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pthread_key_create(3)
NAME
  pthread_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 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

SEE ALSO
  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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