pthread_getspecific() — Get the thread-specific value for a key

Standards

Format

#define _OPEN_THREADS
#include <pthread.h>

int pthread_getspecific(pthread_key_t key, void **value);

#define _UNIX03_THREADS 
#include <pthread.h>
void *pthread_getspecific(pthread_key_t key);

General description

Returns the thread-specific data associated with the specified key for the current thread. If no thread-specific data has been set for key, the NULL value is returned in value.

Many multithreaded applications require storage shared among threads, where each thread has its own unique value. A thread-specific data key is an identifier, created by a thread, for which each thread in the process can set a unique key value.

pthread_key_t is a storage area where the system places the key identifier. To create a key, a thread uses pthread_key_create(). This returns the key identifier into the storage area of type pthread_key_t. At this point, each of the threads in the application has the use of that key, and can set its own unique value by using pthread_setspecific(). A thread can get its own unique value using pthread_getspecific().

Returned value

Example

⁄* CELEBP31 *⁄
#ifndef _OPEN_THREADS
#define _OPEN_THREADS
#endif

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <pthread.h>

#define threads 3
#define BUFFSZ  48
pthread_key_t   key;

void            *threadfunc(void *parm)
{
 int        status;
 void      *value;
 int        threadnum;
 int       *tnum;
 void      *getvalue;
 char       Buffer[BUFFSZ];

 tnum = parm;
 threadnum = *tnum;

 printf("Thread %d executing\n", threadnum);

 if (!(value = malloc(sizeof(Buffer))))
     printf("Thread %d could not allocate storage, errno = %d\n",
                                                  threadnum, errno);
 status = pthread_setspecific(key, (void *) value);
 if ( status <  0) {
    printf("pthread_setspecific failed, thread %d, errno %d",
                                                  threadnum, errno);
    pthread_exit((void *)12);
 }
 printf("Thread %d setspecific value: %d\n", threadnum, value);

 getvalue = 0;
 status = pthread_getspecific(key, &getvalue);
 if ( status <  0) {
    printf("pthread_getspecific failed, thread %d, errno %d",
                                                  threadnum, errno);
    pthread_exit((void *)13);
 }

 if (getvalue != value)
 {
   printf("getvalue not valid, getvalue=%d", (int)getvalue);
   pthread_exit((void *)68);
 }

 pthread_exit((void *)0);
}

void  destr_fn(void *parm)
{

   printf("Destructor function invoked\n");
   free(parm);
}

main() {
 int          getvalue;
 int          status;
 int          i;
 int          threadparm[threads];
 pthread_t    threadid[threads];
 int          thread_stat[threads];


 if ((status = pthread_key_create(&key, destr_fn )) < 0) {
    printf("pthread_key_create failed, errno=%d", errno);
    exit(1);
 }

 ⁄* create 3 threads, pass each its number *⁄
 for (i=0; i<threads; i++) {
    threadparm[i] = i+1;
    status = pthread_create( &threadid[i],
                             NULL,
                             threadfunc,
                             (void *)&threadparm[i]);
    if ( status <  0) {
       printf("pthread_create failed, errno=%d", errno);
       exit(2);
    }
  }

 for ( i=0; i<threads; i++) {
    status = pthread_join( threadid[i], (void *)&thread_stat[i]);
    if ( status <  0) {
       printf("pthread_join failed, thread %d, errno=%d\n", i+1, errno);
    }

    if (thread_stat[i] != 0)   {
        printf("bad thread status, thread %d, status=%d\n", i+1,
                                                   thread_stat[i]);
      }
  }

 exit(0);
}

CELEBP70

⁄* CELEBP70 *⁄
⁄* Example using SUSv3 pthread_getspecific() interface *⁄ 

#define _UNIX03_THREADS  1

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <pthread.h>

#define threads 3
#define BUFFSZ  48
pthread_key_t   key;

void            *threadfunc(void *parm)
{
 int        status;
 void      *value;
 int        threadnum;
 int       *tnum;
 void      *getvalue;
 char       Buffer[BUFFSZ];

 tnum = parm;
 threadnum = *tnum;

 printf("Thread %d executing\n", threadnum);

 if (!(value = malloc(sizeof(Buffer))))
     printf("Thread %d could not allocate storage, errno = %d\n",
                                                  threadnum, errno);
 status = pthread_setspecific(key, (void *) value);
 if ( status <  0) {
    printf("pthread_setspecific failed, thread %d, errno %d",
                                                  threadnum, errno);
    pthread_exit((void *)12);
 }
 printf("Thread %d setspecific value: %d\n", threadnum, value);

 getvalue = 0;
 getvalue = pthread_getspecific(key);
 if ( getvalue ==  0) {
    printf("pthread_getspecific failed, thread %d", threadnum);
    printf("   rc= %d, errno %d, ejr %08x\n",(int)getvalue, errno, __errno2());
    pthread_exit((void *)13);
 } else {
   printf("Success!\n");
   printf("Returned value: %d matches set value: %d\n", getvalue, value);
 }

 if (getvalue != value)
 {
   printf("getvalue not valid, getvalue=%d", (int)getvalue);
   pthread_exit((void *)68);
 }

 pthread_exit((void *)0);
}

void  destr_fn(void *parm)
{

   printf("Destructor function invoked\n");
   free(parm);
}

int main(void)
{
  int          status;
  int          i;
  int          threadparm[threads];
  pthread_t    threadid[threads];
  int          thread_stat[threads];


  if ((status = pthread_key_create(&key, destr_fn )) < 0) {
     printf("pthread_key_create failed, errno=%d", errno);
     exit(1);
  }

  ⁄* create 3 threads, pass each its number *⁄
  for (i=0; i<threads; i++) {
      threadparm[i] = i+1;
     status = pthread_create( &threadid[i],
                               NULL,
                              threadfunc,
                              (void *)&threadparm[i]);
     if ( status <  0) {
        printf("pthread_create failed, errno=%d", errno);
        exit(2);
     }
  }

  for ( i=0; i<threads; i++) {
      status = pthread_join( threadid[i], (void *)&thread_stat[i]);
      if ( status <  0) {
         printf("pthread_join failed, thread %d, errno=%d\n", i+1, errno);
      }

      if (thread_stat[i] != 0)   {
         printf("bad thread status, thread %d, status=%d\n", i+1,
                                                   thread_stat[i]);
      }
  }

  exit(0);
}

Related information

• pthread.h
• pthread_getspecific_d8_np() — Get the thread-specific value for a key
• pthread_key_create() — Create thread-specific data key
• pthread_setspecific() — Set the thread-specific value for a key