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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 standards
  as follows:

  IEEE Std 1003.1c-1995, POSIX System Application Program Interface

PARAMETERS
  key	    Location where the new thread-specific data key will be stored.

  destructor
	    Procedure 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 associated with a key.

DESCRIPTION
  This routine generates a unique, thread-specific data key that is visible
  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 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"
  information with the current thread.	For example, where a routine declares
  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 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 associated
  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 corresponding
  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 destructor 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
  indicating the type of error.	 Possible return values are as follows:

  0	    Successful completion.

  [EAGAIN]  The system lacked the necessary resources to create another
	    thread-specific data key, or the limit on the total number of
	    keys per process (PTHREAD_KEYS_MAX) has been exceeded.

  [ENOMEM]  Insufficient 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

Index for Section 3

Alphabetical listing for P

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