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 signals.
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.
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.
| 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() |
- |
| POSIX |
illumos |
C11 |
| pthread_exit() |
thr_exit() |
thrd_exit() |
| pthread_join() |
thr_join() |
thrd_join() |
| pthread_detach() |
- |
thrd_detach() |
| 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() |
| POSIX |
illumos |
C11 |
| pthread_sigmask() |
thr_sigsetmask() |
- |
| pthread_kill() |
thr_kill() |
- |
| POSIX |
illumos |
c11 |
| pthread_self() |
thr_self() |
thrd_current() |
| pthread_equal() |
- |
thrd_equal() |
| - |
thr_main() |
- |
| 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() |
- |
| POSIX |
illumos |
C11 |
| pthread_cancel() |
- |
- |
| pthread_setcancelstate() |
- |
- |
| pthread_setcanceltype() |
- |
- |
| pthread_testcancel() |
- |
- |
| pthread_cleanup_pop() |
- |
- |
| pthread_cleanup_push() |
- |
- |
| 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() |
| 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() |
| 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() |
- |
- |
| 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() |
- |
| POSIX |
illumos |
C11 |
| pthread_atfork() |
- |
- |
| POSIX |
illumos |
C11 |
| pthread_once() |
- |
call_once() |
| 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.
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 fork().
illumos supports the following three POSIX scheduling policies:
SCHED_OTHER
Traditional Timesharing scheduling policy. It is based on
the timesharing (TS) scheduling class.
SCHED_FIFO
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.
SCHED_RR
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:
SCHED_IA
Threads are scheduled according to the Inter-Active Class
(IA) policy as described in priocntl(2).
SCHED_FSS
Threads are scheduled according to the Fair-Share Class
(FSS) policy as described in priocntl(2).
SCHED_FX
Threads are scheduled according to the Fixed-Priority
Class (FX) policy as described in priocntl(2).
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).
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(5) for descriptions of the following attributes:
| ATTRIBUTE
TYPE |
ATTRIBUTE VALUE |
| MT-Level |
MT-Safe, Fork 1-Safe |
crle(1), fork(2), priocntl(2), libpthread(3LIB),
librt(3LIB), libthread(3LIB), pthread_atfork(3C),
pthread_create(3C), attributes(5), standards(5)
Linker and Libraries Guide