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[glibc.git] / linuxthreads / linuxthreads.texi

@node POSIX Threads
@c @node POSIX Threads, , Top, Top
@chapter POSIX Threads
@c %MENU% The standard threads library

@c This chapter needs more work bigtime. -zw

This chapter describes the pthreads (POSIX threads) library.  This
library provides support functions for multithreaded programs: thread
primitives, synchronization objects, and so forth.  It also implements
POSIX 1003.1b semaphores (not to be confused with System V semaphores).

The threads operations (@samp{pthread_*}) do not use @var{errno}.
Instead they return an error code directly.  The semaphore operations do
use @var{errno}.

@menu
* Basic Thread Operations::     Creating, terminating, and waiting for threads.
* Thread Attributes::           Tuning thread scheduling.
* Cancellation::                Stopping a thread before it's done.
* Cleanup Handlers::            Deallocating resources when a thread is
                                  cancelled.
* Mutexes::                     One way to synchronize threads.
* Condition Variables::         Another way.
* POSIX Semaphores::            And a third way.
* Thread-Specific Data::        Variables with different values in
                                  different threads.
* Threads and Signal Handling:: Why you should avoid mixing the two, and
                                  how to do it if you must.
* Threads and Fork::            Interactions between threads and the 
                                  @code{fork} function.
* Streams and Fork::            Interactions between stdio streams and 
                                  @code{fork}.
* Miscellaneous Thread Functions:: A grab bag of utility routines.
@end menu

@node Basic Thread Operations
@section Basic Thread Operations

These functions are the thread equivalents of @code{fork}, @code{exit},
and @code{wait}.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_create (pthread_t * @var{thread}, pthread_attr_t * @var{attr}, void * (*@var{start_routine})(void *), void * @var{arg})
@code{pthread_create} creates a new thread of control that executes
concurrently with the calling thread. The new thread calls the
function @var{start_routine}, passing it @var{arg} as first argument. The
new thread terminates either explicitly, by calling @code{pthread_exit},
or implicitly, by returning from the @var{start_routine} function. The
latter case is equivalent to calling @code{pthread_exit} with the result
returned by @var{start_routine} as exit code.

The @var{attr} argument specifies thread attributes to be applied to the
new thread. @xref{Thread Attributes}, for details. The @var{attr}
argument can also be @code{NULL}, in which case default attributes are
used: the created thread is joinable (not detached) and has an ordinary
(not realtime) scheduling policy.

On success, the identifier of the newly created thread is stored in the
location pointed by the @var{thread} argument, and a 0 is returned. On
error, a non-zero error code is returned.

This function may return the following errors:
@table @code
@item EAGAIN
Not enough system resources to create a process for the new thread,
or more than @code{PTHREAD_THREADS_MAX} threads are already active.
@end table
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun void pthread_exit (void *@var{retval})
@code{pthread_exit} terminates the execution of the calling thread.  All
cleanup handlers (@pxref{Cleanup Handlers}) that have been set for the
calling thread with @code{pthread_cleanup_push} are executed in reverse
order (the most recently pushed handler is executed first). Finalization
functions for thread-specific data are then called for all keys that
have non-@code{NULL} values associated with them in the calling thread
(@pxref{Thread-Specific Data}).  Finally, execution of the calling
thread is stopped.

The @var{retval} argument is the return value of the thread. It can be
retrieved from another thread using @code{pthread_join}.

The @code{pthread_exit} function never returns.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cancel (pthread_t @var{thread})

@code{pthread_cancel} sends a cancellation request to the thread denoted
by the @var{thread} argument.  If there is no such thread,
@code{pthread_cancel} fails and returns @code{ESRCH}.  Otherwise it
returns 0. @xref{Cancellation}, for details.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_join (pthread_t @var{th}, void **thread_@var{return})
@code{pthread_join} suspends the execution of the calling thread until
the thread identified by @var{th} terminates, either by calling
@code{pthread_exit} or by being cancelled.

If @var{thread_return} is not @code{NULL}, the return value of @var{th}
is stored in the location pointed to by @var{thread_return}.  The return
value of @var{th} is either the argument it gave to @code{pthread_exit},
or @code{PTHREAD_CANCELED} if @var{th} was cancelled.

The joined thread @code{th} must be in the joinable state: it must not
have been detached using @code{pthread_detach} or the
@code{PTHREAD_CREATE_DETACHED} attribute to @code{pthread_create}.

When a joinable thread terminates, its memory resources (thread
descriptor and stack) are not deallocated until another thread performs
@code{pthread_join} on it. Therefore, @code{pthread_join} must be called
once for each joinable thread created to avoid memory leaks.

At most one thread can wait for the termination of a given
thread. Calling @code{pthread_join} on a thread @var{th} on which
another thread is already waiting for termination returns an error.

@code{pthread_join} is a cancellation point. If a thread is canceled
while suspended in @code{pthread_join}, the thread execution resumes
immediately and the cancellation is executed without waiting for the
@var{th} thread to terminate. If cancellation occurs during
@code{pthread_join}, the @var{th} thread remains not joined.

On success, the return value of @var{th} is stored in the location
pointed to by @var{thread_return}, and 0 is returned. On error, one of
the following values is returned:
@table @code
@item ESRCH
No thread could be found corresponding to that specified by @var{th}.
@item EINVAL
The @var{th} thread has been detached, or another thread is already
waiting on termination of @var{th}.
@item EDEADLK
The @var{th} argument refers to the calling thread.
@end table
@end deftypefun

@node Thread Attributes
@section Thread Attributes

@comment pthread.h
@comment POSIX

Threads have a number of attributes that may be set at creation time.
This is done by filling a thread attribute object @var{attr} of type
@code{pthread_attr_t}, then passing it as second argument to
@code{pthread_create}. Passing @code{NULL} is equivalent to passing a
thread attribute object with all attributes set to their default values.

Attribute objects are consulted only when creating a new thread.  The
same attribute object can be used for creating several threads.
Modifying an attribute object after a call to @code{pthread_create} does
not change the attributes of the thread previously created.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_attr_init (pthread_attr_t *@var{attr})
@code{pthread_attr_init} initializes the thread attribute object
@var{attr} and fills it with default values for the attributes. (The
default values are listed below for each attribute.)

Each attribute @var{attrname} (see below for a list of all attributes)
can be individually set using the function
@code{pthread_attr_set@var{attrname}} and retrieved using the function
@code{pthread_attr_get@var{attrname}}.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_attr_destroy (pthread_attr_t *@var{attr})
@code{pthread_attr_destroy} destroys the attribute object pointed to by
@var{attr} releasing any resources associated with it.  @var{attr} is
left in an undefined state, and you must not use it again in a call to
any pthreads function until it has been reinitialized.
@end deftypefun

@findex pthread_attr_setinheritsched
@findex pthread_attr_setschedparam
@findex pthread_attr_setschedpolicy
@findex pthread_attr_setscope
@comment pthread.h
@comment POSIX
@deftypefun int pthread_attr_set@var{attr} (pthread_attr_t *@var{obj}, int @var{value})
Set attribute @var{attr} to @var{value} in the attribute object pointed
to by @var{obj}.  See below for a list of possible attributes and the
values they can take.

On success, these functions return 0.  If @var{value} is not meaningful
for the @var{attr} being modified, they will return the error code
@code{EINVAL}.  Some of the functions have other failure modes; see
below.
@end deftypefun

@findex pthread_attr_getinheritsched
@findex pthread_attr_getschedparam
@findex pthread_attr_getschedpolicy
@findex pthread_attr_getscope
@comment pthread.h
@comment POSIX
@deftypefun int pthread_attr_get@var{attr} (const pthread_attr_t *@var{obj}, int *@var{value})
Store the current setting of @var{attr} in @var{obj} into the variable
pointed to by @var{value}.

These functions always return 0.
@end deftypefun

The following thread attributes are supported:
@table @samp
@item detachstate
Choose whether the thread is created in the joinable state (value
@code{PTHREAD_CREATE_JOINABLE}) or in the detached state
(@code{PTHREAD_CREATE_DETACHED}).  The default is
@code{PTHREAD_CREATE_JOINABLE}.

In the joinable state, another thread can synchronize on the thread
termination and recover its termination code using @code{pthread_join},
but some of the thread resources are kept allocated after the thread
terminates, and reclaimed only when another thread performs
@code{pthread_join} on that thread.

In the detached state, the thread resources are immediately freed when
it terminates, but @code{pthread_join} cannot be used to synchronize on
the thread termination.

A thread created in the joinable state can later be put in the detached
thread using @code{pthread_detach}.

@item schedpolicy
Select the scheduling policy for the thread: one of @code{SCHED_OTHER}
(regular, non-realtime scheduling), @code{SCHED_RR} (realtime,
round-robin) or @code{SCHED_FIFO} (realtime, first-in first-out).
The default is @code{SCHED_OTHER}.
@c Not doc'd in our manual: FIXME.
@c See @code{sched_setpolicy} for more information on scheduling policies.

The realtime scheduling policies @code{SCHED_RR} and @code{SCHED_FIFO}
are available only to processes with superuser privileges.
@code{pthread_attr_setschedparam} will fail and return @code{ENOTSUP} if
you try to set a realtime policy when you are unprivileged.

The scheduling policy of a thread can be changed after creation with
@code{pthread_setschedparam}.

@item schedparam
Change the scheduling parameter (the scheduling priority)
for the thread.  The default is 0.

This attribute is not significant if the scheduling policy is
@code{SCHED_OTHER}; it only matters for the realtime policies
@code{SCHED_RR} and @code{SCHED_FIFO}.

The scheduling priority of a thread can be changed after creation with
@code{pthread_setschedparam}.

@item inheritsched
Choose whether the scheduling policy and scheduling parameter for the
newly created thread are determined by the values of the
@var{schedpolicy} and @var{schedparam} attributes (value
@code{PTHREAD_EXPLICIT_SCHED}) or are inherited from the parent thread
(value @code{PTHREAD_INHERIT_SCHED}).  The default is
@code{PTHREAD_EXPLICIT_SCHED}.

@item scope
Choose the scheduling contention scope for the created thread.  The
default is @code{PTHREAD_SCOPE_SYSTEM}, meaning that the threads contend
for CPU time with all processes running on the machine. In particular,
thread priorities are interpreted relative to the priorities of all
other processes on the machine. The other possibility,
@code{PTHREAD_SCOPE_PROCESS}, means that scheduling contention occurs
only between the threads of the running process: thread priorities are
interpreted relative to the priorities of the other threads of the
process, regardless of the priorities of other processes.

@code{PTHREAD_SCOPE_PROCESS} is not supported in LinuxThreads.  If you
try to set the scope to this value @code{pthread_attr_setscope} will
fail and return @code{ENOTSUP}.
@end table

@node Cancellation
@section Cancellation

Cancellation is the mechanism by which a thread can terminate the
execution of another thread. More precisely, a thread can send a
cancellation request to another thread. Depending on its settings, the
target thread can then either ignore the request, honor it immediately,
or defer it till it reaches a cancellation point.  When threads are
first created by @code{pthread_create}, they always defer cancellation
requests.

When a thread eventually honors a cancellation request, it behaves as if
@code{pthread_exit(PTHREAD_CANCELED)} was called.  All cleanup handlers
are executed in reverse order, finalization functions for
thread-specific data are called, and finally the thread stops executing.
If the cancelled thread was joinable, the return value
@code{PTHREAD_CANCELED} is provided to whichever thread calls
@var{pthread_join} on it. See @code{pthread_exit} for more information.

Cancellation points are the points where the thread checks for pending
cancellation requests and performs them.  The POSIX threads functions
@code{pthread_join}, @code{pthread_cond_wait},
@code{pthread_cond_timedwait}, @code{pthread_testcancel},
@code{sem_wait}, and @code{sigwait} are cancellation points.  In
addition, these system calls are cancellation points:

@multitable @columnfractions .33 .33 .33
@item @t{accept}        @tab @t{open}           @tab @t{sendmsg}
@item @t{close}         @tab @t{pause}          @tab @t{sendto}
@item @t{connect}       @tab @t{read}           @tab @t{system}
@item @t{fcntl}         @tab @t{recv}           @tab @t{tcdrain}
@item @t{fsync}         @tab @t{recvfrom}       @tab @t{wait}
@item @t{lseek}         @tab @t{recvmsg}        @tab @t{waitpid}
@item @t{msync}         @tab @t{send}           @tab @t{write}
@item @t{nanosleep}
@end multitable

@noindent
All library functions that call these functions (such as
@code{printf}) are also cancellation points.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_setcancelstate (int @var{state}, int *@var{oldstate})
@code{pthread_setcancelstate} changes the cancellation state for the
calling thread -- that is, whether cancellation requests are ignored or
not. The @var{state} argument is the new cancellation state: either
@code{PTHREAD_CANCEL_ENABLE} to enable cancellation, or
@code{PTHREAD_CANCEL_DISABLE} to disable cancellation (cancellation
requests are ignored).

If @var{oldstate} is not @code{NULL}, the previous cancellation state is
stored in the location pointed to by @var{oldstate}, and can thus be
restored later by another call to @code{pthread_setcancelstate}.

If the @var{state} argument is not @code{PTHREAD_CANCEL_ENABLE} or
@code{PTHREAD_CANCEL_DISABLE}, @code{pthread_setcancelstate} fails and
returns @code{EINVAL}.  Otherwise it returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_setcanceltype (int @var{type}, int *@var{oldtype})
@code{pthread_setcanceltype} changes the type of responses to
cancellation requests for the calling thread: asynchronous (immediate)
or deferred.  The @var{type} argument is the new cancellation type:
either @code{PTHREAD_CANCEL_ASYNCHRONOUS} to cancel the calling thread
as soon as the cancellation request is received, or
@code{PTHREAD_CANCEL_DEFERRED} to keep the cancellation request pending
until the next cancellation point. If @var{oldtype} is not @code{NULL},
the previous cancellation state is stored in the location pointed to by
@var{oldtype}, and can thus be restored later by another call to
@code{pthread_setcanceltype}.

If the @var{type} argument is not @code{PTHREAD_CANCEL_DEFERRED} or
@code{PTHREAD_CANCEL_ASYNCHRONOUS}, @code{pthread_setcanceltype} fails
and returns @code{EINVAL}.  Otherwise it returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun void pthread_testcancel (@var{void})
@code{pthread_testcancel} does nothing except testing for pending
cancellation and executing it. Its purpose is to introduce explicit
checks for cancellation in long sequences of code that do not call
cancellation point functions otherwise.
@end deftypefun

@node Cleanup Handlers
@section Cleanup Handlers

Cleanup handlers are functions that get called when a thread terminates,
either by calling @code{pthread_exit} or because of
cancellation. Cleanup handlers are installed and removed following a
stack-like discipline.

The purpose of cleanup handlers is to free the resources that a thread
may hold at the time it terminates. In particular, if a thread exits or
is cancelled while it owns a locked mutex, the mutex will remain locked
forever and prevent other threads from executing normally. The best way
to avoid this is, just before locking the mutex, to install a cleanup
handler whose effect is to unlock the mutex. Cleanup handlers can be
used similarly to free blocks allocated with @code{malloc} or close file
descriptors on thread termination.

Here is how to lock a mutex @var{mut} in such a way that it will be
unlocked if the thread is canceled while @var{mut} is locked:

@smallexample
pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_mutex_unlock(&mut);
pthread_cleanup_pop(0);
@end smallexample

Equivalently, the last two lines can be replaced by

@smallexample
pthread_cleanup_pop(1);
@end smallexample

Notice that the code above is safe only in deferred cancellation mode
(see @code{pthread_setcanceltype}). In asynchronous cancellation mode, a
cancellation can occur between @code{pthread_cleanup_push} and
@code{pthread_mutex_lock}, or between @code{pthread_mutex_unlock} and
@code{pthread_cleanup_pop}, resulting in both cases in the thread trying
to unlock a mutex not locked by the current thread. This is the main
reason why asynchronous cancellation is difficult to use.

If the code above must also work in asynchronous cancellation mode,
then it must switch to deferred mode for locking and unlocking the
mutex:

@smallexample
pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_cleanup_pop(1);
pthread_setcanceltype(oldtype, NULL);
@end smallexample

The code above can be rewritten in a more compact and efficient way,
using the non-portable functions @code{pthread_cleanup_push_defer_np}
and @code{pthread_cleanup_pop_restore_np}:

@smallexample
pthread_cleanup_push_defer_np(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_cleanup_pop_restore_np(1);
@end smallexample

@comment pthread.h
@comment POSIX
@deftypefun void pthread_cleanup_push (void (*@var{routine}) (void *), void *@var{arg})

@code{pthread_cleanup_push} installs the @var{routine} function with
argument @var{arg} as a cleanup handler. From this point on to the
matching @code{pthread_cleanup_pop}, the function @var{routine} will be
called with arguments @var{arg} when the thread terminates, either
through @code{pthread_exit} or by cancellation. If several cleanup
handlers are active at that point, they are called in LIFO order: the
most recently installed handler is called first.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun void pthread_cleanup_pop (int @var{execute})
@code{pthread_cleanup_pop} removes the most recently installed cleanup
handler. If the @var{execute} argument is not 0, it also executes the
handler, by calling the @var{routine} function with arguments
@var{arg}. If the @var{execute} argument is 0, the handler is only
removed but not executed.
@end deftypefun

Matching pairs of @code{pthread_cleanup_push} and
@code{pthread_cleanup_pop} must occur in the same function, at the same
level of block nesting.  Actually, @code{pthread_cleanup_push} and
@code{pthread_cleanup_pop} are macros, and the expansion of
@code{pthread_cleanup_push} introduces an open brace @code{@{} with the
matching closing brace @code{@}} being introduced by the expansion of the
matching @code{pthread_cleanup_pop}.

@comment pthread.h
@comment GNU
@deftypefun void pthread_cleanup_push_defer_np (void (*@var{routine}) (void *), void *@var{arg})
@code{pthread_cleanup_push_defer_np} is a non-portable extension that
combines @code{pthread_cleanup_push} and @code{pthread_setcanceltype}.
It pushes a cleanup handler just as @code{pthread_cleanup_push} does,
but also saves the current cancellation type and sets it to deferred
cancellation. This ensures that the cleanup mechanism is effective even
if the thread was initially in asynchronous cancellation mode.
@end deftypefun

@comment pthread.h
@comment GNU
@deftypefun void pthread_cleanup_pop_restore_np (int @var{execute})
@code{pthread_cleanup_pop_restore_np} pops a cleanup handler introduced
by @code{pthread_cleanup_push_defer_np}, and restores the cancellation
type to its value at the time @code{pthread_cleanup_push_defer_np} was
called.
@end deftypefun

@code{pthread_cleanup_push_defer_np} and
@code{pthread_cleanup_pop_restore_np} must occur in matching pairs, at
the same level of block nesting.

The sequence

@smallexample
pthread_cleanup_push_defer_np(routine, arg);
...
pthread_cleanup_pop_defer_np(execute);
@end smallexample

@noindent
is functionally equivalent to (but more compact and efficient than)

@smallexample
@{
  int oldtype;
  pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
  pthread_cleanup_push(routine, arg);
  ...
  pthread_cleanup_pop(execute);
  pthread_setcanceltype(oldtype, NULL);
@}
@end smallexample


@node Mutexes
@section Mutexes

A mutex is a MUTual EXclusion device, and is useful for protecting
shared data structures from concurrent modifications, and implementing
critical sections and monitors.

A mutex has two possible states: unlocked (not owned by any thread),
and locked (owned by one thread). A mutex can never be owned by two
different threads simultaneously. A thread attempting to lock a mutex
that is already locked by another thread is suspended until the owning
thread unlocks the mutex first.

None of the mutex functions is a cancellation point, not even
@code{pthread_mutex_lock}, in spite of the fact that it can suspend a
thread for arbitrary durations. This way, the status of mutexes at
cancellation points is predictable, allowing cancellation handlers to
unlock precisely those mutexes that need to be unlocked before the
thread stops executing. Consequently, threads using deferred
cancellation should never hold a mutex for extended periods of time.

It is not safe to call mutex functions from a signal handler.  In
particular, calling @code{pthread_mutex_lock} or
@code{pthread_mutex_unlock} from a signal handler may deadlock the
calling thread.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_init (pthread_mutex_t *@var{mutex}, const pthread_mutexattr_t *@var{mutexattr})

@code{pthread_mutex_init} initializes the mutex object pointed to by
@var{mutex} according to the mutex attributes specified in @var{mutexattr}.
If @var{mutexattr} is @code{NULL}, default attributes are used instead.

The LinuxThreads implementation supports only one mutex attribute,
the @var{mutex kind}, which is either ``fast'', ``recursive'', or
``error checking''. The kind of a mutex determines whether
it can be locked again by a thread that already owns it.
The default kind is ``fast''.

Variables of type @code{pthread_mutex_t} can also be initialized
statically, using the constants @code{PTHREAD_MUTEX_INITIALIZER} (for
fast mutexes), @code{PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP} (for
recursive mutexes), and @code{PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP}
(for error checking mutexes).

@code{pthread_mutex_init} always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_lock (pthread_mutex_t *mutex))
@code{pthread_mutex_lock} locks the given mutex. If the mutex is
currently unlocked, it becomes locked and owned by the calling thread,
and @code{pthread_mutex_lock} returns immediately. If the mutex is
already locked by another thread, @code{pthread_mutex_lock} suspends the
calling thread until the mutex is unlocked.

If the mutex is already locked by the calling thread, the behavior of
@code{pthread_mutex_lock} depends on the kind of the mutex. If the mutex
is of the ``fast'' kind, the calling thread is suspended.  It will
remain suspended forever, because no other thread can unlock the mutex.
If  the mutex is of the ``error checking'' kind, @code{pthread_mutex_lock}
returns immediately with the error code @code{EDEADLK}.  If the mutex is
of the ``recursive'' kind, @code{pthread_mutex_lock} succeeds and
returns immediately, recording the number of times the calling thread
has locked the mutex. An equal number of @code{pthread_mutex_unlock}
operations must be performed before the mutex returns to the unlocked
state.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_trylock (pthread_mutex_t *@var{mutex})
@code{pthread_mutex_trylock} behaves identically to
@code{pthread_mutex_lock}, except that it does not block the calling
thread if the mutex is already locked by another thread (or by the
calling thread in the case of a ``fast'' mutex). Instead,
@code{pthread_mutex_trylock} returns immediately with the error code
@code{EBUSY}.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_unlock (pthread_mutex_t *@var{mutex})
@code{pthread_mutex_unlock} unlocks the given mutex. The mutex is
assumed to be locked and owned by the calling thread on entrance to
@code{pthread_mutex_unlock}. If the mutex is of the ``fast'' kind,
@code{pthread_mutex_unlock} always returns it to the unlocked state. If
it is of the ``recursive'' kind, it decrements the locking count of the
mutex (number of @code{pthread_mutex_lock} operations performed on it by
the calling thread), and only when this count reaches zero is the mutex
actually unlocked.

On ``error checking'' mutexes, @code{pthread_mutex_unlock} actually
checks at run-time that the mutex is locked on entrance, and that it was
locked by the same thread that is now calling
@code{pthread_mutex_unlock}.  If these conditions are not met,
@code{pthread_mutex_unlock} returns @code{EPERM}, and the mutex remains
unchanged.  ``Fast'' and ``recursive'' mutexes perform no such checks,
thus allowing a locked mutex to be unlocked by a thread other than its
owner. This is non-portable behavior and must not be relied upon.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_destroy (pthread_mutex_t *@var{mutex})
@code{pthread_mutex_destroy} destroys a mutex object, freeing the
resources it might hold. The mutex must be unlocked on entrance. In the
LinuxThreads implementation, no resources are associated with mutex
objects, thus @code{pthread_mutex_destroy} actually does nothing except
checking that the mutex is unlocked.

If the mutex is locked by some thread, @code{pthread_mutex_destroy}
returns @code{EBUSY}.  Otherwise it returns 0.
@end deftypefun

If any of the above functions (except @code{pthread_mutex_init})
is applied to an uninitialized mutex, they will simply return
@code{EINVAL} and do nothing.

A shared global variable @var{x} can be protected by a mutex as follows:

@smallexample
int x;
pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
@end smallexample

All accesses and modifications to @var{x} should be bracketed by calls to
@code{pthread_mutex_lock} and @code{pthread_mutex_unlock} as follows:

@smallexample
pthread_mutex_lock(&mut);
/* operate on x */
pthread_mutex_unlock(&mut);
@end smallexample

Mutex attributes can be specified at mutex creation time, by passing a
mutex attribute object as second argument to @code{pthread_mutex_init}.
Passing @code{NULL} is equivalent to passing a mutex attribute object
with all attributes set to their default values.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutexattr_init (pthread_mutexattr_t *@var{attr})
@code{pthread_mutexattr_init} initializes the mutex attribute object
@var{attr} and fills it with default values for the attributes.

This function always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutexattr_destroy (pthread_mutexattr_t *@var{attr})
@code{pthread_mutexattr_destroy} destroys a mutex attribute object,
which must not be reused until it is
reinitialized. @code{pthread_mutexattr_destroy} does nothing in the
LinuxThreads implementation.

This function always returns 0.
@end deftypefun

LinuxThreads supports only one mutex attribute: the mutex kind, which is
either @code{PTHREAD_MUTEX_FAST_NP} for ``fast'' mutexes,
@code{PTHREAD_MUTEX_RECURSIVE_NP} for ``recursive'' mutexes, or
@code{PTHREAD_MUTEX_ERRORCHECK_NP} for ``error checking'' mutexes.  As
the @code{NP} suffix indicates, this is a non-portable extension to the
POSIX standard and should not be employed in portable programs.

The mutex kind determines what happens if a thread attempts to lock a
mutex it already owns with @code{pthread_mutex_lock}. If the mutex is of
the ``fast'' kind, @code{pthread_mutex_lock} simply suspends the calling
thread forever.  If the mutex is of the ``error checking'' kind,
@code{pthread_mutex_lock} returns immediately with the error code
@code{EDEADLK}.  If the mutex is of the ``recursive'' kind, the call to
@code{pthread_mutex_lock} returns immediately with a success return
code. The number of times the thread owning the mutex has locked it is
recorded in the mutex. The owning thread must call
@code{pthread_mutex_unlock} the same number of times before the mutex
returns to the unlocked state.

The default mutex kind is ``fast'', that is, @code{PTHREAD_MUTEX_FAST_NP}.

@comment pthread.h
@comment GNU
@deftypefun int pthread_mutexattr_setkind_np (pthread_mutexattr_t *@var{attr}, int @var{kind})
@code{pthread_mutexattr_setkind_np} sets the mutex kind attribute in
@var{attr} to the value specified by @var{kind}.

If @var{kind} is not @code{PTHREAD_MUTEX_FAST_NP},
@code{PTHREAD_MUTEX_RECURSIVE_NP}, or
@code{PTHREAD_MUTEX_ERRORCHECK_NP}, this function will return
@code{EINVAL} and leave @var{attr} unchanged.
@end deftypefun

@comment pthread.h
@comment GNU
@deftypefun int pthread_mutexattr_getkind_np (const pthread_mutexattr_t *@var{attr}, int *@var{kind})
@code{pthread_mutexattr_getkind_np} retrieves the current value of the
mutex kind attribute in @var{attr} and stores it in the location pointed
to by @var{kind}.

This function always returns 0.
@end deftypefun

@node Condition Variables
@section Condition Variables

A condition (short for ``condition variable'') is a synchronization
device that allows threads to suspend execution until some predicate on
shared data is satisfied. The basic operations on conditions are: signal
the condition (when the predicate becomes true), and wait for the
condition, suspending the thread execution until another thread signals
the condition.

A condition variable must always be associated with a mutex, to avoid
the race condition where a thread prepares to wait on a condition
variable and another thread signals the condition just before the first
thread actually waits on it.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_init (pthread_cond_t *@var{cond}, pthread_condattr_t *cond_@var{attr})

@code{pthread_cond_init} initializes the condition variable @var{cond},
using the condition attributes specified in @var{cond_attr}, or default
attributes if @var{cond_attr} is @code{NULL}. The LinuxThreads
implementation supports no attributes for conditions, hence the
@var{cond_attr} parameter is actually ignored.

Variables of type @code{pthread_cond_t} can also be initialized
statically, using the constant @code{PTHREAD_COND_INITIALIZER}.

This function always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_signal (pthread_cond_t *@var{cond})
@code{pthread_cond_signal} restarts one of the threads that are waiting
on the condition variable @var{cond}. If no threads are waiting on
@var{cond}, nothing happens. If several threads are waiting on
@var{cond}, exactly one is restarted, but it is not specified which.

This function always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_broadcast (pthread_cond_t *@var{cond})
@code{pthread_cond_broadcast} restarts all the threads that are waiting
on the condition variable @var{cond}. Nothing happens if no threads are
waiting on @var{cond}.

This function always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_wait (pthread_cond_t *@var{cond}, pthread_mutex_t *@var{mutex})
@code{pthread_cond_wait} atomically unlocks the @var{mutex} (as per
@code{pthread_unlock_mutex}) and waits for the condition variable
@var{cond} to be signaled. The thread execution is suspended and does
not consume any CPU time until the condition variable is signaled. The
@var{mutex} must be locked by the calling thread on entrance to
@code{pthread_cond_wait}. Before returning to the calling thread,
@code{pthread_cond_wait} re-acquires @var{mutex} (as per
@code{pthread_lock_mutex}).

Unlocking the mutex and suspending on the condition variable is done
atomically. Thus, if all threads always acquire the mutex before
signaling the condition, this guarantees that the condition cannot be
signaled (and thus ignored) between the time a thread locks the mutex
and the time it waits on the condition variable.

This function always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_timedwait (pthread_cond_t *@var{cond}, pthread_mutex_t *@var{mutex}, const struct timespec *@var{abstime})
@code{pthread_cond_timedwait} atomically unlocks @var{mutex} and waits
on @var{cond}, as @code{pthread_cond_wait} does, but it also bounds the
duration of the wait. If @var{cond} has not been signaled before time
@var{abstime}, the mutex @var{mutex} is re-acquired and
@code{pthread_cond_timedwait} returns the error code @code{ETIMEDOUT}.
The wait can also be interrupted by a signal; in that case
@code{pthread_cond_timedwait} returns @code{EINTR}.

The @var{abstime} parameter specifies an absolute time, with the same
origin as @code{time} and @code{gettimeofday}: an @var{abstime} of 0
corresponds to 00:00:00 GMT, January 1, 1970.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_destroy (pthread_cond_t *@var{cond})
@code{pthread_cond_destroy} destroys the condition variable @var{cond},
freeing the resources it might hold.  If any threads are waiting on the
condition variable, @code{pthread_cond_destroy} leaves @var{cond}
untouched and returns @code{EBUSY}.  Otherwise it returns 0, and
@var{cond} must not be used again until it is reinitialized.

In the LinuxThreads implementation, no resources are associated with
condition variables, so @code{pthread_cond_destroy} actually does
nothing.
@end deftypefun

@code{pthread_cond_wait} and @code{pthread_cond_timedwait} are
cancellation points. If a thread is cancelled while suspended in one of
these functions, the thread immediately resumes execution, relocks the
mutex specified by  @var{mutex}, and finally executes the cancellation.
Consequently, cleanup handlers are assured that @var{mutex} is locked
when they are called.

It is not safe to call the condition variable functions from a signal
handler. In particular, calling @code{pthread_cond_signal} or
@code{pthread_cond_broadcast} from a signal handler may deadlock the
calling thread.

Consider two shared variables @var{x} and @var{y}, protected by the
mutex @var{mut}, and a condition variable @var{cond} that is to be
signaled whenever @var{x} becomes greater than @var{y}.

@smallexample
int x,y;
pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
@end smallexample

Waiting until @var{x} is greater than @var{y} is performed as follows:

@smallexample
pthread_mutex_lock(&mut);
while (x <= y) @{
        pthread_cond_wait(&cond, &mut);
@}
/* operate on x and y */
pthread_mutex_unlock(&mut);
@end smallexample

Modifications on @var{x} and @var{y} that may cause @var{x} to become greater than
@var{y} should signal the condition if needed:

@smallexample
pthread_mutex_lock(&mut);
/* modify x and y */
if (x > y) pthread_cond_broadcast(&cond);
pthread_mutex_unlock(&mut);
@end smallexample

If it can be proved that at most one waiting thread needs to be waken
up (for instance, if there are only two threads communicating through
@var{x} and @var{y}), @code{pthread_cond_signal} can be used as a slightly more
efficient alternative to @code{pthread_cond_broadcast}. In doubt, use
@code{pthread_cond_broadcast}.

To wait for @var{x} to becomes greater than @var{y} with a timeout of 5
seconds, do:

@smallexample
struct timeval now;
struct timespec timeout;
int retcode;

pthread_mutex_lock(&mut);
gettimeofday(&now);
timeout.tv_sec = now.tv_sec + 5;
timeout.tv_nsec = now.tv_usec * 1000;
retcode = 0;
while (x <= y && retcode != ETIMEDOUT) @{
        retcode = pthread_cond_timedwait(&cond, &mut, &timeout);
@}
if (retcode == ETIMEDOUT) @{
        /* timeout occurred */
@} else @{
        /* operate on x and y */
@}
pthread_mutex_unlock(&mut);
@end smallexample

Condition attributes can be specified at condition creation time, by
passing a condition attribute object as second argument to
@code{pthread_cond_init}.  Passing @code{NULL} is equivalent to passing
a condition attribute object with all attributes set to their default
values.

The LinuxThreads implementation supports no attributes for
conditions. The functions on condition attributes are included only for
compliance with the POSIX standard.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_condattr_init (pthread_condattr_t *@var{attr})
@deftypefunx int pthread_condattr_destroy (pthread_condattr_t *@var{attr})
@code{pthread_condattr_init} initializes the condition attribute object
@var{attr} and fills it with default values for the attributes.
@code{pthread_condattr_destroy} destroys the condition attribute object
@var{attr}.

Both functions do nothing in the LinuxThreads implementation.

@code{pthread_condattr_init} and @code{pthread_condattr_destroy} always
return 0.
@end deftypefun

@node POSIX Semaphores
@section POSIX Semaphores

@vindex SEM_VALUE_MAX
Semaphores are counters for resources shared between threads. The
basic operations on semaphores are: increment the counter atomically,
and wait until the counter is non-null and decrement it atomically.

Semaphores have a maximum value past which they cannot be incremented.
The macro @code{SEM_VALUE_MAX} is defined to be this maximum value.  In
the GNU C library, @code{SEM_VALUE_MAX} is equal to @code{INT_MAX}
(@pxref{Range of Type}), but it may be much smaller on other systems.

The pthreads library implements POSIX 1003.1b semaphores.  These should
not be confused with System V semaphores (@code{ipc}, @code{semctl} and
@code{semop}).
@c !!! SysV IPC is not doc'd at all in our manual

All the semaphore functions and macros are defined in @file{semaphore.h}.

@comment semaphore.h
@comment POSIX
@deftypefun int sem_init (sem_t *@var{sem}, int @var{pshared}, unsigned int @var{value})
@code{sem_init} initializes the semaphore object pointed to by
@var{sem}. The count associated with the semaphore is set initially to
@var{value}. The @var{pshared} argument indicates whether the semaphore
is local to the current process (@var{pshared} is zero) or is to be
shared between several processes (@var{pshared} is not zero).

On success @code{sem_init} returns 0.  On failure it returns -1 and sets
@var{errno} to one of the following values:

@table @code
@item EINVAL
@var{value} exceeds the maximal counter value @code{SEM_VALUE_MAX}

@item ENOSYS
@var{pshared} is not zero.  LinuxThreads currently does not support
process-shared semaphores.  (This will eventually change.)
@end table
@end deftypefun

@comment semaphore.h
@comment POSIX
@deftypefun int sem_destroy (sem_t * @var{sem})
@code{sem_destroy} destroys a semaphore object, freeing the resources it
might hold.  If any threads are waiting on the semaphore when
@code{sem_destroy} is called, it fails and sets @var{errno} to
@code{EBUSY}.

In the LinuxThreads implementation, no resources are associated with
semaphore objects, thus @code{sem_destroy} actually does nothing except
checking that no thread is waiting on the semaphore.  This will change
when process-shared semaphores are implemented.
@end deftypefun

@comment semaphore.h
@comment POSIX
@deftypefun int sem_wait (sem_t * @var{sem})
@code{sem_wait} suspends the calling thread until the semaphore pointed
to by @var{sem} has non-zero count. It then atomically decreases the
semaphore count.

@code{sem_wait} is a cancellation point.  It always returns 0.
@end deftypefun

@comment semaphore.h
@comment POSIX
@deftypefun int sem_trywait (sem_t * @var{sem})
@code{sem_trywait} is a non-blocking variant of @code{sem_wait}. If the
semaphore pointed to by @var{sem} has non-zero count, the count is
atomically decreased and @code{sem_trywait} immediately returns 0.  If
the semaphore count is zero, @code{sem_trywait} immediately returns -1
and sets errno to @code{EAGAIN}.
@end deftypefun

@comment semaphore.h
@comment POSIX
@deftypefun int sem_post (sem_t * @var{sem})
@code{sem_post} atomically increases the count of the semaphore pointed to
by @var{sem}. This function never blocks.

@c !!! This para appears not to agree with the code.
On processors supporting atomic compare-and-swap (Intel 486, Pentium and
later, Alpha, PowerPC, MIPS II, Motorola 68k, Ultrasparc), the
@code{sem_post} function is can safely be called from signal handlers.
This is the only thread synchronization function provided by POSIX
threads that is async-signal safe.  On the Intel 386 and earlier Sparc
chips, the current LinuxThreads implementation of @code{sem_post} is not
async-signal safe, because the hardware does not support the required
atomic operations.

@code{sem_post} always succeeds and returns 0, unless the semaphore
count would exceed @code{SEM_VALUE_MAX} after being incremented.  In
that case @code{sem_post} returns -1 and sets @var{errno} to
@code{EINVAL}.  The semaphore count is left unchanged.
@end deftypefun

@comment semaphore.h
@comment POSIX
@deftypefun int sem_getvalue (sem_t * @var{sem}, int * @var{sval})
@code{sem_getvalue} stores in the location pointed to by @var{sval} the
current count of the semaphore @var{sem}.  It always returns 0.
@end deftypefun

@node Thread-Specific Data
@section Thread-Specific Data

Programs often need global or static variables that have different
values in different threads. Since threads share one memory space, this
cannot be achieved with regular variables. Thread-specific data is the
POSIX threads answer to this need.

Each thread possesses a private memory block, the thread-specific data
area, or TSD area for short. This area is indexed by TSD keys. The TSD
area associates values of type @code{void *} to TSD keys. TSD keys are
common to all threads, but the value associated with a given TSD key can
be different in each thread.

For concreteness, the TSD areas can be viewed as arrays of @code{void *}
pointers, TSD keys as integer indices into these arrays, and the value
of a TSD key as the value of the corresponding array element in the
calling thread.

When a thread is created, its TSD area initially associates @code{NULL}
with all keys.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_key_create (pthread_key_t *@var{key}, void (*destr_function) (void *))
@code{pthread_key_create} allocates a new TSD key. The key is stored in
the location pointed to by @var{key}. There is a limit of
@code{PTHREAD_KEYS_MAX} on the number of keys allocated at a given
time. The value initially associated with the returned key is
@code{NULL} in all currently executing threads.

The @var{destr_function} argument, if not @code{NULL}, specifies a
destructor function associated with the key. When a thread terminates
via @code{pthread_exit} or by cancellation, @var{destr_function} is
called on the value associated with the key in that thread. The
@var{destr_function} is not called if a key is deleted with
@code{pthread_key_delete} or a value is changed with
@code{pthread_setspecific}.  The order in which destructor functions are
called at thread termination time is unspecified.

Before the destructor function is called, the @code{NULL} value is
associated with the key in the current thread.  A destructor function
might, however, re-associate non-@code{NULL} values to that key or some
other key.  To deal with this, if after all the destructors have been
called for all non-@code{NULL} values, there are still some
non-@code{NULL} values with associated destructors, then the process is
repeated.  The LinuxThreads implementation stops the process after
@code{PTHREAD_DESTRUCTOR_ITERATIONS} iterations, even if some
non-@code{NULL} values with associated descriptors remain.  Other
implementations may loop indefinitely.

@code{pthread_key_create} returns 0 unless @code{PTHREAD_KEYS_MAX} keys
have already been allocated, in which case it fails and returns
@code{EAGAIN}.
@end deftypefun


@comment pthread.h
@comment POSIX
@deftypefun int pthread_key_delete (pthread_key_t @var{key})
@code{pthread_key_delete} deallocates a TSD key. It does not check
whether non-@code{NULL} values are associated with that key in the
currently executing threads, nor call the destructor function associated
with the key.

If there is no such key @var{key}, it returns @code{EINVAL}.  Otherwise
it returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_setspecific (pthread_key_t @var{key}, const void *@var{pointer})
@code{pthread_setspecific} changes the value associated with @var{key}
in the calling thread, storing the given @var{pointer} instead.

If there is no such key @var{key}, it returns @code{EINVAL}.  Otherwise
it returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun {void *} pthread_getspecific (pthread_key_t @var{key})
@code{pthread_getspecific} returns the value currently associated with
@var{key} in the calling thread.

If there is no such key @var{key}, it returns @code{NULL}.
@end deftypefun

The following code fragment allocates a thread-specific array of 100
characters, with automatic reclaimation at thread exit:

@smallexample
/* Key for the thread-specific buffer */
static pthread_key_t buffer_key;

/* Once-only initialisation of the key */
static pthread_once_t buffer_key_once = PTHREAD_ONCE_INIT;

/* Allocate the thread-specific buffer */
void buffer_alloc(void)
@{
  pthread_once(&buffer_key_once, buffer_key_alloc);
  pthread_setspecific(buffer_key, malloc(100));
@}

/* Return the thread-specific buffer */
char * get_buffer(void)
@{
  return (char *) pthread_getspecific(buffer_key);
@}

/* Allocate the key */
static void buffer_key_alloc()
@{
  pthread_key_create(&buffer_key, buffer_destroy);
@}

/* Free the thread-specific buffer */
static void buffer_destroy(void * buf)
@{
  free(buf);
@}
@end smallexample

@node Threads and Signal Handling
@section Threads and Signal Handling

@comment pthread.h
@comment POSIX
@deftypefun int pthread_sigmask (int @var{how}, const sigset_t *@var{newmask}, sigset_t *@var{oldmask})
@code{pthread_sigmask} changes the signal mask for the calling thread as
described by the @var{how} and @var{newmask} arguments. If @var{oldmask}
is not @code{NULL}, the previous signal mask is stored in the location
pointed to by @var{oldmask}.

The meaning of the @var{how} and @var{newmask} arguments is the same as
for @code{sigprocmask}. If @var{how} is @code{SIG_SETMASK}, the signal
mask is set to @var{newmask}. If @var{how} is @code{SIG_BLOCK}, the
signals specified to @var{newmask} are added to the current signal mask.
If @var{how} is @code{SIG_UNBLOCK}, the signals specified to
@var{newmask} are removed from the current signal mask.

Recall that signal masks are set on a per-thread basis, but signal
actions and signal handlers, as set with @code{sigaction}, are shared
between all threads.

The @code{pthread_sigmask} function returns 0 on success, and one of the
following error codes on error:
@table @code
@item EINVAL
@var{how} is not one of @code{SIG_SETMASK}, @code{SIG_BLOCK}, or @code{SIG_UNBLOCK}

@item EFAULT
@var{newmask} or @var{oldmask} point to invalid addresses
@end table
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_kill (pthread_t @var{thread}, int @var{signo})
@code{pthread_kill} sends signal number @var{signo} to the thread
@var{thread}.  The signal is delivered and handled as described in
@ref{Signal Handling}.

@code{pthread_kill} returns 0 on success, one of the following error codes
on error:
@table @code
@item EINVAL
@var{signo} is not a valid signal number

@item ESRCH
The thread @var{thread} does not exist (e.g. it has already terminated)
@end table
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int sigwait (const sigset_t *@var{set}, int *@var{sig})
@code{sigwait} suspends the calling thread until one of the signals in
@var{set} is delivered to the calling thread. It then stores the number
of the signal received in the location pointed to by @var{sig} and
returns. The signals in @var{set} must be blocked and not ignored on
entrance to @code{sigwait}. If the delivered signal has a signal handler
function attached, that function is @emph{not} called.

@code{sigwait} is a cancellation point.  It always returns 0.
@end deftypefun

For @code{sigwait} to work reliably, the signals being waited for must be
blocked in all threads, not only in the calling thread, since
otherwise the POSIX semantics for signal delivery do not guarantee
that it's the thread doing the @code{sigwait} that will receive the signal.
The best way to achieve this is block those signals before any threads
are created, and never unblock them in the program other than by
calling @code{sigwait}.

Signal handling in LinuxThreads departs significantly from the POSIX
standard. According to the standard, ``asynchronous'' (external) signals
are addressed to the whole process (the collection of all threads),
which then delivers them to one particular thread. The thread that
actually receives the signal is any thread that does not currently block
the signal.

In LinuxThreads, each thread is actually a kernel process with its own
PID, so external signals are always directed to one particular thread.
If, for instance, another thread is blocked in @code{sigwait} on that
signal, it will not be restarted.

The LinuxThreads implementation of @code{sigwait} installs dummy signal
handlers for the signals in @var{set} for the duration of the
wait. Since signal handlers are shared between all threads, other
threads must not attach their own signal handlers to these signals, or
alternatively they should all block these signals (which is recommended
anyway).

@node Threads and Fork
@section Threads and Fork

It's not intuitively obvious what should happen when a multi-threaded POSIX
process calls @code{fork}. Not only are the semantics tricky, but you may
need to write code that does the right thing at fork time even if that code
doesn't use the @code{fork} function. Moreover, you need to be aware of
interaction between @code{fork} and some library features like
@code{pthread_once} and stdio streams.

When @code{fork} is called by one of the threads of a process, it creates a new
process which is copy of the  calling process. Effectively, in addition to
copying certain system objects, the function takes a snapshot of the memory
areas of the parent process, and creates identical areas in the child. 
To make matters more complicated, with threads it's possible for two or more
threads to concurrently call fork to create two or more child processes.

The child process has a copy of the address space of the parent, but it does
not inherit any of its threads. Execution of the child process is carried out
by a new thread which returns from @code{fork} function with a return value of
zero; it is the only thread in the child process.  Because threads are not
inherited across fork, issues arise. At the time of the call to @code{fork},
threads in the parent process other than the one calling @code{fork} may have
been executing critical regions of code.  As a result, the child process may
get a copy of objects that are not in a well-defined state.  This potential
problem affects all components of the program.

Any program component which will continue being used in a child process must
correctly handle its state during @code{fork}. For this purpose, the POSIX
interface provides the special function @code{pthread_atfork} for installing
pointers to handler functions which are called from within @code{fork}.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_atfork (void (*@var{prepare})(void), void (*@var{parent})(void), void (*@var{child})(void))

@code{pthread_atfork} registers handler functions to be called just
before and just after a new process is created with @code{fork}. The
@var{prepare} handler will be called from the parent process, just
before the new process is created. The @var{parent} handler will be
called from the parent process, just before @code{fork} returns. The
@var{child} handler will be called from the child process, just before
@code{fork} returns.

@code{pthread_atfork} returns 0 on success and a non-zero error code on
error.

One or more of the three handlers @var{prepare}, @var{parent} and
@var{child} can be given as @code{NULL}, meaning that no handler needs
to be called at the corresponding point.

@code{pthread_atfork} can be called several times to install several
sets of handlers. At @code{fork} time, the @var{prepare} handlers are
called in LIFO order (last added with @code{pthread_atfork}, first
called before @code{fork}), while the @var{parent} and @var{child}
handlers are called in FIFO order (first added, first called).

If there is insufficient memory available to register the handlers,
@code{pthread_atfork} fails and returns @code{ENOMEM}.  Otherwise it
returns 0.

The functions @code{fork} and @code{pthread_atfork} must not be regarded as
reentrant from the context of the handlers.  That is to say, if a
@code{pthread_atfork} handler invoked from within @code{fork} calls
@code{pthread_atfork} or @code{fork}, the behavior is undefined.

Registering a triplet of handlers is an atomic operation with respect to fork.
If new handlers are registered at about the same time as a fork occurs, either
all three handlers will be called, or none of them will be called.

The handlers are inherited by the child process, and there is no 
way to remove them, short of using @code{exec} to load a new
pocess image.

@end deftypefun

To understand the purpose of @code{pthread_atfork}, recall that
@code{fork} duplicates the whole memory space, including mutexes in
their current locking state, but only the calling thread: other threads
are not running in the child process. Thus, if a mutex is locked by a
thread other than the thread calling @code{fork}, that mutex will remain
locked forever in the child process, possibly blocking the execution of
the child process. Or if some shared data, such as a linked list, was in the
middle of being updated by a thread in the parent process, the child 
will get a copy of the incompletely updated data which it cannot use.

To avoid this, install handlers with @code{pthread_atfork} as follows: have the
@var{prepare} handler lock the mutexes (in locking order), and the
@var{parent} handler unlock the mutexes. The @var{child} handler should reset
the mutexes using @code{pthread_mutex_init}, as well as any other
synchronization objects such as condition variables.

Locking the global mutexes before the fork ensures that all other threads are
locked out of the critical regions of code protected by those mutexes.  Thus
when @code{fork} takes a snapshot of the parent's address space, that snapshot
will copy valid, stable data.  Resetting the synchronization objects in the
child process will ensure they are properly cleansed of any artifacts from the
threading subsystem of the parent process. For example, a mutex may inherit
a wait queue of threads waiting for the lock; this wait queue makes no sense
in the child process. Initializing the mutex takes care of this.

@node Streams and Fork
@section Streams and Fork

The GNU standard I/O library has an internal mutex which guards the internal
linked list of all standard C FILE objects. This mutex is properly taken care
of during @code{fork} so that the child receives an intact copy of the list.
This allows the @code{fopen} function, and related stream-creating functions,
to work correctly in the child process, since these functions need to insert
into the list.

However, the individual stream locks are not completely taken care of.  Thus
unless the multithreaded application takes special precautions in its use of
@code{fork}, the child process might not be able to safely use the streams that
it inherited from the parent.   In general, for any given open stream in the
parent that is to be used by the child process, the application must ensure
that that stream is not in use by another thread when @code{fork} is called.
Otherwise an inconsistent copy of the stream object be produced. An easy way to
ensure this is to use @code{flockfile} to lock the stream prior to calling
@code{fork} and then unlock it with @code{funlockfile} inside the parent
process, provided that the parent's threads properly honor these locks.
Nothing special needs to be done in the child process, since the library
internally resets all stream locks. 

Note that the stream locks are not shared between the parent and child.
For example, even if you ensure that, say, the stream @code{stdout} is properly
treated and can be safely used in the child, the stream locks do not provide
an exclusion mechanism between the parent and child. If both processes write
to @code{stdout}, strangely interleaved output may result regardless of
the explicit use of @code{flockfile} or implicit locks.

Also note that these provisions are a GNU extension; other systems might not
provide any way for streams to be used in the child of a multithreaded process.
POSIX requires that such a child process confines itself to calling only
asynchronous safe functions, which excludes much of the library, including
standard I/O.

@node Miscellaneous Thread Functions
@section Miscellaneous Thread Functions

@comment pthread.h
@comment POSIX
@deftypefun {pthread_t} pthread_self (@var{void})
@code{pthread_self} returns the thread identifier for the calling thread.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_equal (pthread_t thread1, pthread_t thread2)
@code{pthread_equal} determines if two thread identifiers refer to the same
thread.

A non-zero value is returned if @var{thread1} and @var{thread2} refer to
the same thread. Otherwise, 0 is returned.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_detach (pthread_t @var{th})
@code{pthread_detach} puts the thread @var{th} in the detached
state. This guarantees that the memory resources consumed by @var{th}
will be freed immediately when @var{th} terminates. However, this
prevents other threads from synchronizing on the termination of @var{th}
using @code{pthread_join}.

A thread can be created initially in the detached state, using the
@code{detachstate} attribute to @code{pthread_create}. In contrast,
@code{pthread_detach} applies to threads created in the joinable state,
and which need to be put in the detached state later.

After @code{pthread_detach} completes, subsequent attempts to perform
@code{pthread_join} on @var{th} will fail. If another thread is already
joining the thread @var{th} at the time @code{pthread_detach} is called,
@code{pthread_detach} does nothing and leaves @var{th} in the joinable
state.

On success, 0 is returned. On error, one of the following codes is
returned:
@table @code
@item ESRCH
No thread could be found corresponding to that specified by @var{th}
@item EINVAL
The thread @var{th} is already in the detached state
@end table
@end deftypefun

@comment pthread.h
@comment GNU
@deftypefun void pthread_kill_other_threads_np (@var{void})
@code{pthread_kill_other_threads_np} is a non-portable LinuxThreads extension.
It causes all threads in the program to terminate immediately, except
the calling thread which proceeds normally. It is intended to be
called just before a thread calls one of the @code{exec} functions,
e.g. @code{execve}.

Termination of the other threads is not performed through
@code{pthread_cancel} and completely bypasses the cancellation
mechanism. Hence, the current settings for cancellation state and
cancellation type are ignored, and the cleanup handlers are not
executed in the terminated threads.

According to POSIX 1003.1c, a successful @code{exec*} in one of the
threads should automatically terminate all other threads in the program.
This behavior is not yet implemented in LinuxThreads.  Calling
@code{pthread_kill_other_threads_np} before @code{exec*} achieves much
of the same behavior, except that if @code{exec*} ultimately fails, then
all other threads are already killed.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_once (pthread_once_t *once_@var{control}, void (*@var{init_routine}) (void))

The purpose of @code{pthread_once} is to ensure that a piece of
initialization code is executed at most once. The @var{once_control}
argument points to a static or extern variable statically initialized
to @code{PTHREAD_ONCE_INIT}.

The first time @code{pthread_once} is called with a given
@var{once_control} argument, it calls @var{init_routine} with no
argument and changes the value of the @var{once_control} variable to
record that initialization has been performed. Subsequent calls to
@code{pthread_once} with the same @code{once_control} argument do
nothing.

If a thread is cancelled while executing @var{init_routine}
the state of the @var{once_control} variable is reset so that
a future call to @code{pthread_once} will call the routine again.

If the process forks while one or more threads are executing
@code{pthread_once} initialization routines, the states of their respective
@var{once_control} variables will appear to be reset in the child process so
that if the child calls @code{pthread_once}, the routines will be executed.

@code{pthread_once} always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_setschedparam (pthread_t target_@var{thread}, int @var{policy}, const struct sched_param *@var{param})

@code{pthread_setschedparam} sets the scheduling parameters for the
thread @var{target_thread} as indicated by @var{policy} and
@var{param}. @var{policy} can be either @code{SCHED_OTHER} (regular,
non-realtime scheduling), @code{SCHED_RR} (realtime, round-robin) or
@code{SCHED_FIFO} (realtime, first-in first-out). @var{param} specifies
the scheduling priority for the two realtime policies.  See
@code{sched_setpolicy} for more information on scheduling policies.

The realtime scheduling policies @code{SCHED_RR} and @code{SCHED_FIFO}
are available only to processes with superuser privileges.

On success, @code{pthread_setschedparam} returns 0.  On error it returns
one of the following codes:
@table @code
@item EINVAL
@var{policy} is not one of @code{SCHED_OTHER}, @code{SCHED_RR},
@code{SCHED_FIFO}, or the priority value specified by @var{param} is not
valid for the specified policy

@item EPERM
Realtime scheduling was requested but the calling process does not have
sufficient privileges.

@item ESRCH
The @var{target_thread} is invalid or has already terminated

@item EFAULT
@var{param} points outside the process memory space
@end table
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_getschedparam (pthread_t target_@var{thread}, int *@var{policy}, struct sched_param *@var{param})

@code{pthread_getschedparam} retrieves the scheduling policy and
scheduling parameters for the thread @var{target_thread} and stores them
in the locations pointed to by @var{policy} and @var{param},
respectively.

@code{pthread_getschedparam} returns 0 on success, or one of the
following error codes on failure:
@table @code
@item ESRCH
The @var{target_thread} is invalid or has already terminated.

@item EFAULT
@var{policy} or @var{param} point outside the process memory space.

@end table
@end deftypefun