pthread(3T)

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 restricts 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 or -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 register set and a program counter) and a sequence of instructions to execute.

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. A kernel 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 provides a function which will be executed by the thread. Optionally, the call may explicitly specify some attributes for the thread (see PTHREAD ATTRIBUTES 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.
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. However, the attribute set may be used in multiple pthread_create() calls.

Note that only the "detachstate", "schedparam", "schedpolicy", and "processor" attributes of a thread may be effected subsequent to thread creation. However, this is done by the pthread_detach(), pthread_setschedparam(), and pthread_processor_bind_np() functions, respectively.

pthread_attr_init(): Initializes an attribute set for use in the pthread_create() call.
pthread_attr_destroy(): Destroys the content of an attribute set.

pthread_attr_getdetachstate(),
pthread_attr_getguardsize(),
pthread_attr_getinheritsched(),
pthread_attr_getprocessor_np(),
pthread_attr_getschedparam(),
pthread_attr_getschedpolicy(),
pthread_attr_getscope(),
pthread_attr_getstackaddr(),
pthread_attr_getstacksize(),
pthread_attr_setdetachstate(),
pthread_attr_setguardsize(),
pthread_attr_setinheritsched(),
pthread_attr_setprocessor_np(),
pthread_attr_setschedparam(),
pthread_attr_setschedpolicy(),
pthread_attr_setscope(),
pthread_attr_setstackaddr(),
pthread_attr_setstacksize()

These pthread_attr_get/set<attribute>() functions get/set the associated attribute in the attribute set. See the manpages for these functions for descriptions of the attributes.
pthread_default_stacksize_np()
This is used to set the default stacksize for threads created in subsequent attribute set initializations (calls to pthread_attr_init()) or in pthread_create() where no attributes are supplied.

CANCELLATION

Certain applications may desire to terminate a particular thread without causing the entire process to exit. A thread may be canceled by another thread in the same process while the cancellation target thread executes a system call or particular library routine.

When a thread issues a cancel request against another thread, the target thread can check to see if a request is pending against it by the pthread_testcancel() interface. When called with a request pending, the target thread terminates after executing any cleanup handlers which may have been installed. Cleanup handlers may be used to delete any dynamic storage allocated by the canceled thread, to unlock a mutex, or other operations.

Typically, the cancellation type for a thread is deferred. That is, cancellation requests are held pending until the thread reaches a cancellationpoint which is simply one of a list of library functions and system calls (see lists below).

The thread may set its cancellation type to asynchronous. In this case cancellation requests are acted upon at any time. This can be used effectively in compute-bound threads which do not call any functions that are cancellation points.

pthread_cancel(): Cancel execution of a given thread.
pthread_testcancel(): Called by a thread to process pending cancel requests.

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()

Refer to thread_safety(5) for the list of cancellation points in the pthread library, system functions, and libc.

For libc functions, whether the thread is cancelled depends upon what action is performed while executing the function. If the thread blocks while inside the function, a cancellation point is created (i.e., the thread may be cancelled). Other libraries may have cancellation points. Check the associated documentation for details.

The list of cancellation points will vary from release to release. In general, if a function can return with an EINTR error, chances are that it is a cancellation point.

SCHEDULING

Threads may individually control their scheduling policy and priorities. Threads may also suspend their own execution, or that of other threads. Finally, threads are given some control over allocation of processor resources.

pthread_suspend(): This function is used to temporarily stop the execution of a thread.

pthread_continue(),
pthread_resume_np()

These functions cause a previously suspended thread to continue execution.

pthread_num_processor_np(),
pthread_processor_bind_np(),
pthread_processor_id_np()

These functions are used to interrogate processor configuration and to bind a thread to a specific processor.

pthread_getconcurrency(),
pthread_setconcurrency()

These functions are used to control the actual concurrency for unbound threads.

pthread_getschedparam(),
pthread_setschedparam()

These functions are used to manipulate the scheduling policy and priority for a thread.

sched_get_priority_max(),
sched_get_priority_min()

These functions are used to interrogate the priority range for a given scheduling policy.
sched_yield
This function is used by a thread to yield the processor to other threads of equal or greater priority.

COMMUNICATION & SYNCHRONIZATION

Multi-threaded applications concurrently execute instructions. Access to process-wide (or interprocess) shared resources (memory, file descriptors, etc.) requires mechanisms for coordination or synchronization among threads. The libpthread library offers synchronization primitives necessary to create a deterministic application. A multithreaded application ensures determinism by forcing asynchronous thread contexts to synchronize, or serialize, access to data structures and resources managed and manipulated during run-time. These are mutual-exclusion (mutex) locks, condition variables, and read-write locks. The HP-UX operating system also provides POSIX semaphores (see next section).

Mutexes furnish the means to exclusively guard data structures from concurrent modification. Their protocol precludes more than one thread which has locked the mutex from changing the contents of the protected structure until the locker performs an analogous mutex unlock. A mutex can be initialized in two ways: by a call to pthread_mutex_init(); or by assignment of PTHREAD_MUTEX_INITIALIZER.

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.

pthread_mutex_lock(),
pthread_mutex_trylock(),
pthread_mutex_unlock()

Lock/unlock a mutex.

pthread_mutex_getprioceiling(),
pthread_mutex_setprioceiling()

Manipulate mutex locking priorities.

pthread_mutexattr_init(),
pthread_mutexattr_destroy(),
pthread_mutexattr_getprioceiling(),
pthread_mutexattr_getprotocol(),
pthread_mutexattr_getpshared(),
pthread_mutexattr_gettype(),
pthread_mutexattr_getspin_np(),
pthread_mutexattr_setprioceiling(),
pthread_mutexattr_setprotocol(),
pthread_mutexattr_setpshared(),
pthread_mutexattr_settype(),
pthread_mutexattr_setspin_np()

Manage mutex attributes used for pthread_mutex_init(). Only the "prioceiling" attribute can be changed for an exiting mutex.

pthread_mutex_getyieldfreq_np(),
pthread_mutex_setyieldfreq_np()

These functions, together with the spin attributes, are used to tune mutex performance to the specific application.

pthread_cond_init(),
pthread_cond_destroy()

Initialize/destroy contents of a read-write lock.

pthread_cond_signal(),
pthread_cond_broadcast(),
pthread_cond_timedwait(),
pthread_cond_wait()

Wait upon or signal occurrence of a condition variable.

pthread_condattr_init(),
pthread_condattr_destroy(),
pthread_condattr_getpshared(),
pthread_condattr_setpshared()

Manage condition variable attributes used for pthread_cond_init().

pthread_rwlock_init(),
pthread_rwlock_destroy()

Initialize/destroy contents of a read-write lock.

pthread_rwlock_rdlock(),
pthread_rwlock_tryrdlock(),
pthread_rwlock_wrlock(),
pthread_rwlock_trywrlock(),
pthread_rwlock_unlock()

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.

sem_post(),
sem_wait(),
sem_trywait()

Increment/decrement semaphore value (possibly blocking).

SIGNALS

In a multithreaded process, all threads share signal actions. That is, a signal handler established by one thread is used in all threads. However, each thread has a separate signal mask, by which it can selectively block signals.

Signals can be sent to other threads within the same process, or to other processes. When a signal is sent to the process, exactly one thread which does not have that signal blocked will handle the signal. When sent to a thread within the same process, that thread will handle the signal, perhaps later if the signal is blocked. Signals whose action is to terminate, stop, or continue will terminate, stop, or continue the entire process, respectively, even if directed at a particular thread.

pthread_kill(): Sends a signal to the given thread.
pthread_sigmask(): Blocks selected signals for the thread.

sigwait(),
sigwaitinfo(),
sigtimedwait()

These functions synchronously wait for given signals.

THREAD-SPECIFIC DATA

Thread-specific data (TSD) is global data that is private or specific to a thread. Each thread has a different value for the same thread-specific data variable. The global errno is a perfect example of thread-specific global data.

Each thread-specific data item is associated with a key. The key is shared by all threads. However, when a thread references the key, it references its own private copy of the data.

pthread_key_create(),
pthread_key_destroy()

These functions manage the thread-specific data keys.

pthread_getspecific(),
pthread_setspecific()

These functions retrieve and assign the data value associated with a key.

The HP-UX compiler supports a thread local storage (TLS) storage class. (This is not a POSIX standard feature.) TLS is identical to TSD, except functions are not required to create/set/get values. TLS variables are accessed just like normal global variables. TLS variables can be declared using the following syntax:

 __thread     int     zyx;

The keyword __thread tells the compiler that zyx is a TLS variable. Now each thread can set or get TLS with statements such as:

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. This is true for modules linked statically at startup. In case of dynamically loaded liabraries (with sh_load()), TLS space for a thread will be allocated when the TLS variables are accessed by it. If few threads actually use a large TLS area, it may be wise to use the POSIX TSD instead (above).

REENTRANT LIBC & STDIO

Because they return pointers to library-internal static data, a number of libc functions cannot be used in multithreaded programs. This is because calling these functions in a thread will overwrite the results of previous calls in other threads. Alternate functions, having the suffix _r (for reentrant), are provided within libc for threaded programming.

Also, some primitives for synchronization of standard I/O operations are provided.

asctime_r(),
ctime_r(),
getgrgid_t(),
getgrnam_r(),
getlogin_r(),
getpwnam_r(),
getpwuid_r(),
gmtime_t(),
localtime_r(),
rand_r(),
readdir_r(),
strtok_r(),
ttyname_r()

Provide reentrant versions of previously existing libc functions.

flockfile(),
ftrylockfile(),
funlock()

Provide explicit synchronization for standard I/O streams.

MISCELLANEOUS FUNCTIONS

The section summarizes some miscellaneous pthread-related functions not covered in the preceding sections.

pthread_atfork(): Establish special functions to be called just prior to and just subsequent to a fork() operation.
pthread_equal(): Tests whether two pthread_t values represent the same pthread.
pthread_once(): Executes given function just once in a process, regardless of how many threads make the same call. (Useful for one-time data initialization.)
pthread_self(): Returns identifier (pthread_t) of calling thread.

THREAD DEBUGGING

Debugging of multithreaded programs is supported in the standard HP-UX debugger, dde. When any thread is to be stopped due to a debugger event, the debugger will stop all threads. The register state, stack, and data for any thread can be interrogated and manipulated.

See the dde(1) manpage and built-in graphical help system for more information.

TRACING FACILITIES

HP-UX provides a tracing facility for pthread operations. To use it, you must link your application using the tracing version of the library:

cc -D_POSIX_C_SOURCE=199506L -o myapp myapp.c -lpthread_tr -lcl

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.

THR_TRACE_ASYNC

By default, trace records are buffered and only written to the file when the buffer is full. If this variable is set to any non-NULL value, data is immediately written to the trace file.

THR_TRACE_EVENTS

By default, all pthread events are traced. If this variable is defined, only the categories defined will be traced. Each category is separated by a ':'. The possible trace categories are:

thread:cond:mutex:rwlock

For example, to only trace thread and mutex operations set the THR_TRACE_EVENTS variable to:

thread:mutex

Details of the trace file record format can be found in /usr/include/sys/trace_thread.h.

See the ttv(1) manpage and built-in graphical help system for more information on the use of the trace information.

PERFORMANCE CONSIDERATIONS

Often, an application is designed to be multithreaded to improve performance over its single-threaded counterparts. However, the multithreaded approach requires some attention to issues not always of concern in the single-threaded case. These are issues traditionally associated with the programming of multiprocessor systems.

The design must employ a lock granularity appropriate to the data structures and access patterns. Coarse-grained locks, which protect relatively large amounts of data, can lead to undesired lock contention, reducing the potential parallelism of the application. On the other hand, employing very fine-grained locks, which protect very small amounts of data, can consume processor cycles with too much locking activity.

The use of thread-specific data (TSD) or thread-localstorage (TLS) must be traded off, as described above (see THREAD-SPECIFIC DATA).

Mutex spin and yield frequency attributes can be used to tune mutex behavior to the application. See pthread_mutexattr_setspin_np(3T) and pthread_mutex_setyieldfreq_np(3T) for more information.

The default stacksize attribute can be set to improve system thread caching behavior. See pthread_default_stacksize_np(3T) for more information.

Because multiple threads are actually running simultaneously, they can be accessing the same data from multiple processors. The hardware processors coordinate their caching of data such that no processor is using stale data. When one processor accesses the data (especially for write operations), the other processors must flush the stale data from their caches. If multiple processors repeatedly read/write the same data, this can lead to cache-thrashing which slows execution of the instruction stream. This can also occur when threads access separate data items which just happen to reside in the same hardware-cachable unit (called a cache line). This latter situation is called false-sharing which can be avoided by spacing data such that popular items are not stored close together.

GLOSSARY

The following definitions were extracted from the text ThreadTime by Scott J. Norton and Mark D. DiPasquale, Prentice-Hall, ISBN 0-13-190067-6, 1996.

Application Programming Interface (API)

An interface is the conduit that provides access to an entity or communication between entities. In the programming world, an interface describes how access (or communication) with a function should take place. Specifically, the number of parameters, their names and purpose describe how to access a function. An API is the facility that provides access to a function.

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. Only a restricted set of functions may safely be called by a signal handler. These functions are listed in section 3.3.1.3 of the POSIX.1c standard.

Asynchronous Signal

An asynchronous signal is a signal that has been generated due to an external event. Signals sent by kill() and signals generated due to timer expiration or asynchronous I/O completion are all examples of asynchronously generated signals. Asynchronous signals are delivered to the process. All signals can be generated asynchronously.

Atfork Handler

Application-provided and registered functions that are called before and after a fork() operation. These functions generally acquire all mutex locks before the fork() and release these mutex locks in both the parent and child processes after the fork().

Atomic Operation

An operation or sequence of events that is guaranteed to complete as if it were one instruction.

Barrier

A synchronization primitive that causes a certain number of threads to wait or rendezvous at specified points in an application. Barriers are used when a application needs to ensure that all threads have completed some operation before proceeding onto the next task.

Bound Thread

A user thread that is directly bound to a kernel-scheduled entity. These threads contain a system scheduling scope and are scheduled directly by the kernel.

Cache Thrashing

Cache thrashing is a situation in which a thread executes on different processors, causing cached data to be moved to and from the different processor caches. Cache thrashing can cause severe performance degradation.

Cancellation Cleanup Handler

An application-provided and registered function that is called when a thread is canceled. These functions generally perform thread cleanup actions during thread cancellation. These handlers are similar to signal handlers.

Condition Variable

A condition variable is a synchronization primitive used to allow a thread to wait for an event. Condition variables are often used in producer-consumer problems where a producer must provide something to one or more consumers.

Context Switch

The act of removing the currently running thread from the processor and running another thread. A context switch saves the register state of the currently running thread and restores the register state of the thread chosen to execute next.

Critical Section

A section of code that must complete atomically and uninterrupted. A critical section of code is generally one in which some global resource (variables, data structures, linked lists, etc.) is modified. The operation being performed must complete atomically so that other threads do not see the critical section in an inconsistent state.

Deadlock

A deadlock occurs when one or more threads can no longer execute. For example, thread A holds lock 1 and is blocked on lock 2. Meanwhile, thread B holds lock 2 and is blocked on lock 1. Threads A and B 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.

Joinable Thread

A thread whose termination can be waited for by another thread. Joinable threads can return an exit status to a joining thread. Joinable threads maintain some state after termination until they are joined by another thread.

Kernel Mode

A mode of operation where all operations are allowed. While a thread is executing a system call it is executing in kernel mode.

Kernel Space

The kernel program exists in this space. Kernel code is executed in this space at the highest privilege level. In general, there are two privilege levels: one for user code (user mode) and the other for kernel code (kernel mode).

Kernel Stack

When a thread makes a system call, it executes in kernel mode. While in kernel mode, it does not use the stack allocated for use by the application. Instead, a separate kernel stack is used while in the system call. Each kernel-scheduled entity, whether a process, kernel thread or lightweight process, contains a kernel stack. See Stack for a generic description of a stack.

Kernel Thread

Kernel threads are created by the thread functions in the threads library. Kernel threads are kernel-scheduled entities that are visible to the operating system kernel. A kernel thread typically supports one or more user threads. Kernel threads execute kernel code or system calls on behalf of user threads. Some systems may call the equivalent of a kernel thread a lightweightprocess. See Thread for a generic description of a thread.

Lightweight Process

A kernel-scheduled entity. Some systems may call the equivalent of a lightweight process a kernel thread. Each process contains one or more lightweight process. How many lightweight processes a process contains depends on whether and how the process is multithreaded. See Thread for a generic description of a thread.

Multiprocessor

A system with two or more processors (CPUs). Multiprocessors allow multithreaded applications to obtain true parallelism.

Multithreading

A programming model that allows an application to have multiple threads of execution. Multithreading allows an application to have concurrency and parallelism (on multiprocessor systems).

Mutex

A mutex is a mutual exclusion synchronization primitive. Mutexes provide threads with the ability to regulate or serialize access to process shared data and resources. When a thread locks a mutex, other threads trying to lock the mutex block until the owning thread unlocks the mutex.

POSIX

Portable Operating System Interface. POSIX defines a set of standards that multiple vendors conform to in order to provide for application portability. The Pthreads standard (POSIX 1003.1c) provides a set of portable multithreading APIs to application developers.

Priority Inversion

A situation where a low-priority thread has acquired a resource that is needed by a higher priority thread. As the resource cannot be acquired, the higher priority thread must wait for the resource. The 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.

Process Control Block (PCB)

This structure holds the register context of a process.

Process Structure

The operating system maintains a process structure for each process in the system. This structure represents the actual process internally in the system. A sample of process structure information includes the process ID, the process' set of open files, and the signal vector. The process structure and the values contained within it are part of the context of a process.

Program Counter (PC)

The program counter is part of the register context of a process. It holds the address of the current instruction to be executed.

Race Condition

When the result of two or more threads performing an operation depends on unpredictable timing factors, this is a race condition.

Read-Write Lock

A read-write lock is a synchronization primitive. Read-write locks provide threads with the ability to regulate or serialize access to process-shared data and resources. Read-write locks allow multiple readers to concurrently acquire the read lock whereas only one writer at a time may acquire the write lock. These locks are useful for shared data that is mostly read and only rarely written.

Reentrant Function

A reentrant function is one that when called by multiple threads, behaves as if the function was called serially, one after another, by the different threads. These functions may execute in parallel.

Scheduling Allocation Domain

The set of processors on which a thread is scheduled. The size of this domain may dynamically change over time. Threads may also be moved from one domain to another.

Scheduling Contention Scope

The scheduling contention scope defines the group of threads that a thread competes with for access to resources. The contention scope is most often associated with access to a processor. However, this scope may also be used when threads compete for other resources. Threads with the system scope compete for access to resources with all other threads in the system. Threads with the process scope compete for access to resources with other process scope threads in the process.

Scheduling Policy

A scheduling policy is a set of rules used to determine how and when multiple threads are scheduled to execute. The scheduling policy also determines how long a thread is allowed to execute.

Scheduling Priority

A scheduling priority is a numeric priority value assigned to threads in certain scheduling policies. Threads with higher priorities are given preference when scheduling decisions are made.

Semaphore

A semaphore is similar to a mutex. A semaphore regulates access to one or more shared objects. A semaphore has a value associated with it. The value is generally set to the number of shared resources regulated by the semaphore. When a semaphore has a value of one, it is a binary semaphore. A mutex is essentially a binary semaphore. When a semaphore has a value greater than one, it is known as a countingsemaphore. A counting semaphore can be locked by multiple threads simultaneously. Each time the semaphore is locked, the value is decremented by one. After the value reaches zero, new attempts to lock the semaphore cause the locking thread to block until the semaphore is unlocked by another thread.

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. Signals can be generated synchronously and asynchronously.

Signal Mask

A signal mask determines which signals a thread accepts and which ones are blocked from delivery. If a synchronous signal is blocked from delivery, it is held pending until either the thread unblocks the signal or the thread terminates. If an asynchronous signal delivered to the process is blocked from delivery by a thread, the signal may be handled by a different thread in the process that does not have the signal blocked.

Signal Vector

A signal vector is a table contained in each process that describes the action that should be taken when a signal is delivered to a thread within the process. Each signal has one of three potential behaviors: ignore the signal, execute a signal-handling function, or perform the default action of the signal (usually process termination).

Single-Threaded

means that there is only one flow of control (one thread) through the program code; only one instruction is executed at a time.

Spinlock

A synchronization primitive similar to a mutex. If the lock cannot be acquired, instead of blocking, the thread wishing to acquire the lock spins in a loop until the lock can be acquired. Spinlocks can be easily used improperly and can severely degrade performance if used on a single processor system.

Spurious Wakeup

A spurious wakeup occurs when a thread is incorrectly unblocked, even though the event it was waiting for has not occurred. A condition wait that is interrupted and returns because the blocked thread received a normal signal is an example of a spurious wakeup.

Stack

A stack is used by a thread to make function calls (and return from those calls), to pass arguments to a function call, and to create the space for local variables when in that function call. Bound threads have a user stack and a kernel stack. Unbound threads have only a user stack.

Synchronous Signal

A synchronous signal is a signal that has been generated due to some action of a specific thread. For example, when a thread does a divide by zero, causes a floating point exception, or executes an illegal instruction, a signal is generated synchronously. Synchronous signals are delivered to the thread that caused the signal to be sent.

Traditional Process

This is a single-threaded entity that can be scheduled to execute on a processor.

Thread

A thread is an independent flow of control within a process, composed of a context (which includes a register set and program counter) and a sequence of instructions to execute.

Thread Local Storage (TLS)

Thread local storage is essentially thread-specific data requiring support from the compilers. With TLS, an application can allocate the actual data as thread-specific data rather than using thread-specific data keys. Additionally, TLS does not require the thread to make a function call to obtain thread-specific data. The thread can access the data directly.

Thread-Safe Function

A thread-safe function is one that may be safely called by multiple threads at the same time. If the function accesses shared data or resources, this access is regulated by a mutex or some other form of 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. A sample of thread structure information includes the thread ID, the scheduling policy and priority, and the signal mask. The thread structure and the values contained within it are part of the context of a thread.

User Mode

A mode of operation where a subset of operations are allowed. While a thread is executing an applications code, it is executing in user mode. When the thread makes a system call, it changes modes and executes in kernel mode until the system call completes.

User Space

The user code exists in this space. User code is executed in this space at the normal privilege level. In general, there are two privilege levels: one for user code (user mode) and the other for kernel code (kernel mode).

User Stack

When a thread is executing code in user space, it needs to use a stack to make function calls, pass parameters, and create local variables. While in user mode, a thread does not use the kernel stack. Instead, a separate user stack is allocated for use by each user thread. See Stack for a generic description of a stack.

User Thread

When pthread_create() is called, a user thread is created. Whether a kernel-scheduled entity (kernel thread or lightweight process) is also created depends on the user thread's scheduling contention scope. When a bound thread is created, both a user thread and a kernel-scheduled entity are created. When an unbound thread is created, generally only a user thread is created. See Thread for a generic description of a thread.