Tribblix: manual page: pthread_key

PTHREAD_KEY_CREATE(3C) Standard C Library Functions PTHREAD_KEY_CREATE(3C)

NAME

pthread_key_create, pthread_key_create_once_np - create thread-
specific data key

SYNOPSIS

cc -mt [ flag... ] file... -lpthread [ library... ]
#include <pthread.h>

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

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

DESCRIPTION

The pthread_key_create() function creates 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 is associated with the new key in
all active threads. Upon thread creation, the value NULL is
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 function pointed to is called with the current associated
value as its sole argument. Destructors can be called in any order.

If, after all the destructors have been called for all keys with
non-NULL values, there are still some keys with non-NULL values, the
process will be repeated. If, after at least
PTHREAD_DESTRUCTOR_ITERATIONS iterations of destructor calls for
outstanding non-NULL values, there are still some keys with non-NULL
values, the process is continued, even though this might result in an
infinite loop.

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 pthread_key_create_once_np() function is identical to the
pthread_key_create() function except that the key referred to by *key
must be statically initialized with the value PTHREAD_ONCE_KEY_NP
before calling pthread_key_create_once_np(), and the key is created
exactly once. This function call is equivalent to using
pthread_once(3C) to call a onetime initialization function that calls
pthread_key_create() to create the data key.

RETURN VALUES

If successful, the pthread_key_create() and
pthread_key_create_once_np() functions store the newly created key
value at *key and return 0. Otherwise, an error number is returned to
indicate the error.

ERRORS

The pthread_key_create() and pthread_key_create_once_np() functions
will 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() and pthread_key_create_once_np() functions
will not return an error value of EINTR.

EXAMPLES

Example 1: Call thread-specific data in the function from more than

one thread without special initialization.

In the following example, the thread-specific data in the 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.

/* cc -mt thisfile.c */

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

static void *thread_function(void *);
static void show_tsd(void);
static void cleanup(void*);

#define MAX_THREADS 20

static pthread_key_t tsd_key = PTHREAD_ONCE_KEY_NP;

int
main(int argc, char *argv[])
{
pthread_t tid[MAX_THREADS];
int num_threads;
int i;

if ((num_threads = argc - 1) > MAX_THREADS)
num_threads = MAX_THREADS;
for (i = 0; i < num_threads; i++)
pthread_create(&tid[i], NULL, thread_function, argv[i+1]);
for (i = 0; i < num_threads; i++)
pthread_join(tid[i], NULL);
return (0);
}

static void *
thread_function(void *arg)
{
char *data;

pthread_key_create_once_np(&tsd_key, cleanup);
data = malloc(strlen(arg) + 1);
strcpy(data, arg);
pthread_setspecific(tsd_key, data);
show_tsd();
return (NULL);
}

static void
show_tsd()
{
void *tsd = pthread_getspecific(tsd_key);

printf("tsd for %d = %s\n", pthread_self(), (char *)tsd);
}

/* application-specific clean-up function */
static void
cleanup(void *tsd)
{
printf("freeing tsd for %d = %s\n", pthread_self(), (char *)tsd);
free(tsd);
}

ATTRIBUTES