This is libc.info, produced by makeinfo version 4.2 from libc.texinfo.

INFO-DIR-SECTION GNU libraries
START-INFO-DIR-ENTRY
* Libc: (libc).                 C library.
END-INFO-DIR-ENTRY

   This file documents the GNU C library.

   This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library
Reference Manual', for Version 2.3.x.

   Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002
Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software Needs Free Documentation" and
"GNU Lesser General Public License", the Front-Cover texts being (a)
(see below), and with the Back-Cover Texts being (b) (see below).  A
copy of the license is included in the section entitled "GNU Free
Documentation License".

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.


File: libc.info,  Node: Basic Thread Operations,  Next: Thread Attributes,  Up: POSIX Threads

Basic Thread Operations
=======================

   These functions are the thread equivalents of `fork', `exit', and
`wait'.

 - Function: int pthread_create (pthread_t * THREAD, pthread_attr_t *
          ATTR, void * (*START_ROUTINE)(void *), void * ARG)
     `pthread_create' creates a new thread of control that executes
     concurrently with the calling thread. The new thread calls the
     function START_ROUTINE, passing it ARG as first argument. The new
     thread terminates either explicitly, by calling `pthread_exit', or
     implicitly, by returning from the START_ROUTINE function. The
     latter case is equivalent to calling `pthread_exit' with the result
     returned by START_ROUTINE as exit code.

     The ATTR argument specifies thread attributes to be applied to the
     new thread. *Note Thread Attributes::, for details. The ATTR
     argument can also be `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 THREAD argument, and a 0 is
     returned. On error, a non-zero error code is returned.

     This function may return the following errors:
    `EAGAIN'
          Not enough system resources to create a process for the new
          thread, or more than `PTHREAD_THREADS_MAX' threads are
          already active.

 - Function: void pthread_exit (void *RETVAL)
     `pthread_exit' terminates the execution of the calling thread.  All
     cleanup handlers (*note Cleanup Handlers::) that have been set for
     the calling thread with `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-`NULL' values associated with
     them in the calling thread (*note Thread-Specific Data::).
     Finally, execution of the calling thread is stopped.

     The RETVAL argument is the return value of the thread. It can be
     retrieved from another thread using `pthread_join'.

     The `pthread_exit' function never returns.

 - Function: int pthread_cancel (pthread_t THREAD)
     `pthread_cancel' sends a cancellation request to the thread denoted
     by the THREAD argument.  If there is no such thread,
     `pthread_cancel' fails and returns `ESRCH'.  Otherwise it returns
     0. *Note Cancellation::, for details.

 - Function: int pthread_join (pthread_t TH, void **thread_RETURN)
     `pthread_join' suspends the execution of the calling thread until
     the thread identified by TH terminates, either by calling
     `pthread_exit' or by being canceled.

     If THREAD_RETURN is not `NULL', the return value of TH is stored
     in the location pointed to by THREAD_RETURN.  The return value of
     TH is either the argument it gave to `pthread_exit', or
     `PTHREAD_CANCELED' if TH was canceled.

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

     When a joinable thread terminates, its memory resources (thread
     descriptor and stack) are not deallocated until another thread
     performs `pthread_join' on it. Therefore, `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 `pthread_join' on a thread TH on which another thread is
     already waiting for termination returns an error.

     `pthread_join' is a cancellation point. If a thread is canceled
     while suspended in `pthread_join', the thread execution resumes
     immediately and the cancellation is executed without waiting for
     the TH thread to terminate. If cancellation occurs during
     `pthread_join', the TH thread remains not joined.

     On success, the return value of TH is stored in the location
     pointed to by THREAD_RETURN, and 0 is returned. On error, one of
     the following values is returned:
    `ESRCH'
          No thread could be found corresponding to that specified by
          TH.

    `EINVAL'
          The TH thread has been detached, or another thread is already
          waiting on termination of TH.

    `EDEADLK'
          The TH argument refers to the calling thread.


File: libc.info,  Node: Thread Attributes,  Next: Cancellation,  Prev: Basic Thread Operations,  Up: POSIX Threads

Thread Attributes
=================

   Threads have a number of attributes that may be set at creation time.
This is done by filling a thread attribute object ATTR of type
`pthread_attr_t', then passing it as second argument to
`pthread_create'. Passing `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 `pthread_create' does not
change the attributes of the thread previously created.

 - Function: int pthread_attr_init (pthread_attr_t *ATTR)
     `pthread_attr_init' initializes the thread attribute object ATTR
     and fills it with default values for the attributes. (The default
     values are listed below for each attribute.)

     Each attribute ATTRNAME (see below for a list of all attributes)
     can be individually set using the function
     `pthread_attr_setATTRNAME' and retrieved using the function
     `pthread_attr_getATTRNAME'.

 - Function: int pthread_attr_destroy (pthread_attr_t *ATTR)
     `pthread_attr_destroy' destroys the attribute object pointed to by
     ATTR releasing any resources associated with it.  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.

 - Function: int pthread_attr_setattr (pthread_attr_t *OBJ, int VALUE)
     Set attribute ATTR to VALUE in the attribute object pointed to by
     OBJ.  See below for a list of possible attributes and the values
     they can take.

     On success, these functions return 0.  If VALUE is not meaningful
     for the ATTR being modified, they will return the error code
     `EINVAL'.  Some of the functions have other failure modes; see
     below.

 - Function: int pthread_attr_getattr (const pthread_attr_t *OBJ, int
          *VALUE)
     Store the current setting of ATTR in OBJ into the variable pointed
     to by VALUE.

     These functions always return 0.

   The following thread attributes are supported:
`detachstate'
     Choose whether the thread is created in the joinable state (value
     `PTHREAD_CREATE_JOINABLE') or in the detached state
     (`PTHREAD_CREATE_DETACHED').  The default is
     `PTHREAD_CREATE_JOINABLE'.

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

     In the detached state, the thread resources are immediately freed
     when it terminates, but `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 `pthread_detach'.

`schedpolicy'
     Select the scheduling policy for the thread: one of `SCHED_OTHER'
     (regular, non-realtime scheduling), `SCHED_RR' (realtime,
     round-robin) or `SCHED_FIFO' (realtime, first-in first-out).  The
     default is `SCHED_OTHER'.

     The realtime scheduling policies `SCHED_RR' and `SCHED_FIFO' are
     available only to processes with superuser privileges.
     `pthread_attr_setschedparam' will fail and return `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 `pthread_setschedparam'.

`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
     `SCHED_OTHER'; it only matters for the realtime policies
     `SCHED_RR' and `SCHED_FIFO'.

     The scheduling priority of a thread can be changed after creation
     with `pthread_setschedparam'.

`inheritsched'
     Choose whether the scheduling policy and scheduling parameter for
     the newly created thread are determined by the values of the
     SCHEDPOLICY and SCHEDPARAM attributes (value
     `PTHREAD_EXPLICIT_SCHED') or are inherited from the parent thread
     (value `PTHREAD_INHERIT_SCHED').  The default is
     `PTHREAD_EXPLICIT_SCHED'.

`scope'
     Choose the scheduling contention scope for the created thread.  The
     default is `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, `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.

     `PTHREAD_SCOPE_PROCESS' is not supported in LinuxThreads.  If you
     try to set the scope to this value, `pthread_attr_setscope' will
     fail and return `ENOTSUP'.

`stackaddr'
     Provide an address for an application managed stack.  The size of
     the stack must be at least `PTHREAD_STACK_MIN'.

`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 `PTHREAD_STACK_MIN', `pthread_attr_setstacksize' will fail
     and return `EINVAL'.

`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
     STACKADDR with the size of the memory area, STACKSIZE, measured in
     bytes.

     If the value of STACKSIZE is less than `PTHREAD_STACK_MIN', or
     greater than the system's maximum stack size, or if the value of
     STACKADDR lacks the proper alignment, `pthread_attr_setstack' will
     fail and return `EINVAL'.

`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 `stackaddr'
     attribute has been set), then the `guardsize' attribute is ignored.

     If the value exceeds the `stacksize', `pthread_atrr_setguardsize'
     will fail and return `EINVAL'.


File: libc.info,  Node: Cancellation,  Next: Cleanup Handlers,  Prev: Thread Attributes,  Up: POSIX Threads

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 `pthread_create', they always defer cancellation
requests.

   When a thread eventually honors a cancellation request, it behaves
as if `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
`PTHREAD_CANCELED' is provided to whichever thread calls PTHREAD_JOIN
on it. See `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 `pthread_join', `pthread_cond_wait',
`pthread_cond_timedwait', `pthread_testcancel', `sem_wait', and
`sigwait' are cancellation points.  In addition, these system calls are
cancellation points:

accept                   open                     sendmsg
close                    pause                    sendto
connect                  read                     system
fcntl                    recv                     tcdrain
fsync                    recvfrom                 wait
lseek                    recvmsg                  waitpid
msync                    send                     write
nanosleep                                         

All library functions that call these functions (such as `printf') are
also cancellation points.

 - Function: int pthread_setcancelstate (int STATE, int *OLDSTATE)
     `pthread_setcancelstate' changes the cancellation state for the
     calling thread - that is, whether cancellation requests are
     ignored or not. The STATE argument is the new cancellation state:
     either `PTHREAD_CANCEL_ENABLE' to enable cancellation, or
     `PTHREAD_CANCEL_DISABLE' to disable cancellation (cancellation
     requests are ignored).

     If OLDSTATE is not `NULL', the previous cancellation state is
     stored in the location pointed to by OLDSTATE, and can thus be
     restored later by another call to `pthread_setcancelstate'.

     If the STATE argument is not `PTHREAD_CANCEL_ENABLE' or
     `PTHREAD_CANCEL_DISABLE', `pthread_setcancelstate' fails and
     returns `EINVAL'.  Otherwise it returns 0.

 - Function: int pthread_setcanceltype (int TYPE, int *OLDTYPE)
     `pthread_setcanceltype' changes the type of responses to
     cancellation requests for the calling thread: asynchronous
     (immediate) or deferred.  The TYPE argument is the new
     cancellation type: either `PTHREAD_CANCEL_ASYNCHRONOUS' to cancel
     the calling thread as soon as the cancellation request is
     received, or `PTHREAD_CANCEL_DEFERRED' to keep the cancellation
     request pending until the next cancellation point. If OLDTYPE is
     not `NULL', the previous cancellation state is stored in the
     location pointed to by OLDTYPE, and can thus be restored later by
     another call to `pthread_setcanceltype'.

     If the TYPE argument is not `PTHREAD_CANCEL_DEFERRED' or
     `PTHREAD_CANCEL_ASYNCHRONOUS', `pthread_setcanceltype' fails and
     returns `EINVAL'.  Otherwise it returns 0.

 - Function: void pthread_testcancel (VOID)
     `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.


File: libc.info,  Node: Cleanup Handlers,  Next: Mutexes,  Prev: Cancellation,  Up: POSIX Threads

Cleanup Handlers
================

   Cleanup handlers are functions that get called when a thread
terminates, either by calling `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
`malloc' or close file descriptors on thread termination.

   Here is how to lock a mutex MUT in such a way that it will be
unlocked if the thread is canceled while MUT is locked:

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

   Equivalently, the last two lines can be replaced by

     pthread_cleanup_pop(1);

   Notice that the code above is safe only in deferred cancellation mode
(see `pthread_setcanceltype'). In asynchronous cancellation mode, a
cancellation can occur between `pthread_cleanup_push' and
`pthread_mutex_lock', or between `pthread_mutex_unlock' and
`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:

     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);

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

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

 - Function: void pthread_cleanup_push (void (*ROUTINE) (void *), void
          *ARG)
     `pthread_cleanup_push' installs the ROUTINE function with argument
     ARG as a cleanup handler. From this point on to the matching
     `pthread_cleanup_pop', the function ROUTINE will be called with
     arguments ARG when the thread terminates, either through
     `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.

 - Function: void pthread_cleanup_pop (int EXECUTE)
     `pthread_cleanup_pop' removes the most recently installed cleanup
     handler. If the EXECUTE argument is not 0, it also executes the
     handler, by calling the ROUTINE function with arguments ARG. If
     the EXECUTE argument is 0, the handler is only removed but not
     executed.

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

 - Function: void pthread_cleanup_push_defer_np (void (*ROUTINE) (void
          *), void *ARG)
     `pthread_cleanup_push_defer_np' is a non-portable extension that
     combines `pthread_cleanup_push' and `pthread_setcanceltype'.  It
     pushes a cleanup handler just as `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.

 - Function: void pthread_cleanup_pop_restore_np (int EXECUTE)
     `pthread_cleanup_pop_restore_np' pops a cleanup handler introduced
     by `pthread_cleanup_push_defer_np', and restores the cancellation
     type to its value at the time `pthread_cleanup_push_defer_np' was
     called.

   `pthread_cleanup_push_defer_np' and `pthread_cleanup_pop_restore_np'
must occur in matching pairs, at the same level of block nesting.

   The sequence

     pthread_cleanup_push_defer_np(routine, arg);
     ...
     pthread_cleanup_pop_defer_np(execute);

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

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


File: libc.info,  Node: Mutexes,  Next: Condition Variables,  Prev: Cleanup Handlers,  Up: POSIX Threads

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
`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 `pthread_mutex_lock' or `pthread_mutex_unlock' from
a signal handler may deadlock the calling thread.

 - Function: int pthread_mutex_init (pthread_mutex_t *MUTEX, const
          pthread_mutexattr_t *MUTEXATTR)
     `pthread_mutex_init' initializes the mutex object pointed to by
     MUTEX according to the mutex attributes specified in MUTEXATTR.
     If MUTEXATTR is `NULL', default attributes are used instead.

     The LinuxThreads implementation supports only one mutex attribute,
     the 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 `pthread_mutex_t' can also be initialized
     statically, using the constants `PTHREAD_MUTEX_INITIALIZER' (for
     timed mutexes), `PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP' (for
     recursive mutexes), `PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP' (for
     fast mutexes(, and `PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP' (for
     error checking mutexes).

     `pthread_mutex_init' always returns 0.

 - Function: int pthread_mutex_lock (pthread_mutex_t *mutex))
     `pthread_mutex_lock' locks the given mutex. If the mutex is
     currently unlocked, it becomes locked and owned by the calling
     thread, and `pthread_mutex_lock' returns immediately. If the mutex
     is already locked by another thread, `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 `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,
     `pthread_mutex_lock' returns immediately with the error code
     `EDEADLK'.  If the mutex is of the "recursive" type,
     `pthread_mutex_lock' succeeds and returns immediately, recording
     the number of times the calling thread has locked the mutex. An
     equal number of `pthread_mutex_unlock' operations must be
     performed before the mutex returns to the unlocked state.

 - Function: int pthread_mutex_trylock (pthread_mutex_t *MUTEX)
     `pthread_mutex_trylock' behaves identically to
     `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,
     `pthread_mutex_trylock' returns immediately with the error code
     `EBUSY'.

 - Function: int pthread_mutex_timedlock (pthread_mutex_t *MUTEX, const
          struct timespec *ABSTIME)
     The `pthread_mutex_timedlock' is similar to the
     `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 ABSTIME is reached.

     This function can only be used on standard ("timed") and "error
     checking" mutexes.  It behaves just like `pthread_mutex_lock' for
     all other types.

     If the mutex is successfully locked, the function returns zero.
     If the time specified in ABSTIME is reached without the mutex
     being locked, `ETIMEDOUT' is returned.

     This function was introduced in the POSIX.1d revision of the POSIX
     standard.

 - Function: int pthread_mutex_unlock (pthread_mutex_t *MUTEX)
     `pthread_mutex_unlock' unlocks the given mutex. The mutex is
     assumed to be locked and owned by the calling thread on entrance to
     `pthread_mutex_unlock'. If the mutex is of the "fast" type,
     `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 `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, `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
     `pthread_mutex_unlock'.  If these conditions are not met,
     `pthread_mutex_unlock' returns `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.

 - Function: int pthread_mutex_destroy (pthread_mutex_t *MUTEX)
     `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 `pthread_mutex_destroy' actually does
     nothing except checking that the mutex is unlocked.

     If the mutex is locked by some thread, `pthread_mutex_destroy'
     returns `EBUSY'.  Otherwise it returns 0.

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

   A shared global variable X can be protected by a mutex as follows:

     int x;
     pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;

   All accesses and modifications to X should be bracketed by calls to
`pthread_mutex_lock' and `pthread_mutex_unlock' as follows:

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

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

 - Function: int pthread_mutexattr_init (pthread_mutexattr_t *ATTR)
     `pthread_mutexattr_init' initializes the mutex attribute object
     ATTR and fills it with default values for the attributes.

     This function always returns 0.

 - Function: int pthread_mutexattr_destroy (pthread_mutexattr_t *ATTR)
     `pthread_mutexattr_destroy' destroys a mutex attribute object,
     which must not be reused until it is reinitialized.
     `pthread_mutexattr_destroy' does nothing in the LinuxThreads
     implementation.

     This function always returns 0.

   LinuxThreads supports only one mutex attribute: the mutex type,
which is either `PTHREAD_MUTEX_ADAPTIVE_NP' for "fast" mutexes,
`PTHREAD_MUTEX_RECURSIVE_NP' for "recursive" mutexes,
`PTHREAD_MUTEX_TIMED_NP' for "timed" mutexes, or
`PTHREAD_MUTEX_ERRORCHECK_NP' for "error checking" mutexes.  As the
`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 `pthread_mutex_lock'. If the mutex is of the
"fast" type, `pthread_mutex_lock' simply suspends the calling thread
forever.  If the mutex is of the "error checking" type,
`pthread_mutex_lock' returns immediately with the error code `EDEADLK'.
If the mutex is of the "recursive" type, the call to
`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
`pthread_mutex_unlock' the same number of times before the mutex
returns to the unlocked state.

   The default mutex type is "timed", that is, `PTHREAD_MUTEX_TIMED_NP'.

 - Function: int pthread_mutexattr_settype (pthread_mutexattr_t *ATTR,
          int TYPE)
     `pthread_mutexattr_settype' sets the mutex type attribute in ATTR
     to the value specified by TYPE.

     If TYPE is not `PTHREAD_MUTEX_ADAPTIVE_NP',
     `PTHREAD_MUTEX_RECURSIVE_NP', `PTHREAD_MUTEX_TIMED_NP', or
     `PTHREAD_MUTEX_ERRORCHECK_NP', this function will return `EINVAL'
     and leave ATTR unchanged.

     The standard Unix98 identifiers `PTHREAD_MUTEX_DEFAULT',
     `PTHREAD_MUTEX_NORMAL', `PTHREAD_MUTEX_RECURSIVE', and
     `PTHREAD_MUTEX_ERRORCHECK' are also permitted.


 - Function: int pthread_mutexattr_gettype (const pthread_mutexattr_t
          *ATTR, int *TYPE)
     `pthread_mutexattr_gettype' retrieves the current value of the
     mutex type attribute in ATTR and stores it in the location pointed
     to by TYPE.

     This function always returns 0.


File: libc.info,  Node: Condition Variables,  Next: POSIX Semaphores,  Prev: Mutexes,  Up: POSIX Threads

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.

 - Function: int pthread_cond_init (pthread_cond_t *COND,
          pthread_condattr_t *cond_ATTR)
     `pthread_cond_init' initializes the condition variable COND, using
     the condition attributes specified in COND_ATTR, or default
     attributes if COND_ATTR is `NULL'. The LinuxThreads implementation
     supports no attributes for conditions, hence the COND_ATTR
     parameter is actually ignored.

     Variables of type `pthread_cond_t' can also be initialized
     statically, using the constant `PTHREAD_COND_INITIALIZER'.

     This function always returns 0.

 - Function: int pthread_cond_signal (pthread_cond_t *COND)
     `pthread_cond_signal' restarts one of the threads that are waiting
     on the condition variable COND. If no threads are waiting on COND,
     nothing happens. If several threads are waiting on COND, exactly
     one is restarted, but it is not specified which.

     This function always returns 0.

 - Function: int pthread_cond_broadcast (pthread_cond_t *COND)
     `pthread_cond_broadcast' restarts all the threads that are waiting
     on the condition variable COND. Nothing happens if no threads are
     waiting on COND.

     This function always returns 0.

 - Function: int pthread_cond_wait (pthread_cond_t *COND,
          pthread_mutex_t *MUTEX)
     `pthread_cond_wait' atomically unlocks the MUTEX (as per
     `pthread_unlock_mutex') and waits for the condition variable COND
     to be signaled. The thread execution is suspended and does not
     consume any CPU time until the condition variable is signaled. The
     MUTEX must be locked by the calling thread on entrance to
     `pthread_cond_wait'. Before returning to the calling thread,
     `pthread_cond_wait' re-acquires MUTEX (as per
     `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.

 - Function: int pthread_cond_timedwait (pthread_cond_t *COND,
          pthread_mutex_t *MUTEX, const struct timespec *ABSTIME)
     `pthread_cond_timedwait' atomically unlocks MUTEX and waits on
     COND, as `pthread_cond_wait' does, but it also bounds the duration
     of the wait. If COND has not been signaled before time ABSTIME,
     the mutex MUTEX is re-acquired and `pthread_cond_timedwait'
     returns the error code `ETIMEDOUT'.  The wait can also be
     interrupted by a signal; in that case `pthread_cond_timedwait'
     returns `EINTR'.

     The ABSTIME parameter specifies an absolute time, with the same
     origin as `time' and `gettimeofday': an ABSTIME of 0 corresponds
     to 00:00:00 GMT, January 1, 1970.

 - Function: int pthread_cond_destroy (pthread_cond_t *COND)
     `pthread_cond_destroy' destroys the condition variable COND,
     freeing the resources it might hold.  If any threads are waiting
     on the condition variable, `pthread_cond_destroy' leaves COND
     untouched and returns `EBUSY'.  Otherwise it returns 0, and COND
     must not be used again until it is reinitialized.

     In the LinuxThreads implementation, no resources are associated
     with condition variables, so `pthread_cond_destroy' actually does
     nothing.

   `pthread_cond_wait' and `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  MUTEX, and finally executes the cancellation.
Consequently, cleanup handlers are assured that MUTEX is locked when
they are called.

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

   Consider two shared variables X and Y, protected by the mutex MUT,
and a condition variable COND that is to be signaled whenever X becomes
greater than Y.

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

   Waiting until X is greater than Y is performed as follows:

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

   Modifications on X and Y that may cause X to become greater than Y
should signal the condition if needed:

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

   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 X
and Y), `pthread_cond_signal' can be used as a slightly more efficient
alternative to `pthread_cond_broadcast'. In doubt, use
`pthread_cond_broadcast'.

   To wait for X to becomes greater than Y with a timeout of 5 seconds,
do:

     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);

   Condition attributes can be specified at condition creation time, by
passing a condition attribute object as second argument to
`pthread_cond_init'.  Passing `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.

 - Function: int pthread_condattr_init (pthread_condattr_t *ATTR)
 - Function: int pthread_condattr_destroy (pthread_condattr_t *ATTR)
     `pthread_condattr_init' initializes the condition attribute object
     ATTR and fills it with default values for the attributes.
     `pthread_condattr_destroy' destroys the condition attribute object
     ATTR.

     Both functions do nothing in the LinuxThreads implementation.

     `pthread_condattr_init' and `pthread_condattr_destroy' always
     return 0.


File: libc.info,  Node: POSIX Semaphores,  Next: Thread-Specific Data,  Prev: Condition Variables,  Up: POSIX Threads

POSIX Semaphores
================

   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 `SEM_VALUE_MAX' is defined to be this maximum
value.  In the GNU C library, `SEM_VALUE_MAX' is equal to `INT_MAX'
(*note 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 (`ipc', `semctl' and
`semop').

   All the semaphore functions and macros are defined in `semaphore.h'.

 - Function: int sem_init (sem_t *SEM, int PSHARED, unsigned int VALUE)
     `sem_init' initializes the semaphore object pointed to by SEM. The
     count associated with the semaphore is set initially to VALUE. The
     PSHARED argument indicates whether the semaphore is local to the
     current process (PSHARED is zero) or is to be shared between
     several processes (PSHARED is not zero).

     On success `sem_init' returns 0.  On failure it returns -1 and sets
     ERRNO to one of the following values:

    `EINVAL'
          VALUE exceeds the maximal counter value `SEM_VALUE_MAX'

    `ENOSYS'
          PSHARED is not zero.  LinuxThreads currently does not support
          process-shared semaphores.  (This will eventually change.)

 - Function: int sem_destroy (sem_t * SEM)
     `sem_destroy' destroys a semaphore object, freeing the resources it
     might hold.  If any threads are waiting on the semaphore when
     `sem_destroy' is called, it fails and sets ERRNO to `EBUSY'.

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

 - Function: int sem_wait (sem_t * SEM)
     `sem_wait' suspends the calling thread until the semaphore pointed
     to by SEM has non-zero count. It then atomically decreases the
     semaphore count.

     `sem_wait' is a cancellation point.  It always returns 0.

 - Function: int sem_trywait (sem_t * SEM)
     `sem_trywait' is a non-blocking variant of `sem_wait'. If the
     semaphore pointed to by SEM has non-zero count, the count is
     atomically decreased and `sem_trywait' immediately returns 0.  If
     the semaphore count is zero, `sem_trywait' immediately returns -1
     and sets errno to `EAGAIN'.

 - Function: int sem_post (sem_t * SEM)
     `sem_post' atomically increases the count of the semaphore pointed
     to by SEM. This function never blocks.

     On processors supporting atomic compare-and-swap (Intel 486,
     Pentium and later, Alpha, PowerPC, MIPS II, Motorola 68k,
     Ultrasparc), the `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 `sem_post' is not async-signal safe, because the
     hardware does not support the required atomic operations.

     `sem_post' always succeeds and returns 0, unless the semaphore
     count would exceed `SEM_VALUE_MAX' after being incremented.  In
     that case `sem_post' returns -1 and sets ERRNO to `EINVAL'.  The
     semaphore count is left unchanged.

 - Function: int sem_getvalue (sem_t * SEM, int * SVAL)
     `sem_getvalue' stores in the location pointed to by SVAL the
     current count of the semaphore SEM.  It always returns 0.


File: libc.info,  Node: Thread-Specific Data,  Next: Threads and Signal Handling,  Prev: POSIX Semaphores,  Up: POSIX Threads

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 `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 `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 `NULL'
with all keys.

 - Function: int pthread_key_create (pthread_key_t *KEY, void
          (*destr_function) (void *))
     `pthread_key_create' allocates a new TSD key. The key is stored in
     the location pointed to by KEY. There is a limit of
     `PTHREAD_KEYS_MAX' on the number of keys allocated at a given
     time. The value initially associated with the returned key is
     `NULL' in all currently executing threads.

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

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

     `pthread_key_create' returns 0 unless `PTHREAD_KEYS_MAX' keys have
     already been allocated, in which case it fails and returns
     `EAGAIN'.

 - Function: int pthread_key_delete (pthread_key_t KEY)
     `pthread_key_delete' deallocates a TSD key. It does not check
     whether non-`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 KEY, it returns `EINVAL'.  Otherwise it
     returns 0.

 - Function: int pthread_setspecific (pthread_key_t KEY, const void
          *POINTER)
     `pthread_setspecific' changes the value associated with KEY in the
     calling thread, storing the given POINTER instead.

     If there is no such key KEY, it returns `EINVAL'.  Otherwise it
     returns 0.

 - Function: void * pthread_getspecific (pthread_key_t KEY)
     `pthread_getspecific' returns the value currently associated with
     KEY in the calling thread.

     If there is no such key KEY, it returns `NULL'.

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

     /* 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);
     }


File: libc.info,  Node: Threads and Signal Handling,  Next: Threads and Fork,  Prev: Thread-Specific Data,  Up: POSIX Threads

Threads and Signal Handling
===========================

 - Function: int pthread_sigmask (int HOW, const sigset_t *NEWMASK,
          sigset_t *OLDMASK)
     `pthread_sigmask' changes the signal mask for the calling thread as
     described by the HOW and NEWMASK arguments. If OLDMASK is not
     `NULL', the previous signal mask is stored in the location pointed
     to by OLDMASK.

     The meaning of the HOW and NEWMASK arguments is the same as for
     `sigprocmask'. If HOW is `SIG_SETMASK', the signal mask is set to
     NEWMASK. If HOW is `SIG_BLOCK', the signals specified to NEWMASK
     are added to the current signal mask.  If HOW is `SIG_UNBLOCK',
     the signals specified to 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 `sigaction', are shared
     between all threads.

     The `pthread_sigmask' function returns 0 on success, and one of the
     following error codes on error:
    `EINVAL'
          HOW is not one of `SIG_SETMASK', `SIG_BLOCK', or `SIG_UNBLOCK'

    `EFAULT'
          NEWMASK or OLDMASK point to invalid addresses

 - Function: int pthread_kill (pthread_t THREAD, int SIGNO)
     `pthread_kill' sends signal number SIGNO to the thread THREAD.
     The signal is delivered and handled as described in *Note Signal
     Handling::.

     `pthread_kill' returns 0 on success, one of the following error
     codes on error:
    `EINVAL'
          SIGNO is not a valid signal number

    `ESRCH'
          The thread THREAD does not exist (e.g. it has already
          terminated)

 - Function: int sigwait (const sigset_t *SET, int *SIG)
     `sigwait' suspends the calling thread until one of the signals in
     SET is delivered to the calling thread. It then stores the number
     of the signal received in the location pointed to by SIG and
     returns. The signals in SET must be blocked and not ignored on
     entrance to `sigwait'. If the delivered signal has a signal handler
     function attached, that function is _not_ called.

     `sigwait' is a cancellation point.  It always returns 0.

   For `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 `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 `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 `sigwait' on
that signal, it will not be restarted.

   The LinuxThreads implementation of `sigwait' installs dummy signal
handlers for the signals in 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).

