NetBSD/lib/libpthread/pthread_key_create.3

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.\" $FreeBSD: src/lib/libpthread/man/pthread_key_create.3,v 1.12 2002/09/16 19:29:28 mini Exp $
.\"
.Dd July 9, 2010
.Dt PTHREAD_KEY_CREATE 3
.Os
.Sh NAME
.Nm pthread_key_create
.Nd thread-specific data
.Sh LIBRARY
.Lb libpthread
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_key_create "pthread_key_t *key" "void (*destructor)(void *)"
.Ft int
.Fn pthread_key_delete "pthread_key_t key"
.Sh DESCRIPTION
The
.Fn pthread_key_create
function creates a thread-specific data key visible to all threads in the
process.
Key values are opaque objects used to locate thread-specific data.
The same key value may be used by different threads,
but the values bound to the key by
.Fn pthread_setspecific
are maintained on a per-thread basis and
persist for the life of the calling thread.
.Pp
Upon key creation, the value
.Dv NULL
is associated with the new key in all active threads.
Upon thread creation, the value
.Dv NULL
is associated with all
defined keys in the new thread.
.Pp
An optional destructor function may be associated with each key value.
At thread exit, if a key value has a
.Pf non- Dv NULL
destructor pointer, and the thread has a
.Pf non- Dv NULL
value associated with the key, the function pointed
to is called with the current associated value as its sole argument.
The order of destructor calls is unspecified if more
than one destructor exists for a thread when it exits.
.Pp
If, after all the destructors have been called for all
.Pf non- Dv NULL
values with associated destructors, there are still some
.Pf non- Dv NULL
values with associated destructors, then the process is repeated.
If, after at least
.Dv PTHREAD_DESTRUCTOR_ITERATIONS
iterations of destructor calls for outstanding
.Pf non- Dv NULL
values, there are still some
.Pf non- Dv NULL
values with
associated destructors, the implementation stops calling destructors.
.Pp
The
.Fn pthread_key_delete
function deletes a thread-specific data key previously returned by
.Fn pthread_key_create .
The thread-specific data values associated with
.Fa key
need not be
.Dv NULL
at the time of the call.
It is the responsibility of the application to free any
application storage or perform any cleanup actions for data structures
related to the deleted key or associated thread-specific data in any threads;
this cleanup can be done either before or after
.Fn pthread_key_delete
is called.
Any attempt to use
.Fa key
following the call to
.Fn pthread_key_delete
results in undefined behavior.
.Pp
The
.Fn pthread_key_delete
function itself is callable from within destructor functions,
but destructor functions are not invoked by the function.
Any destructor function that may have been associated with
.Fa key
will no longer be called upon thread exit.
.Sh RETURN VALUES
If successful, the
.Fn pthread_key_create
function will store the newly created key value at the location specified by
.Fa key
and returns zero.
Also
.Fn pthread_key_delete
will return zero upon success.
Upon failure both functions return an error number to indicate the cause.
.Sh ERRORS
The
.Fn pthread_key_create
may fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The system lacked the necessary resources to create another thread-specific
data key, or the system-imposed limit on the total number of keys per process
.Dv PTHREAD_KEYS_MAX
would be exceeded.
.It Bq Er ENOMEM
Insufficient memory exists to create the key.
.El
.Pp
The
.Fn pthread_key_delete
function may fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa key
value is invalid.
.El
.Sh SEE ALSO
.Xr pthread_getspecific 3
.Sh STANDARDS
These functions conform to
.St -p1003.1-2001 .
.Sh BUGS
The current specifications are flawed and
do not permit a clean implementation without potential problems.
The current implementation in
.Nx
addresses these problems by not supporting key reuse.