man-pages/man7/pthreads.7

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.TH PTHREADS 7  2005-06-07 "Linux 2.6.12" "Linux Programmer's Manual"
.SH NAME
pthreads \- POSIX threads
.SH DESCRIPTION
POSIX.1 specifies a set of interfaces (functions, header files) for
threaded programming commonly known as POSIX threads, or Pthreads.
A single process can contain multiple threads,
all of which are executing the same program.
These threads share the same global memory (data and heap segments),
but each thread has its own stack (automatic variables).

POSIX.1 also requires that threads share a range of other attributes
(i.e., these attributes are process-wide rather than per-thread):
.IP \- 3
process ID
.IP \- 3
parent process ID
.IP \- 3
process group ID and session ID
.IP \- 3
controlling terminal
.IP \- 3
user and group IDs
.IP \- 3
open file descriptors
.IP \- 3
record locks (see
.BR fcntl (2))
.IP \- 3
signal dispositions
.IP \- 3
file mode creation mask
.RB ( umask (2))
.IP \- 3
current directory
.RB ( chdir (2))
and
root directory
.RB ( chroot (2))
.IP \- 3
interval timers
.RB ( setitimer (2))
and POSIX timers
.RB ( timer_create ())
.IP \- 3
nice value
.RB ( setpriority (2))
.IP \- 3
resource limits
.RB ( setrlimit (2))
.IP \- 3
measurements of the consumption of CPU time
.RB ( times (2))
and resources
.RB ( getrusage (2))
.PP
As well as the stack, POSIX.1 specifies that various other
attributes are distinct for each thread, including:
.IP \- 3
thread ID (the
.I pthread_t
data type)
.IP \- 3
signal mask
.RM ( pthread_sigmask ())
.IP \- 3
the
.I errno
variable
.IP \- 3
alternate signal stack
.RB ( sigaltstack (2))
.IP \- 3
real-time scheduling policy and priority
.RB ( sched_setscheduler (2)
and
.BR sched_setparam (2))
.PP
The following Linux-specific features are also per-thread:
.IP \- 3
capabilities (see
.BR capabilities (7))
.IP \- 3
CPU affinity
.RB ( sched_setaffinity (2))
.SS "Compiling on Linux"
On Linux, programs that use the Pthreads API should be compiled using
.IR "cc \-pthread" .
.SS "Linux Implementations of POSIX Threads"
Over time, two threading implementations have been provided by
the GNU C library on Linux:
.IP \- 3
.B LinuxThreads.
This is the original (now obsolete) Pthreads implementation.
.IP \- 3
.B NPTL
(Native POSIX Threads Library)
This is the modern Pthreads implementation.
By comparison with LinuxThreads, NPTL provides closer conformance to
the requirements of the POSIX.1 specification and better performance
when creating large numbers of threads.
NPTL requires features that are present in the Linux 2.6 kernel.
.PP
Both of these are so-called 1:1 implementations, meaning that each
thread maps to a kernel scheduling entity.

Both threading implementations employ the Linux
.BR clone (2)
system call.
In NPTL, thread synchronisation primitives (mutexes,
thread joining, etc.) are implemented using the Linux
.BR futex (2)
system call.
.PP
Modern GNU C libraries provide both LinuxThreads and NPTL, with the
latter being the default (if supported by the underlying kernel).
.SS LinuxThreads
The notable features of this implementation are the following:
.IP \- 3
In addition to the main (initial) thread, 
and the threads that the program creates using
.BR pthread_create (),
the implementation creates a "manager" thread.
This thread handles thread creation and termination.
(Problems can result if this thread is inadvertently killed.)
.IP \- 3
Signals are used internally by the implementation.
On Linux 2.2 and later, the first three real-time signals are used.
On older Linux kernels, SIGUSR1 and SIGUSR2 are used.
Applications must avoid the use of whichever set of signals is
employed by the implementation.
.IP \- 3
Threads do not share process IDs.
(In effect, LinuxThreads threads are implemented as processes which share
more information than usual, but which do not share a common process ID.)
LinuxThreads threads (including the manager thread)
are visible as separate processes using
.BR ps (1).
.PP
The LinuxThreads implementation deviates from the POSIX.1
specification in a number of ways, including the following:
.IP \- 3
Calls to
.BR getpid (2)
return a different value in each thread.
.IP \- 3
Calls to
.BR getppid (2)
in threads other than the main thread return the process ID of the
manager thread; instead
.BR getppid (2)
in these threads should return the same value as
.BR getppid (2)
in the main thread.
.IP \- 3
When one thread creates a new child process using
.BR fork (2),
any thread should be able to
.BR wait (2)
on the child.
However, the implementation only allows the thread that
created the child to
.BR wait (2)
on it.
.IP \- 3
When a thread calls
.BR execve (2),
all other threads are terminated (as required by POSIX.1).
However, the resulting process has the same PID as the thread that called
.BR execve (2):
it should have the same PID as the main thread.
.IP \- 3
Threads do not share user and group IDs.
This can cause complications with set-user-ID programs and
can cause failures in Pthreads functions if an application
changes its credentials using
.BR seteuid (2)
or similar.
.IP \- 3
Threads do not share a common session ID and process group ID.
.IP \- 3
Threads do not share record locks created using
.BR fcntl (2).
.IP \- 3
The information returned by
.BR times (2)
and
.BR getrusage (2)
is per-thread rather than process-wide.
.IP \- 3
Threads do not share semaphore undo values (see
.BR semop (2)).
.IP \- 3
Threads do not share interval timers.
.IP \- 3
Threads do not share a common nice value.
.IP \- 3
POSIX.1 distinguishes the notions of signals that are directed
to the process as a whole and signals are directed to individual
threads.
According to POSIX.1, a process-directed signal (sent using
.BR kill (2),
for example) should be handled by a single,
arbitrarily selected thread within the process.
LinuxThreads does not support the notion of process-directed signals:
signals may only be sent to specific threads.
.IP \- 3
Threads have distinct alternate signal stack settings.
However, a new thread's alternate signal stack settings
are copied from the thread that created it, so that
the threads initially share an alternate signal stack.
(A new thread should start with no alternate signal stack defined.
If two threads handle signals on their shared alternate signal
stack at the same time, unpredictable program failures are
likely to occur.)
.SS NPTL
With NPTL, all of the threads in a process are placed
in the same thread group;
all members of a thread groups share the same PID.
NPTL does not employ a manager thread.
NPTL makes internal use of the first two real-time signals;
these signals cannot be used in applications.

NPTL still has a few non-conformances with POSIX.1:
.IP \- 3
Threads have distinct alternate signal stack settings.
However, a new thread's alternate signal stack settings
are copied from the the thread that created it, so that
the threads initially share an alternate signal stack.
.IP \- 3
Threads do not share a common nice value.
.IP \- 3
Only the main thread is permitted to start a new session using
.BR setsid (2).
.\" FIXME why is there this limitation on setsid()?
.IP \- 3
Only the main thread is permitted to make the process into a
process group leader using
.BR setpgid (2).
.\" FIXME why is there this limitation on setpgid()?
.PP
Some NPTL non-conformances only occur with older kernels:
.IP \- 3
The information returned by
.BR times (2)
and
.BR getrusage (2)
is per-thread rather than process-wide (fixed in kernel 2.6.9).
.IP \- 3
Threads do not share resource limits (fixed in kernel 2.6.10).
.IP \- 3
Threads do not share interval timers (fixed in kernel 2.6.12).
.SS "Determining the Threading Implementation"
Since glibc 2.3.2, the
.BR getconf (1)
command can be used to determine
the system's default threading implementation, for example:
.nf
.in +4

bash$ getconf GNU_LIBPTHREAD_VERSION
NPTL 2.3.4
.in -4
.fi
.PP
With older glibc versions, a command such as the following should
be sufficient to determine the default threading implementation:
.nf
.in +4

bash$ $( ldd /bin/ls | grep libc.so | awk '{print $3}' ) | \\
                egrep \-i 'threads|ntpl'
        Native POSIX Threads Library by Ulrich Drepper et al
.in -4
.fi
.SS "Selecting the Threading Implementation: LD_ASSUME_KERNEL"
On systems with a glibc that supports both LinuxThreads and NPTL,
the LD_ASSUME_KERNEL environment variable can be used to override
the dynamic linker's default choice of threading implementation.
This variable tells the dynamic linker to assume that it is
running on top of a particular kernel version.
By specifying a kernel version that does not
provide the support required by NPTL, we can force the use
of LinuxThreads.
(The most likely reason for doing this is to run a
(broken) application that depends on some non-conformant behavior
in LinuxThreads.)
For example:
.nf
.in +4

bash$ $( LD_ASSUME_KERNEL=2.2.5 ldd /bin/ls | grep libc.so | \\
                awk '{print $3}' ) | egrep \-i 'threads|ntpl'
        linuxthreads-0.10 by Xavier Leroy
.in -4
.fi
.SH "SEE ALSO"
.BR clone (2),
.BR futex (2),
.BR gettid (2),
.BR futex (4),
and various Pthreads manual pages, for example:
.BR pthread_atfork (3),
.BR pthread_cleanup_push (3),
.BR pthread_cond_signal (3),
.BR pthread_cond_wait (3),
.BR pthread_create (3),
.BR pthread_detach (3),
.BR pthread_equal (3),
.BR pthread_exit (3),
.BR pthread_key_create (3),
.BR pthread_kill (3),
.BR pthread_mutex_lock (3),
.BR pthread_mutex_unlock (3),
.BR pthread_once (3),
.BR pthread_setcancelstate (3),
.BR pthread_setcanceltype (3),
.BR pthread_setspecific (3),
.BR pthread_sigmask (3),
and
.BR pthread_testcancel (3).