THREADS(7) Standards, Environments, and Macros THREADS(7)


threads, pthreads - POSIX pthreads, c11, and illumos threads concepts



gcc -D_REENTRANT [ flag... ] file... [ library... ]

#include <pthread.h>

gcc -std=c11 -D_REENTRANT [ flag... ] file... [ library... ]

#include <threads.h>

gcc -D_REENTRANT [ flag... ] file... [ library... ]

#include <sched.h>

#include <thread.h>


A thread is an independent source of execution within a process. Every
process is created with a single thread, which calls the main function. A
process may have multiple threads, all of which are scheduled
independently by the system and may run concurrently. Threads within a
process all use the same address space and as a result can access all
data in the process; however, each thread is created with its own
attributes and its own stack. When a thread is created, it inherits the
signal mask of the thread which created it, but it has no pending

All threads of execution have their own, independent life time, though it
is ultimately bounded by the life time of the process. If the process
terminates for any reason, whether due to a call to exit(3C), the receipt
of a fatal signal, or some other reason, then all threads within the
process are terminated. Threads may themselves exit and status
information of them may be obtained, for more information, see the
pthread_detach(3C), pthread_join(3C), and pthread_exit(3C) functions, and
their equivalents as described in the tables later on in the manual.

Most hardware platforms do not have any special synchronization for data
objects which may be accessed concurrently from multiple threads of
execution. To avoid such problems, programs may use atomic operations
(see atomic_ops(3C)) and locking primitives, such as mutexes,
readers/writer locks, condition variables, and semaphores. Note, that
depending on the hardware platform, memory synchronization may be
necessary, for more information, see membar_ops(3C).

POSIX, C11, and illumos threads each have their own implementation within
libc(3LIB). All implementations are interoperable, their functionality
similar, and can be used within the same application. Only POSIX threads
are guaranteed to be fully portable to other POSIX-compliant
environments. C11 threads are an optional part of ISO C11 and may not
exist on every ISO C11 platform. POSIX, C11, and illumos threads require
different source and include files. See SYNOPSIS.


Most of the POSIX and illumos threading functions have counterparts with
each other. POSIX function names, with the exception of the semaphore
names, have a "pthread" prefix. Function names for similar POSIX and
illumos functions have similar endings. Typically, similar POSIX and
illumos functions have the same number and use of arguments.


POSIX pthreads and illumos threads differ in the following ways:

o POSIX threads are more portable.

o POSIX threads establish characteristics for each thread
according to configurable attribute objects.

o POSIX pthreads implement thread cancellation.

o POSIX pthreads enforce scheduling algorithms.

o POSIX pthreads allow for clean-up handlers for fork(2) calls.

o illumos threads can be suspended and continued.

o illumos threads implement daemon threads, for whose demise the
process does not wait.

Comparison to C11 Threads
C11 threads are not as functional as either POSIX or illumos threads. C11
threads only support intra-process locking and do not have any form of
readers/writer locking or semaphores. In general, POSIX threads will be
more portable than C11 threads, all POSIX-compliant systems support
pthreads; however, not all C environments support C11 Threads.

In addition to lacking other common synchronization primitives, the
ISO/IEC standard for C11 threads does not have rich error semantics. In
an effort to not extend the set of error numbers standardized in ISO/IEC
C11, none of the routines set errno and instead multiple distinguishable
errors, aside from the equivalent to ENOMEM and EBUSY, are all squashed
into one. As such, users of the platform are encouraged to use POSIX
threads, unless a portability concern dictates otherwise.


The following table compares the POSIX pthreads, C11 threads, and illumos
threads functions. When a comparable interface is not available either
in POSIX pthreads, C11 threads or illumos threads, a hyphen (-) appears
in the column.

Functions Related to Creation

POSIX illumos C11
pthread_create() thr_create() thrd_create()
pthread_attr_init() - -
pthread_attr_setdetachstate() - -
pthread_attr_getdetachstate() - -
pthread_attr_setinheritsched() - -
pthread_attr_getinheritsched() - -
pthread_attr_setschedparam() - -
pthread_attr_getschedparam() - -
pthread_attr_setschedpolicy() - -
pthread_attr_getschedpolicy() - -
pthread_attr_setscope() - -
pthread_attr_getscope() - -
pthread_attr_setstackaddr() - -
pthread_attr_getstackaddr() - -
pthread_attr_setstacksize() - -
pthread_attr_getstacksize() - -
pthread_attr_getguardsize() - -
pthread_attr_setguardsize() - -
pthread_attr_destroy() - -
- thr_min_stack() -

Functions Related to Exit

POSIX illumos C11
pthread_exit() thr_exit() thrd_exit()
pthread_join() thr_join() thrd_join()
pthread_detach() - thrd_detach()

Functions Related to Thread Specific Data

POSIX illumos C11
pthread_key_create() thr_keycreate() tss_create()
pthread_setspecific() thr_setspecific() tss_set()
pthread_getspecific() thr_getspecific() tss_get()
pthread_key_delete() - tss_delete()

Functions Related to Signals

POSIX illumos C11
pthread_sigmask() thr_sigsetmask() -
pthread_kill() thr_kill() -

Functions Related to IDs

POSIX illumos c11
pthread_self() thr_self() thrd_current()
pthread_equal() - thrd_equal()
- thr_main() -

Functions Related to Scheduling

POSIX illumos C11
- thr_yield() thrd_yield()
- thr_suspend() -
- thr_continue() -
pthread_setconcurrency() thr_setconcurrency() -
pthread_getconcurrency() thr_getconcurrency() -
pthread_setschedparam() thr_setprio() -
pthread_setschedprio() thr_setprio() -
pthread_getschedparam() thr_getprio() -

Functions Related to Cancellation

POSIX illumos C11
pthread_cancel() - -
pthread_setcancelstate() - -
pthread_setcanceltype() - -
pthread_testcancel() - -
pthread_cleanup_pop() - -
pthread_cleanup_push() - -

Functions Related to Mutexes

POSIX illumos c11
pthread_mutex_init() mutex_init() mtx_init()
pthread_mutexattr_init() - -
pthread_mutexattr_setpshared() - -
pthread_mutexattr_getpshared() - -
pthread_mutexattr_setprotocol() - -
pthread_mutexattr_getprotocol() - -
pthread_mutexattr_setprioceiling() - -
pthread_mutexattr_getprioceiling() - -
pthread_mutexattr_settype() - -
pthread_mutexattr_gettype() - -
pthread_mutexattr_setrobust() - -
pthread_mutexattr_getrobust() - -
pthread_mutexattr_destroy() - mtx_destroy()
pthread_mutex_setprioceiling() - -
pthread_mutex_getprioceiling() - -
pthread_mutex_lock() mutex_lock() mtx_lock()
pthread_mutex_timedlock() - mtx_timedlock()
pthread_mutex_trylock() mutex_trylock() mtx_trylock()
pthread_mutex_unlock() mutex_unlock() mtx_unlock()
pthread_mutex_destroy() mutex_destroy() mtx_destroy()

Functions Related to Condition Variables

POSIX illumos C11
pthread_cond_init() cond_init() cnd_init()
pthread_condattr_init() - -
pthread_condattr_setpshared() - -
pthread_condattr_getpshared() - -
pthread_condattr_destroy() - -
pthread_cond_wait() cond_wait() cnd_wait()
pthread_cond_timedwait() cond_timedwait() cond_timedwait()
pthread_cond_signal() cond_signal() cnd_signal()
pthread_cond_broadcast() cond_broadcast() cnd_broadcast()
pthread_cond_destroy() cond_destroy() cnd_destroy()

Functions Related to Reader/Writer Locking

POSIX illumos C11
pthread_rwlock_init() rwlock_init() -
pthread_rwlock_rdlock() rw_rdlock() -
pthread_rwlock_tryrdlock() rw_tryrdlock() -
pthread_rwlock_wrlock() rw_wrlock() -
pthread_rwlock_trywrlock() rw_trywrlock() -
pthread_rwlock_unlock() rw_unlock() -
pthread_rwlock_destroy() rwlock_destroy() -
pthread_rwlockattr_init() - -
pthread_rwlockattr_destroy() - -
pthread_rwlockattr_getpshared() - -
pthread_rwlockattr_setpshared() - -

Functions Related to Semaphores

POSIX illumos C11
sem_init() sema_init() -
sem_open() - -
sem_close() - -
sem_wait() sema_wait() -
sem_trywait() sema_trywait() -
sem_post() sema_post() -
sem_getvalue() - -
sem_unlink() - -
sem_destroy() sema_destroy() -

Functions Related to fork() Clean Up

POSIX illumos C11
pthread_atfork() - -

Functions Related to Limits

POSIX illumos C11
pthread_once() - call_once()

Functions Related to Debugging

POSIX illumos C11
- thr_stksegment() -



Multithreaded behavior is asynchronous, and therefore, optimized for
concurrent and parallel processing. As threads, always from within the
same process and sometimes from multiple processes, share global data
with each other, they are not guaranteed exclusive access to the shared
data at any point in time. Securing mutually exclusive access to shared
data requires synchronization among the threads. Both POSIX and illumos
implement four synchronization mechanisms: mutexes, condition variables,
reader/writer locking (optimized frequent-read occasional-write mutex),
and semaphores, where as C11 threads only implement two mechanisms:
mutexes and condition variables.

Synchronizing multiple threads diminishes their concurrency. The coarser
the grain of synchronization, that is, the larger the block of code that
is locked, the lesser the concurrency.

MT fork()
If a threads program calls fork(2), it implicitly calls fork1(2), which
replicates only the calling thread. Should there be any outstanding
mutexes throughout the process, the application should call
pthread_atfork(3C) to wait for and acquire those mutexes prior to calling


POSIX Threads

illumos supports the following three POSIX scheduling policies:

Traditional Timesharing scheduling policy. It is based on
the timesharing (TS) scheduling class.

First-In-First-Out scheduling policy. Threads scheduled to
this policy, if not preempted by a higher priority, will
proceed until completion. Such threads are in real-time
(RT) scheduling class. The calling process must have a
effective user ID of 0.

Round-Robin scheduling policy. Threads scheduled to this
policy, if not preempted by a higher priority, will
execute for a time period determined by the system. Such
threads are in real-time (RT) scheduling class and the
calling process must have a effective user ID of 0.

In addition to the POSIX-specified scheduling policies above, illumos
also supports these scheduling policies:

Threads are scheduled according to the Inter-Active Class
(IA) policy as described in priocntl(2).

Threads are scheduled according to the Fair-Share Class
(FSS) policy as described in priocntl(2).

Threads are scheduled according to the Fixed-Priority Class
(FX) policy as described in priocntl(2).

illumos Threads
Only scheduling policy supported is SCHED_OTHER, which is timesharing,
based on the TS scheduling class.


In a multithreaded application, EINTR can be returned from blocking
system calls when another thread calls forkall(2).


-mt compiler option
The -mt compiler option compiles and links for multithreaded code. It
compiles source files with -D_REENTRANT and augments the set of support
libraries properly.

Users of other compilers such as gcc and clang should manually set
-D_REENTRANT on the compilation line. There are no other libraries or
flags necessary.


See attributes(7) for descriptions of the following attributes:

|MT-Level | MT-Safe, Fork 1-Safe |


crle(1), fork(2), priocntl(2), pthread_atfork(3C), pthread_create(3C),
libpthread(3LIB), librt(3LIB), libthread(3LIB), attributes(7),

Linker and Libraries Guide

illumos March 27, 2016 THREADS(7)