pthread_getspecific()--Get Thread Local Storage Value by Key

 #include <pthread.h>
 void *pthread_getspecific(pthread_key_t key);

The pthread_getspecific() function retrieves the thread local storage value associated with the key. pthread_getspecific() may be called from a data destructor.

The thread local storage value is a variable of type void * that is local to a thread, but global to all of the functions called within that thread. It is accessed by the key.

Authorities and Locks

None.

Parameters

key

(Input) The thread local storage key returned from pthread_key_create()

Return Value

value

pthread_getspecific() was successful. value is set to indicate the current thread specific data pointer stored at the key location. 

NULL

pthread_getspecific() returned the null thread specific data value stored at the key location or the key was out of range.

Error Conditions

None.

Related Information

• The <pthread.h> header file. See Header files for Pthread functions.
  
  
• pthread_key_create()--Create Thread Local Storage 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 <stdio.h>
#include "check.h"

#define         NUMTHREADS     3
pthread_key_t   tlsKey = 0;

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


void showGlobal(void)
{
  void                 *global;
  pthread_id_np_t       tid;

  global  = pthread_getspecific(tlsKey);
  pthread_getunique_np((pthread_t *)global, &tid);
  printf("showGlobal: global data stored for thread 0x%.8x%.8x\n",
         tid);
}

void *threadfunc(void *parm)
{
  int           rc;
  int          *myThreadDataStructure;
  pthread_t     me = pthread_self();

  printf("Inside secondary thread\n");

  myThreadDataStructure = malloc(sizeof(pthread_t) + sizeof(int) * 10);
  memcpy(myThreadDataStructure, &me, sizeof(pthread_t));
  pthread_setspecific(tlsKey, myThreadDataStructure);
  showGlobal();
  pthread_exit(NULL);
}

int main(int argc, char **argv)
{
  pthread_t             thread[NUMTHREADS];
  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("Create %d threads using joinable attributes\n",
         NUMTHREADS);
  for (i=0; i<NUMTHREADS; ++i) {
    rc = pthread_create(&thread[i], NULL, threadfunc, NULL);
    checkResults("pthread_create()\n", rc);
  }

  printf("Join to threads\n");
  for (i=0; i<NUMTHREADS; ++i) {
    rc = pthread_join(thread[i], NULL);
    checkResults("pthread_join()\n", rc);
  }

  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/TPGETS0
Create a thread local storage key
Create 3 threads using joinable attributes
Join to threads
Inside secondary thread
showGlobal: global data stored for thread 0x000000000000000b
In the globalDestructor
Inside secondary thread
showGlobal: global data stored for thread 0x000000000000000d
In the globalDestructor
Inside secondary thread
showGlobal: global data stored for thread 0x000000000000000c
In the globalDestructor
Delete a thread local storage key
Main completed