PTHREADS(7) Linux Programmer's Manual PTHREADS(7)NAMEpthreads - POSIX threads
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 vari‐
ables).
POSIX.1 also requires that threads share a range of other attributes
(i.e., these attributes are process-wide rather than per-thread):
- process ID
- parent process ID
- process group ID and session ID
- controlling terminal
- user and group IDs
- open file descriptors
- record locks (see fcntl(2))
- signal dispositions
- file mode creation mask (umask(2))
- current directory (chdir(2)) and root directory (chroot(2))
- interval timers (setitimer(2)) and POSIX timers (timer_create())
- nice value (setpriority(2))
- resource limits (setrlimit(2))
- measurements of the consumption of CPU time (times(2)) and resources
(getrusage(2))
As well as the stack, POSIX.1 specifies that various other attributes
are distinct for each thread, including:
- thread ID (the pthread_t data type)
- signal mask (pthread_sigmask())
- the errno variable
- alternate signal stack (sigaltstack(2))
- real-time scheduling policy and priority (sched_setscheduler(2) and
sched_setparam(2))
The following Linux-specific features are also per-thread:
- capabilities (see capabilities(7))
- CPU affinity (sched_setaffinity(2))
Compiling on Linux
On Linux, programs that use the Pthreads API should be compiled using
cc -pthread.
Linux Implementations of POSIX Threads
Over time, two threading implementations have been provided by the GNU
C library on Linux:
- LinuxThreads This is the original (now obsolete) Pthreads implemen‐
tation.
- 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.
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 clone(2) system call.
In NPTL, thread synchronisation primitives (mutexes, thread joining,
etc.) are implemented using the Linux futex(2) system call.
Modern GNU C libraries provide both LinuxThreads and NPTL, with the
latter being the default (if supported by the underlying kernel).
LinuxThreads
The notable features of this implementation are the following:
- In addition to the main (initial) thread, and the threads that the
program creates using pthread_create(), the implementation creates a
"manager" thread. This thread handles thread creation and termina‐
tion. (Problems can result if this thread is inadvertently killed.)
- 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.
- 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 pro‐
cesses using ps(1).
The LinuxThreads implementation deviates from the POSIX.1 specification
in a number of ways, including the following:
- Calls to getpid(2) return a different value in each thread.
- Calls to getppid(2) in threads other than the main thread return the
process ID of the manager thread; instead getppid(2) in these
threads should return the same value as getppid(2) in the main
thread.
- When one thread creates a new child process using fork(2), any
thread should be able to wait(2) on the child. However, the imple‐
mentation only allows the thread that created the child to wait(2)
on it.
- When a thread calls 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 execve(2): it should have the same PID
as the main thread.
- Threads do not share user and group IDs. This can cause complica‐
tions with set-user-ID programs and can cause failures in Pthreads
functions if an application changes its credentials using seteuid(2)
or similar.
- Threads do not share a common session ID and process group ID.
- Threads do not share record locks created using fcntl(2).
- The information returned by times(2) and getrusage(2) is per-thread
rather than process-wide.
- Threads do not share semaphore undo values (see semop(2)).
- Threads do not share interval timers.
- Threads do not share a common nice value.
- 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 kill(2), for example) should be handled by a single, arbitrar‐
ily selected thread within the process. LinuxThreads does not sup‐
port the notion of process-directed signals: signals may only be
sent to specific threads.
- 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 alter‐
nate signal stack defined. If two threads handle signals on their
shared alternate signal stack at the same time, unpredictable pro‐
gram failures are likely to occur.)
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:
- Threads do not share a common nice value.
Some NPTL non-conformances only occur with older kernels:
- The information returned by times(2) and getrusage(2) is per-thread
rather than process-wide (fixed in kernel 2.6.9).
- Threads do not share resource limits (fixed in kernel 2.6.10).
- Threads do not share interval timers (fixed in kernel 2.6.12).
- Only the main thread is permitted to start a new session using set‐
sid(2) (fixed in kernel 2.6.16).
- Only the main thread is permitted to make the process into a process
group leader using setpgid(2) (fixed in kernel 2.6.16).
- 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 (fixed in kernel 2.6.16).
Determining the Threading Implementation
Since glibc 2.3.2, the getconf(1) command can be used to determine the
system's default threading implementation, for example:
bash$ getconf GNU_LIBPTHREAD_VERSION
NPTL 2.3.4
With older glibc versions, a command such as the following should be
sufficient to determine the default threading implementation:
bash$ $( ldd /bin/ls | grep libc.so | awk '{print $3}' ) | \
egrep -i 'threads|ntpl'
Native POSIX Threads Library by Ulrich Drepper et al
Selecting the Threading Implementation: LD_ASSUME_KERNEL
On systems with a glibc that supports both LinuxThreads and NPTL, the
LD_ASSUME_KERNEL environment variable can be used to override the
dynamic linker's default choice of threading implementation. This
variable tells the dynamic linker to assume that it is running on top
of a particular kernel version. By specifying a kernel version that
does not provide the support required by NPTL, we can force the use of
LinuxThreads. (The most likely reason for doing this is to run a (bro‐
ken) application that depends on some non-conformant behavior in Linux‐
Threads.) For example:
bash$ $( 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
SEE ALSOclone(2), futex(2), gettid(2), futex(7), and various Pthreads manual
pages, for example: pthread_atfork(3), pthread_cleanup_push(3),
pthread_cond_signal(3), pthread_cond_wait(3), pthread_create(3),
pthread_detach(3), pthread_equal(3), pthread_exit(3), pthread_key_cre‐
ate(3), pthread_kill(3), pthread_mutex_lock(3),
pthread_mutex_unlock(3), pthread_once(3), pthread_setcancelstate(3),
pthread_setcanceltype(3), pthread_setspecific(3), pthread_sigmask(3),
and pthread_testcancel(3).
Linux 2.6.12 2005-06-07 PTHREADS(7)
