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-@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
-                                  canceled.
-* 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 canceled.
-
-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 canceled.
-
-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_setdetachstate
-@findex pthread_attr_setguardsize
-@findex pthread_attr_setinheritsched
-@findex pthread_attr_setschedparam
-@findex pthread_attr_setschedpolicy
-@findex pthread_attr_setscope
-@findex pthread_attr_setstack
-@findex pthread_attr_setstackaddr
-@findex pthread_attr_setstacksize
-@comment pthread.h
-@comment POSIX
-@deftypefun int pthread_attr_setattr (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_getdetachstate
-@findex pthread_attr_getguardsize
-@findex pthread_attr_getinheritsched
-@findex pthread_attr_getschedparam
-@findex pthread_attr_getschedpolicy
-@findex pthread_attr_getscope
-@findex pthread_attr_getstack
-@findex pthread_attr_getstackaddr
-@findex pthread_attr_getstacksize
-@comment pthread.h
-@comment POSIX
-@deftypefun int pthread_attr_getattr (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}.
-
-@item stackaddr
-Provide an address for an application managed stack.  The size of the
-stack must be at least @code{PTHREAD_STACK_MIN}.
-
-@item stacksize
-Change the size of the stack created for the thread.  The value defines
-the minimum stack size, in bytes.
-
-If the value exceeds the system's maximum stack size, or is smaller
-than @code{PTHREAD_STACK_MIN}, @code{pthread_attr_setstacksize} will
-fail and return @code{EINVAL}.
-
-@item stack
-Provide both the address and size of an application managed stack to
-use for the new thread.  The base of the memory area is @var{stackaddr}
-with the size of the memory area, @var{stacksize}, measured in bytes.
-
-If the value of @var{stacksize} is less than @code{PTHREAD_STACK_MIN},
-or greater than the system's maximum stack size, or if the value of
-@var{stackaddr} lacks the proper alignment, @code{pthread_attr_setstack}
-will fail and return @code{EINVAL}.
-
-@item guardsize
-Change the minimum size in bytes of the guard area for the thread's
-stack.  The default size is a single page.  If this value is set, it
-will be rounded up to the nearest page size.  If the value is set to 0,
-a guard area will not be created for this thread.  The space allocated
-for the guard area is used to catch stack overflow.  Therefore, when
-allocating large structures on the stack, a larger guard area may be
-required to catch a stack overflow.
-
-If the caller is managing their own stacks (if the @code{stackaddr}
-attribute has been set), then the @code{guardsize} attribute is ignored.
-
-If the value exceeds the @code{stacksize}, @code{pthread_atrr_setguardsize}
-will fail and return @code{EINVAL}.
-@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 canceled 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 canceled 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_restore_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 type}, which is either ``fast'', ``recursive'', or
-``error checking''. The type of a mutex determines whether
-it can be locked again by a thread that already owns it.
-The default type is ``fast''.
-
-Variables of type @code{pthread_mutex_t} can also be initialized
-statically, using the constants @code{PTHREAD_MUTEX_INITIALIZER} (for
-timed mutexes), @code{PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP} (for
-recursive mutexes), @code{PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP}
-(for fast 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 type of the mutex. If the mutex
-is of the ``fast'' type, 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'' type, @code{pthread_mutex_lock}
-returns immediately with the error code @code{EDEADLK}.  If the mutex is
-of the ``recursive'' type, @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.
-@c This doesn't discuss PTHREAD_MUTEX_TIMED_NP mutex attributes. FIXME
-@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_timedlock (pthread_mutex_t *@var{mutex}, const struct timespec *@var{abstime})
-The @code{pthread_mutex_timedlock} is similar to the
-@code{pthread_mutex_lock} function but instead of blocking for in
-indefinite time if the mutex is locked by another thread, it returns
-when the time specified in @var{abstime} is reached.
-
-This function can only be used on standard (``timed'') and ``error
-checking'' mutexes.  It behaves just like @code{pthread_mutex_lock} for
-all other types.
-
-If the mutex is successfully locked, the function returns zero.  If the
-time specified in @var{abstime} is reached without the mutex being locked,
-@code{ETIMEDOUT} is returned.
-
-This function was introduced in the POSIX.1d revision of the POSIX standard.
-@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'' type,
-@code{pthread_mutex_unlock} always returns it to the unlocked state. If
-it is of the ``recursive'' type, 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 type, which is
-either @code{PTHREAD_MUTEX_ADAPTIVE_NP} for ``fast'' mutexes,
-@code{PTHREAD_MUTEX_RECURSIVE_NP} for ``recursive'' mutexes,
-@code{PTHREAD_MUTEX_TIMED_NP} for ``timed'' 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 type 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'' type, @code{pthread_mutex_lock} simply suspends the calling
-thread forever.  If the mutex is of the ``error checking'' type,
-@code{pthread_mutex_lock} returns immediately with the error code
-@code{EDEADLK}.  If the mutex is of the ``recursive'' type, 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 type is ``timed'', that is, @code{PTHREAD_MUTEX_TIMED_NP}.
-@c This doesn't describe how a ``timed'' mutex behaves. FIXME
-
-@comment pthread.h
-@comment POSIX
-@deftypefun int pthread_mutexattr_settype (pthread_mutexattr_t *@var{attr}, int @var{type})
-@code{pthread_mutexattr_settype} sets the mutex type attribute in
-@var{attr} to the value specified by @var{type}.
-
-If @var{type} is not @code{PTHREAD_MUTEX_ADAPTIVE_NP},
-@code{PTHREAD_MUTEX_RECURSIVE_NP}, @code{PTHREAD_MUTEX_TIMED_NP}, or
-@code{PTHREAD_MUTEX_ERRORCHECK_NP}, this function will return
-@code{EINVAL} and leave @var{attr} unchanged.
-
-The standard Unix98 identifiers @code{PTHREAD_MUTEX_DEFAULT},
-@code{PTHREAD_MUTEX_NORMAL}, @code{PTHREAD_MUTEX_RECURSIVE},
-and @code{PTHREAD_MUTEX_ERRORCHECK} are also permitted.
-
-@end deftypefun
-
-@comment pthread.h
-@comment POSIX
-@deftypefun int pthread_mutexattr_gettype (const pthread_mutexattr_t *@var{attr}, int *@var{type})
-@code{pthread_mutexattr_gettype} retrieves the current value of the
-mutex type attribute in @var{attr} and stores it in the location pointed
-to by @var{type}.
-
-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 canceled 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.  The mutexes are not usable after
-the @code{fork} and must be initialized with @code{pthread_mutex_init}
-in the child process.  This is a limitation of the current
-implementation and might or might not be present in future versions.
-
-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
-
-@comment pthread.h
-@comment POSIX
-@deftypefun int pthread_setconcurrency (int @var{level})
-@code{pthread_setconcurrency} is unused in LinuxThreads due to the lack
-of a mapping of user threads to kernel threads.  It exists for source
-compatibility.  It does store the value @var{level} so that it can be
-returned by a subsequent call to @code{pthread_getconcurrency}.  It takes
-no other action however.
-@end deftypefun
-
-@comment pthread.h
-@comment POSIX
-@deftypefun int pthread_getconcurrency ()
-@code{pthread_getconcurrency} is unused in LinuxThreads due to the lack
-of a mapping of user threads to kernel threads.  It exists for source
-compatibility.  However, it will return the value that was set by the
-last call to @code{pthread_setconcurrency}.
-@end deftypefun