Single-threaded C programs have two basic classes of data—local data and global data. For multithreaded C programs a third class is added—thread-specific data (TSD). This is very much like global data, except that it is private to a thread.
Thread-specific data is maintained on a per-thread basis. TSD is the only way to define and refer to data that is private to a thread. Each thread-specific data item is associated with a key that is global to all threads in the process. Using the key, a thread can access a pointer (void *) that is maintained per-thread.
Use pthread_key_create(3THR) to allocate a key that is used to identify thread-specific data in a process. The key is global to all threads in the process, and all threads initially have the value NULL associated with the key when it is created.
Call pthread_key_create() once for each key before using the key. There is no implicit synchronization.
Once a key has been created, each thread can bind a value to the key. The values are specific to the threads and are maintained for each thread independently. The per-thread binding is deallocated when a thread terminates if the key was created with a destructor function.
Prototype: int pthread_key_create(pthread_key_t *key, void (*destructor) (void *));
#include <pthread.h> pthread_key_t key; int ret; /* key create without destructor */ ret = pthread_key_create(&key, NULL); /* key create with destructor */ ret = pthread_key_create(&key, destructor);
When pthread_key_create() returns successfully, the allocated key is stored in the location pointed to by key. The caller must ensure that the storage and access to this key are properly synchronized.
An optional destructor function, destructor, can be used to free stale storage. When a key has a non-NULL destructor function and the thread has a non-NULL value associated with that key, the destructor function is called with the current associated value when the thread exits. The order in which the destructor functions are called is unspecified.
pthread_key_create() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occur, pthread_key_create() fails and returns the corresponding value.