linuxthreads.texi

Axis 221 camera embedded programing interface

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

@c This chapter needs more work bigtime. -zw

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

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

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

@node Basic Thread Operations
@section Basic Thread Operations

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@node Thread Attributes
@section Thread Attributes

@comment pthread.h
@comment POSIX

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

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

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

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

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

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

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

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

These functions always return 0.
@end deftypefun

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

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

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

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

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

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

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

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

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

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

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

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

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

@node Cancellation
@section Cancellation

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

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

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

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

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

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

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

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

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

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