pthread.3t (2010 09)

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pthread(3T) pthread(3T)

(Pthread Library)

NAME

pthread - introduction to POSIX.1c threads

DESCRIPTION

The POSIX.1c library developed by HP enables the creation of processes that can exploit application and

multiprocessor platform parallelism. The pthread library libpthread consists of over 90 standardized

interfaces for developing concurrent applications and synchronizing their actions within processes or

between them. This manual page presents an overview of libpthread including terminology and how to

compile and link programs which use threads.

COMPILATION SUMMARY

A multithreaded application must deﬁne the appropriate POSIX revision level (199506) at compile time

and link against the pthread library with

-lpthread. For example:

cc -D_POSIX_C_SOURCE=199506L -o myapp myapp.c -lpthread

All program sources must also include the header ﬁle

<pthread.h>.

Note: If

-lc is explicitly speciﬁed in the link line, then it must be after the

-lpthread. Refer to

pthread_stubs(5) for more details.

Note: When explicitly specifying ANSI compilation (with "-Aa"), deﬁning the POSIX revision level res-

tricts the program to using interfaces within the POSIX namespaces. If interfaces in the larger X/Open

namespace are to be called, either of the compiler options,

-D_XOPEN_SOURCE_EXTENDED

-D_HPUX_SOURCE, must be speciﬁed in addition to

-D_POSIX_C_SOURCE=199506L. Alternatively,

compiling with -Ae (or not specifying "-A") will implicitly specify

-D_HPUX_SOURCE

Note: Some documentation will recommend the use of

-D_REENTRANT for compilation. Although this

also functions properly, it is considered an obsolescent form.

THREAD OVERVIEW

A thread is an independent ﬂow of control within a process, composed of a context (which includes a

All processes consist of at least one thread. Multi-threaded processes contain several threads. All

threads share the common address space allocated for the process. A program using the POSIX pthread

APIs creates and manipulates what are called user threads .Akernel thread is a kernel-schedulable

entity which may support one or more user threads. At HP-UX release 11i Version 1.6 and forward, the

HP-UX threads implementation supports many-to-any as well as one-to-one mapping between user and

kernel threads.

Each thread is assigned a unique identiﬁer of type pthread_t upon creation. The thread id is a process-

private value and implementation-dependent. It is considered to be an opaque handle for the thread. Its

value should not be used by the application.

NOTES ON INTERFACES

The HP-UX system provides some non-standard extensions to the pthread API. These will always have a

distinguishing sufﬁx of

_np or _NP (non-portable).

The programmer should always consult the manpages for the functions being used. Some standard-

speciﬁed functions are not available or may have no effect in some implementations.

THREAD CREATION/DESTRUCTION

A program creates a thread using the

pthread_create() function. When the thread has completed

its work, it may optionally call the pthread_exit() function, or simply return from its initial function.

A thread can detect the completion of another by using the pthread_join() function.

pthread_create() Creates a thread and assigns a unique identiﬁer, pthread_t . The caller pro-

vides a function which will be executed by the thread. Optionally, the call

may explicitly specify some attributes for the thread (see PTHREAD ATTRI-

BUTES below).

pthread_exit() Called by a thread when it completes. This function does not return.

pthread_join() This is analogous to wait(), but for pthreads. Any thread may join any

other thread in the process, there is no parent/child relationship. It returns

when a speciﬁed thread terminates, and the thread resources have been

reaped.

HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

Summary of content (12 pages)

PAGE 1
pthread(3T) pthread(3T) (Pthread Library) NAME pthread - introduction to POSIX.1c threads DESCRIPTION The POSIX.1c library developed by HP enables the creation of processes that can exploit application and multiprocessor platform parallelism. The pthread library libpthread consists of over 90 standardized interfaces for developing concurrent applications and synchronizing their actions within processes or between them.
PAGE 2
pthread(3T) pthread(3T) (Pthread Library) pthread_detach() Makes it unnecessary to "join" the thread. Thread resources are reaped by the system at the time the thread terminates. PTHREAD ATTRIBUTES A set of thread attributes may be provided to pthread_create(). Any changes from default values must be made to the attribute set before the call to pthread_create() is made. Subsequent changes to the attribute set do not affect the created thread.
PAGE 3
pthread(3T) pthread(3T) (Pthread Library) pthread_setcancelstate(), pthread_setcanceltype() Set the characteristics of cancellation for the thread. Cancellation may be enabled or disabled, or it may be synchronous or deferred. pthread_cleanup_pop(), pthread_cleanup_push() Register or remove cancellation cleanup handlers. Refer to thread_safety(5) for the list of cancellation points in the pthread library, system functions, and libc.
PAGE 4
pthread(3T) pthread(3T) (Pthread Library) Condition Variables are used by a thread to wait for the occurrence of some event. A thread detecting or causing such an event can signal or broadcast that occurrence to the waiting thread or threads. Read-Write locks permit concurrent read access by multiple threads to structures guarded by a read-write lock, but write access by only a single thread. pthread_mutex_init(), pthread_mutex_destroy() Initialize/destroy contents of a mutex lock.
PAGE 5
pthread(3T) pthread(3T) (Pthread Library) Lock/unlock a read-write lock. pthread_rwlockattr_init(), pthread_rwlockattr_destroy(), pthread_rwlockattr_getpshared(), pthread_rwlockattr_setpshared() Manage read-write lock attributes used for pthread_rwlock_init(). POSIX 1.b SEMAPHORES The semaphore functions specified in the POSIX 1.b standard can also be used for synchronization in a multithreaded application. sem_init(), sem_destroy() Initialize/destroy contents of a semaphore.
PAGE 6
pthread(3T) pthread(3T) (Pthread Library) zyx = 21; Each thread will have a different value associated with zyx. TLS variables can be statically initialized. Uninitialized TLS variables will be set to zero. Dynamically loaded libraries (with shl_load()) can declare and use TLS variables. TLS does have a cost in thread creation/termination operations, as TLS space for each thread must be allocated and initialized, regardless of whether it will ever use the variables.
PAGE 7
pthread(3T) pthread(3T) (Pthread Library) When the application is executed, it produces a per-thread file of pthread events. This is used as input to the ttv thread trace visualizer facility available in the HP/PAK performance application kit . There are environment variables defined to control trace data files: THR_TRACE_DIR Where to place the trace data files. If this is not defined, the files go to the current working directory.
PAGE 8
pthread(3T) pthread(3T) (Pthread Library) Async-Cancel Safe A function that may be called by a thread with the cancelability state set to PTHREAD_CANCEL_ENABLE and the cancelability type set to PTHREAD_CANCEL_ASYNCHRONOUS. If a thread is canceled in one of these functions, no state is left in the function. These functions generally do not acquire resources to perform the function’s task. Async-Signal Safe An async-signal safe function is a function that may be called by a signal handler.
PAGE 9
pthread(3T) pthread(3T) (Pthread Library) are permanently deadlocked. Deadlocks can occur with any number of resource holding threads. An interactive deadlock involves two or more threads. A recursive (or self) deadlock involves only one thread. Detached Thread A thread whose resources are automatically released by the system when the thread terminates. A detached thread cannot be joined by another thread. Consequently, detached threads cannot return an exit status.
PAGE 10
pthread(3T) pthread(3T) (Pthread Library) end result is that a low-priority thread blocks a high-priority thread. Process A process can be thought of as a container for one or more threads of execution, an address space, and shared process resources. All processes have at least one thread. Each thread in the process executes within the process’ address space. Examples of process-shared resources are open file descriptors, message queue descriptors, mutexes, and semaphores.
PAGE 11
pthread(3T) pthread(3T) (Pthread Library) Shared Object A shared object is a tangible entity that exists in the address space of a process and is accessible by all threads within the process. In the context of multithreaded programming, "shared objects" are global variables, file descriptors, and other such objects that require access by threads to be synchronized. Signal A signal is a simplified IPC mechanism that allows a process or thread to be notified of an event.
PAGE 12
pthread(3T) pthread(3T) (Pthread Library) synchronization. Thread-Specific Data (TSD) Thread-specific data is global data that is specific to a thread. All threads access the same data variable. However, each thread has its own thread-specific value associated with this variable. errno is an example of thread-specific data. Thread Structure The operating system maintains a thread structure for each thread in the system. This structure represents the actual thread internally in the system.