used from all threads */

  printf("- The key can now be used from all threads\n");
  printf("- in the process to storage thread local\n");
  printf("- (but global to all functions in that thread)\n");
  printf("- storage\n");

  printf("Delete a thread local storage key\n");
  rc = pthread_key_delete(tlsKey);
  checkResults("pthread_key_delete()\n", rc);
  /* The key and any remaining values are now gone. */
  printf("Main completed\n");
  return 0;
}

Output:

Enter Testcase - QP0WTEST/TPKEYC0
Create a thread local storage key
- The key can now be used from all threads
- in the process to storage thread local
- (but global to all functions in that thread)
- storage
Delete a thread local storage key
Main completed