pthreads

pthreads - POSIX threads

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):

- 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(3))

- 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(3))

- 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 R 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 implementation.

- 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 synchronization 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(3), the implementation creates a "manager" thread. This thread handles thread creation and termination. (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 processes 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 implementation 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 complications 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, arbitrarily selected thread within the process. LinuxThreads does not support 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 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.)

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 setsid(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).

Note the following further points about the NPTL implementation:

- If the stack size soft resource limit (see the description of RLIMIT_STACK in setrlimit(2)) is set to a value other than R unlimited , then this value defines the default stack size for new threads. To be effective, this limit must be set before the program is executed, perhaps using the ulimit -s shell built-in command (limit stacksize in the C shell).

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 (broken) application that depends on some non-conformant behavior in LinuxThreads.) 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

clone(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_create(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).