path: root/lib/libpthread/man/pthreads.3

.\" $OpenBSD: pthreads.3,v 1.14 2001/10/30 23:41:05 marc Exp $
.\" David Leonard <d@openbsd.org>, 1998. Public domain.
.Dd August 17, 1998
.Dt PTHREADS 3
.Os
.Sh NAME
.Nm pthreads
.Nd POSIX 1003.1c thread interface
.Sh DESCRIPTION
A thread is a flow of control within a process.
Each thread represents a minimal amount of state;
normally just the cpu state and a signal mask.
All other process state (such as memory, file descriptors)
is shared among all of the threads in the process.
.Pp
In
.Ox ,
threads are implemented in a user-level library.
A program using these routines must be linked with the
.Fl pthread
option.
.Pp
The
.Dv SIGINFO
signal can be sent to a threaded process to have the library show the state of
all of its threads. The information is sent to the process'
controlling tty and describes each thread's
ID,
state (see
.Sx Thread states ) ,
current priority,
flags (see
.Sx Thread flags ) ,
signal mask, and name (as set by
.Xr pthread_set_name_np 3 ) .
If the environment variable 
.Ev PTHREAD_DEBUG
is defined additional information is displayed.
.Pp
.Ss Thread states
Threads can be in one of these states:
.Bl -tag -offset indent -width Dv -compact
.It cond_wait
Executing
.Xr pthread_cond_wait 3
or
.Xr pthread_cond_timedwait 3 .
.It dead
Waiting for resource deallocation by the thread garbage collector.
.It deadlock
Waiting for a resource held by the thread itself.
.It fdlr_wait
File descriptor read lock wait.
.It fdlw_wait
File descriptor write lock wait.
.It fdr_wait
Executing one of
.Xr accept 2 ,
.Xr read 2 ,
.Xr readv 2 ,
.Xr recvfrom 2 ,
.Xr recvmsg 2 .
.It fdw_wait
Executing one of
.Xr connect 2 ,
.Xr sendmsg 2 ,
.Xr sendto 2 ,
.Xr write 2 ,
.Xr writev 2 .
.It file_wait
Executing
.Xr flockfile 3
or similar.
.It join
Executing
.Xr pthread_join 3 .
.It mutex_wait
Executing
.Xr pthread_mutex_lock 3 .
.It poll_wait
Executing
.Xr poll 2 .
.It running
Scheduled for, or engaged in, program execution.
.It select_wait
Executing
.Xr select 2 .
.It sigsuspend
Executing
.Xr sigsuspend 2 .
.It sigwait
Executing
.Xr sigwait 3 .
.It sleep_wait
Executing
.Xr sleep 3
or
.Xr nanosleep 2 .
.It spinblock
Waiting for a machine-level atomic lock.
.It suspended
Suspended with
.Xr pthread_suspend_np 3 .
.It wait_wait
Executing
.Xr wait4 2
or similar.
.El
.Ss Thread flags
The first three flags are one of:
.Bl -tag -offset indent -width 3en -compact
.It "p"
Private, system thread (e.g., the garbage collector).
.It "E, C, or c"
Thread is exiting (E), has a cancellation pending (C) (see
.Xr pthread_cancel 3 ) ,
or is at a cancellation point (c).
.It "t"
Thread is being traced.
.El
The next 7 flags refer to thread attributes:
.Bl -tag -offset indent -width 3en -compact
.It "C"
Thread is in the
.Dv CONDQ .
.It "R"
Thread is in the
.Dv WORKQ .
.It "W"
Thread is in the
.Dv WAITQ .
.It "p"
Thread is in the
.Dv PRIOQ .
.It "d"
Thread has been detached (see
.Xr pthread_detach 3 ) .
.It "i"
Thread inherits scheduler properties.
.It "f"
Thread will save floating point context.
.El
.Ss Scheduling algorithm
The scheduling algorithm used by the user-level thread library is
roughly as follows:
.Bl -enum -compact
.It
Threads each have a time slice credit which is debited
by the actual time the thread spends in running.
Freshly scheduled threads are given a time slice credit of 100000 usec.
.It
Give an incremental priority update to run-enabled threads that
have not run since the last time that an incremental priority update
was given to them.
.It
Choose the next run-enabled thread with the highest priority,
that became inactive least recently, and has
the largest remaining time slice.
.El
.Pp
When all threads are blocked, the process also blocks.
When there are no threads remaining,
the process terminates with an exit code of zero.
.Ss Thread stacks
Each thread has (or should have) a different stack, whether it be provided by a
user attribute, or provided automatically by the system.
If a thread overflows its stack, unpredictable results may occur.
System-allocated stacks (including that of the initial thread)
are typically allocated in such a way that a
.Dv SIGSEGV
signal is delivered to the process when a stack overflows.
.Pp
Signals handlers are normally run on the stack of the currently executing
thread.
Hence, if you want to handle the
.Dv SIGSEGV
signal, you should make use of
.Xr sigaltstack 2
or
.Xr sigprocmask 2 .
.Sh ENVIRONMENT
.Bl -tag -width "PTHREAD_DEBUG"
.It Ev PTHREAD_DEBUG
Enables verbose
.Dv SIGINFO
signal output.
.It Ev LIBC_R_DEBUG
Display thread status every time the garbage collection thread runs,
approximately once every 10 seconds.   The status display verbosity is
controled by the
.Ev PTHREAD_DEBUG
environment variable.
.El
.Sh SEE ALSO
.Xr pthread_cleanup_pop 3 ,
.Xr pthread_cleanup_push 3 ,
.Xr pthread_cond_broadcast 3 ,
.Xr pthread_cond_destroy 3 ,
.Xr pthread_cond_init 3 ,
.Xr pthread_cond_signal 3 ,
.Xr pthread_cond_timedwait 3 ,
.Xr pthread_cond_wait 3 ,
.Xr pthread_create 3 ,
.Xr pthread_detach 3 ,
.Xr pthread_equal 3 ,
.Xr pthread_exit 3 ,
.Xr pthread_getspecific 3 ,
.Xr pthread_join 3 ,
.Xr pthread_key_create 3 ,
.Xr pthread_key_delete 3 ,
.Xr pthread_mutex_destroy 3 ,
.Xr pthread_mutex_init 3 ,
.Xr pthread_mutex_lock 3 ,
.Xr pthread_mutex_trylock 3 ,
.Xr pthread_mutex_unlock 3 ,
.Xr pthread_once 3 ,
.Xr pthread_rwlock_destroy 3 ,
.Xr pthread_rwlock_init 3 ,
.Xr pthread_rwlock_rdlock 3 ,
.Xr pthread_rwlock_unlock 3 ,
.Xr pthread_rwlock_wrlock 3 ,
.Xr pthread_rwlockattr_destroy 3 ,
.Xr pthread_rwlockattr_getpshared 3 ,
.Xr pthread_rwlockattr_init 3 ,
.Xr pthread_rwlockattr_setpshared 3 ,
.Xr pthread_self 3 ,
.Xr pthread_setspecific 3
.Sh STANDARDS
The user-level thread library provides functions that
conform to ISO/IEC 9945-1 ANSI/IEEE
.Pq Dq Tn POSIX
Std 1003.1 Second Edition 1996-07-12.
.Sh AUTHORS
John Birrell
.Pa ( jb@freebsd.org )
wrote the majority of the user level thread library.
.\" David Leonard did a fair bit too, but is far too modest.
.Sh BUGS
The library contains a scheduler that uses the
process virtual interval timer to pre-empt running threads.
This means that using
.Xr setitimer 2
to alter the process virtual timer will have undefined effects. The
.Dv SIGVTALRM
will never be delivered to threads in a process.
.Pp
Some pthread functions fail to work correctly when linked using the
.Fl g
option to
.Xr cc 1
or
.Xr gcc 1 .
The problems do not occur when linked using the
.Fl ggdb
option.