pthread_key_create(3)pthread_key_create(3)NAMEpthread_key_create - Generates a unique thread-specific data key
SYNOPSIS
#include <pthread.h>
int pthread_key_create(
pthread_key_t *key,
void (*destructor)(void *) );
LIBRARY
DECthreads POSIX 1003.1c Library (libpthread.so)
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
IEEE Std 1003.1c-1995, POSIX System Application Program Interface
PARAMETERS
Location where the new thread-specific data key will be stored. Proce‐
dure called to destroy a thread-specific data value associated with the
created key when a thread terminates. Note that the argument to the
destructor for the user-specified routine is the non-NULL value associ‐
ated with a key.
DESCRIPTION
This routine generates a unique, thread-specific data key that is visi‐
ble to all threads in the process. The variable key provided by this
routine is an opaque object used to locate thread-specific data.
Although the same key value can be used by different threads, the val‐
ues bound to the key by pthread_setspecific(3) are maintained on a per-
thread basis and persist for the life of the calling thread.
DECthreads imposes a maximum number of thread-specific data keys, equal
to the symbolic constant PTHREAD_KEYS_MAX.
Thread-specific data allows client software to associate “static”
information with the current thread. For example, where a routine
declares a variable static in a single-threaded program, a multi‐
threaded version of the program might create a thread-specific data key
to store the same variable.
This routine generates and returns a new key value. The key reserves a
cell within each thread. Each call to this routine creates a new cell
that is unique within an application invocation. Keys must be generated
from initialization code that is guaranteed to be called only once
within each process. (See the pthread_once(3) description for more
information.)
When a thread terminates, its thread-specific data is automatically
destroyed; however, the key remains unless destroyed by a call to
pthread_key_delete(3). An optional destructor function can be associ‐
ated with each key. At thread exit, if a key has a non-NULL destructor
pointer, and the thread has a non-NULL value associated with that key,
the destructor function is called with the current associated value as
its sole argument. Note that the order in which thread-specific data
destructors are called at thread termination is undefined.
Before each destructor is called, the thread's value for the corre‐
sponding key is set to NULL. After the destructors have been called for
all non-NULL values with associated destructors, if there are still
some non-NULL values with associated destructors, then this sequence of
actions is repeated. If there are still non-NULL values for any key
with a destructor after four repetitions of this sequence, DECthreads
terminates the thread. At this point, any key values that represent
allocated heap will be lost. Note that this occurs only when a destruc‐
tor performs some action that creates a new value for some key. Your
program's destructor code should attempt to avoid this sort of circu‐
larity.
RETURN VALUES
If an error condition occurs, this routine returns an integer value
indicating the type of error. Possible return values are as follows:
Successful completion. The system lacked the necessary resources to
create another thread-specific data key, or the limit on the total num‐
ber of keys per process (PTHREAD_KEYS_MAX) has been exceeded. Insuffi‐
cient memory exists to create the key.
ERRORS
None
SEE ALSO
Functions: pthread_getspecific(3), pthread_key_delete(3),
pthread_once(3), pthread_setspecific(3)
Manuals: Guide to DECthreads and Programmer's Guide
pthread_key_create(3)
