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pthread_key_create(3) Library Functions Manual pthread_key_create(3) NAME pthread_key_create - Generates a unique thread-specific data key. LIBRARY DECthreads POSIX 1003.1c Library (libpthread.so) SYNOPSIS #include <pthread.h> int pthread_key_create( pthread_key_t *key, void (*destructor)(void *)); 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 The new thread-specific data key. Procedure called to destroy a thread-specific data value associated with the created key when the thread terminates. Note that the argument to the destructor for the user-specified routine is the non-NULL value associated 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. Al- though the same key value can be used by different threads, the values 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" in- formation with the current thread. For example, where a routine de- clares a variable static in a single-threaded program, a multithreaded 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 gener- ated from initialization code that is guaranteed to be called only once within each process. (See the pthread_once(3) description for more in- formation.) When a thread terminates, its thread-specific data is automatically de- stroyed; 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 de- structor performs some action that creates a new value for some key. Your program's destructor code should attempt to avoid this sort of circularity. RETURN VALUES If an error condition occurs, this routine returns an integer value in- dicating 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 RELATED INFORMATION Functions: pthread_getspecific(3), pthread_key_delete(3), pthread_once(3), pthread_setspecific(3) Manuals: Guide to DECthreads and Programmer's Guide delim off pthread_key_create(3)