pthread_key_create example c c++

NAME

pthread_key_create - thread-specific data key creation

SYNOPSIS

[THR]  #include <pthread.h>

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

DESCRIPTION

The pthread_key_create() function shall create 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 shall be associated with the new key in all active threads. Upon thread creation, the value NULL shall be 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 value of the key is set to NULL, and then the function pointed to is called with the previously associated value as its sole argument. The order of destructor calls is unspecified if more than one destructor exists for a thread when it exits. (pthread_key_create)
If, after all the destructors have been called for all non-NULL values with associated destructors, there are still some non-NULL values with associated destructors, then the process is repeated. If, after at least {PTHREAD_DESTRUCTOR_ITERATIONS} iterations of destructor calls for outstanding non-NULL values, there are still some non-NULL values with associated destructors, implementations may stop calling destructors, or they may continue calling destructors until no non-NULL values with associated destructors exist, even though this might result in an infinite loop.

RETURN VALUE

If successful, the pthread_key_create() function shall store the newly created key value at *key and shall return zero. Otherwise, an error number shall be returned to indicate the error.

ERRORS

The pthread_key_create() function shall fail if:

[EAGAIN]

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 exceeded.

[ENOMEM]

Insufficient memory exists to create the key.

The pthread_key_create() function shall not return an error code of [EINTR].

Example of pthread_key_create

Note: By using the code examples, you agree to the terms of the Code license and disclaimer information.

#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