Threads and processes

A process is a program that is running. When a program starts, the system gives it a block of private memory that only it can see, a table of the files and other resources it holds open, and a name the system tracks it by. That running program, plus everything the system keeps for it, is a process. When the program finishes, its process goes away.

A process can also do several things at once by running more than one thread. A thread is a single line of execution — one sequence of steps the system is working through. A process always has at least one thread (its initial execution is its first thread), and it can start more. Every thread in a process shares the same private memory and the same open resources. What each thread keeps to itself is its own place in its sequence: each runs its own steps, at its own pace, possibly in parallel with the others.

The actors of the system #

Processes and threads are what carry out work on the system. Every action — opening a file, sending data over a network, starting another program — is performed by some thread. When the system decides whether an action is allowed, the question it answers is "is this thread allowed?" When it records that something happened, it records which thread did it.

A thread always acts as someone — a person, a service, or the system itself. Peios carries that identity along with the thread on an object called a token (see Tokens). Two facts matter here:

• Every thread is always acting as someone. There is no "nobody" state.
• When one process starts another, the new process begins acting as the same identity as the process that started it.

What a process has #

The things the system keeps for every process:

This is not the full list — a process also carries other per-process state, such as its Process Security Block (PSB), which holds its security-related settings.

Where to start #

Continue with The process and thread model for how a thread and a process relate, and why the thread — not the process — is the more basic unit, with a "process" being one particular way of using them.

This topic also covers:

• Creating processes — how a process starts another, and what the new one begins with.
• Process lifecycle — how a process ends, and how the system cleans up after it.
• Process relationships and job control — the process tree, process groups, and sessions (distinct from the logon sessions in Logon sessions).

A running program has two separable parts: the threads, which run, and the process, which contains them.

The thread is the part that runs — one line of execution, a single sequence of steps the system works through. A machine with several cores runs that many threads genuinely in parallel, one per core; a single core runs many threads by switching between them. Which thread runs, on which core, and for how long is decided by the system's scheduler — a subject of its own. Some tools and logs call a thread a task; it is the same thing.

The process is the boundary around a group of threads. It does not run — it contains. A process holds the things its threads share: their private memory, their open files and connections, and their identity. Every thread inside one process sees the same memory and the same open resources; threads in two different processes do not. That separation is what stops one program from reaching into another's memory or files.

The thread does the work; the process is the shared space the work happens in. A program that does one thing at a time is a process with a single thread. A program that does several things at once is a process with several threads, all sharing one space.

Why the thread is the more basic idea #

Threads, not processes, are the fundamental unit: what the system runs is threads, and a "process" is the answer to the question "what does this thread share, and with whom?" Bundle some threads together so they share one memory, one set of open resources, and one identity, and that bundle is a process. Give a thread its own fresh, separate memory instead of sharing an existing one, and you have made a new process.

This matters because it explains creation. Making a new thread and making a new process are the same mechanism with a different answer to "how much does the new line of execution share with the one that made it?" Share everything, and the result is another thread in the same process. Share nothing, and the result is a new process. Everything in between is possible too. How the operation is actually performed is covered in Creating processes.

What threads share, and what they don't #

Within one process, every thread shares:

Each thread keeps its own place in its sequence — which step it is on right now. That is the point of having more than one: each thread makes progress through its own work independently of the others.

There is one exception to the shared identity. A single thread can temporarily act as a different principal — for example, a service handling a request can act as the requester for that one piece of work, then revert. Only that one thread is affected, and only until it reverts. This is covered in Impersonation; here it is enough to know that "every thread in a process shares the process's identity" is the default, not an unbreakable rule.

Naming a process #

The system gives each process a number called its process ID, or PID, so that people and other programs can refer to it — to inspect it, signal it, or wait for it to finish. Threads are individually identifiable too, by a thread ID — see thread operations.

A PID is short and convenient, but it only names a process while that process exists. Once a process ends, its PID can later be reused for a completely different process. A PID tells you which process is which right now, but it is useless as a lasting record — two unrelated processes can hold the same PID at different times.

For that, every process also has a Process GUID (globally unique identifier): a name that is never reused, assigned to each process when it starts and fixed for the rest of its life. Where the PID is the convenient short handle for the moment, the Process GUID is the permanent one — it identifies that one specific process and no other, even long after it has ended. This is what lets a record of something that happened on the system name exactly the right process, with no chance of confusing two that happened to share a PID.

Both the PID and the GUID refer to a process. Neither says who the process is acting as — that is a separate question, answered by its identity rather than by any number or name.

Where to go next #

To see how a process or a thread is created — and what a new one starts out with — read Creating processes.

Every process on the system was started by another process — a process that is already running has to create it. Creation is built from two operations, which combine to cover every case.

Splitting: one process becomes two #

A running process can split into a copy of itself. The original keeps going, and a second process appears beside it that begins as a near-identical copy. The original is called the parent and the copy the child.

The child starts with:

• Its own private memory, copied from the parent. From the moment of the split the two memories are separate — a change one process makes is invisible to the other.
• Copies of the parent's open resources — the files and connections the parent had open, the child has open too.
• The same identity — the child begins acting as the same principal as its parent. (It inherits the parent's token; how the token carries across a split is in Token lifecycle.)

It also gets things of its own that are not copied: a new PID, and a new Process GUID — the child is its own distinct process, not a continuation of the parent. A few pieces of per-process state start fresh rather than being inherited, too.

This operation is called fork.

Replacing: same process, different program #

A process can also replace the program it is running with a different one. It remains the same process — same PID, same Process GUID, same identity — but from that point on it runs entirely different code. Whatever the previous program was doing is discarded, replaced wholesale by the new one.

This operation is called exec.

Putting them together #

The two operations combine to launch a different program as a new process: a process splits, and the child immediately replaces itself with the program to run. This split-then-replace pair is how most programs are started — when you run a command, something forks and the child execs your command. There is also a single combined step, spawn, that does both at once for the common case.

Underneath, splitting is one setting of a more general operation, clone, that lets the creator choose how much the new line of execution shares with it. Share everything, and the result is another thread in the same process; share almost nothing, and the result is a new process — the two ends of the range from the process and thread model. Everyday code rarely uses the general form directly; it makes threads or spawns programs, which are particular settings of it.

The exact contract — every clone sharing flag, precisely what fork passes to a child, and the variants of exec — is collected in the process creation reference.

Getting hold of the new process #

When a process creates a child, it usually needs to keep track of it — to wait for it to finish, or to send it a signal later. For this it can hold a handle to the child: a reference to that one specific process, called a pidfd. It is the same kind of handle the system hands out for an open file.

A process can be given a pidfd for its child at the moment it creates it, or ask for one for a process that already exists. Either way the pidfd is tied to that one specific process and can never be mistaken for another, so waiting on it or signalling through it always reaches the intended target.

This is what makes a pidfd safer than a bare PID. A PID can be reused once the process it named has ended; code that holds a bare PID risks later acting on a completely different process that inherited the number. A pidfd never has that problem — it refers to the one process it was created for and nothing else, for as long as the program holds it.

Where to go next #

A process that has been created eventually ends. How it finishes, and how the system cleans up after it, is covered in Process lifecycle.

Every process eventually ends. A process's life has three parts — it is created, it runs, and it finishes — and the system ensures that when it finishes, its result is collected and its resources are reclaimed.

The states it passes through #

While it exists, a process is in one of a handful of states, and the system moves it between them. Most of the time it is either running — executing, or ready to run the moment a core is free — or sleeping, paused while it waits for something to happen, such as data to arrive or a timer to fire. A sleeping process uses no processor time until what it is waiting for is ready; most processes spend most of their lives asleep.

A few states are worth knowing by name:

• Stopped — suspended (for example by job control when a job is paused at the terminal); it does nothing until told to resume.
• Traced — stopped under the control of a debugger that has attached to it, so the debugger can inspect and step it.
• Uninterruptible sleep — waiting on something the system will not interrupt, almost always a brief piece of disk or device I/O. A process here cannot be woken or even killed until the operation finishes; it normally lasts an instant, and a process stuck in it is a sign that hardware or a filesystem is stuck too.
• Zombie — finished, but still held as a result waiting to be collected (see below).

Once its result has been collected the process is gone, removed from the system entirely.

Two ways a process ends #

A process ends in one of two ways.

Most often it ends on its own: it finishes its work and stops. A process that ends this way leaves behind an exit status — a small record of how it went, usually just "succeeded" or "failed" (and, on failure, a number giving a rough reason). The act of a process ending itself is called exit.

The other way is that a process is stopped from outside — something tells it to stop, and it ends whether or not its work was done. (The messages that do this are signals, a subject of their own.) A process ended this way also leaves an exit status, marked to show it was stopped rather than finishing on its own.

Either way, a record of the process remains until its result is collected.

Collecting the result #

Whenever a child changes state — ends, is stopped, or resumes — the system sends its parent a signal, SIGCHLD, prompting it to check. The parent then reads the child's exit status. This is called waiting for the process.

Between the moment a process ends and the moment its parent collects the result, the finished process is a zombie: no longer running, but with the system still holding its exit status so the parent can read it. Despite the name, a zombie is harmless — it does no work and uses almost nothing; it is a result waiting to be collected. The instant the parent collects it, the zombie is gone for good.

If a parent never collects, zombies accumulate — finished processes whose results no one read. That is a fault in the parent, not normal behaviour.

A parent holding a pidfd for its child can wait on it directly — the dependable way to be told the exact moment the child ends.

The exact calls — every wait variant, and how an exit status is read — are in the process lifecycle reference.

When the parent ends first #

A parent does not always outlive its children. If a parent ends while one of its children is still running, that child becomes an orphan.

Orphans are not lost. The system's first process — PID 1, which on Peios is peinit — adopts every orphan and stands in as its parent from then on: when the orphan eventually ends, peinit collects its result, so no finished process is ever left uncollected. Adoption is automatic; the orphan keeps running unaffected, only with a new parent.

PID 1 is the catch-all, but a process can arrange to catch orphans within its own subtree. By marking itself a subreaper (with prctl(PR_SET_CHILD_SUBREAPER)), a process becomes the one that adopts any orphan descended from it — its grandchildren and below — instead of letting them reparent all the way up to PID 1. Service managers and container managers use this to keep responsibility for everything they launched: when something deep in their subtree loses its immediate parent, it reparents to the manager, which still learns when it finally ends.

What the system cleans up #

When a process ends, the system reclaims what it was using. Its private memory is freed and its open files and connections are closed.

It also releases its identity. A process acts as a principal, carried on its token; when the process ends, its hold on that token is released. If other processes were sharing the same identity — for example, siblings from the same login — the identity lives on for them, and the system clears it away only once nothing is using it any more. The details are in Token lifecycle.

The process's PID becomes available for the system to assign to a future process. Its Process GUID is not reused — it remains a permanent marker of that one process, so records of what it did still point unambiguously back to it long after it is gone.

Where to go next #

Processes are arranged into a family tree and grouped together in ways that matter for running them. That is Process relationships and job control.

Every process is related to others in two different ways: by descent — who started whom — and by grouping — which processes are handled together when they are controlled at a terminal.

The family tree #

Every process was started by another, so every process has a parent: the process that created it. A process can refer to its parent by the parent's PID, sometimes called the PPID (parent process ID).

Follow the parents upward and they all lead back to the same place — PID 1, the first process, which the system starts at boot. Every other process descends from it, directly or through a chain of parents, so the processes on a running system form a single family tree rooted at PID 1. This is the same tree that makes orphan adoption work: when a process loses its parent, PID 1 takes over the role, as covered in the lifecycle.

A few processes at the very root of the tree are special. PID 1 is the first process started at boot — on Peios, peinit — and the ancestor of all the rest; the system protects it, refusing to kill it with any signal it has not chosen to handle, because losing it would bring down the whole tree. Beneath the userspace processes the kernel runs threads of its own: PID 0 is the idle process (what a core runs when it has nothing else to do), and PID 2, kthreadd, is the parent of every kernel thread. The kernel's threads — among them the worker threads (kworker) that run deferred work and the threads that service hardware interrupts — appear in a process listing as the system's own work rather than as programs loaded from disk.

Process groups #

Several processes often do one job together. In a pipeline — the output of one command feeding into the next — each command is its own process, but they form one job: you stop, pause, or resume the whole job at once, not a piece at a time.

To make that possible, related processes are collected into a process group. A process group is a set of processes treated as a unit, so that an action — most often a signal, like the one that stops a job — can be delivered to all of them together instead of one by one.

Sessions and the terminal #

Process groups are themselves collected into a larger unit: a session, tied to the terminal the work is running at.

A terminal can only take input for one job at a time, so within the session one process group is the foreground group: the one connected to the keyboard, the one typing reaches, the one a stop-or-interrupt keystroke acts on. Any other groups are in the background, running without holding the keyboard. Moving a job between foreground and background — and starting, stopping, and resuming jobs — is job control.

A different kind of "session" #

The word "session" has two meanings here, and they are unrelated:

• A job-control session, described above, organises processes at a terminal — which groups share a terminal, which one is in the foreground. It says nothing about who anyone is.
• A logon session records one sign-in — a single authentication event — and ties together every identity token that came from it. It is part of the identity model, covered in Logon sessions.

They often line up in practice — signing in at a terminal creates a logon session, and the processes run there share a job-control session — but they are separate things answering separate questions: "which processes share this terminal?" versus "where did this identity come from?"

Where to go next #

Beyond its place in the tree and its groups, every process carries a bundle of its security-related settings — its Process Security Block. That is The Process Security Block.

A process's identity — who it is acting as — is carried on its token, and can change moment to moment, since a thread can impersonate another principal. A process has a second aspect that is independent of identity: what it is. What program is it running? How trusted is that program? How hardened is it against attack? Who is allowed to operate on it?

Those facts are gathered in one place — the Process Security Block, or PSB. Every process has one. Where the token answers who this process is acting as, the PSB answers what this process is. Unlike the token, the PSB never changes when a thread impersonates: impersonation changes who, never what.

What the PSB holds #

• Its permanent name. The process's Process GUID — the never-reused identifier — lives on the PSB.
• How trusted its program is. When a process starts running its program, the system checks the program's cryptographic signature and records from it how trusted the program is. This is the process's PIP (Process Integrity Protection) label, and it decides which other processes are allowed to inspect, signal, or interfere with this one. The barrier is based on what program is running, not on who is running it: even a fully privileged process cannot disturb a more-trusted one. The full treatment is Process integrity protection.
• How it is hardened. A set of mitigations — restrictions the process carries on what it may do with its own memory and code, so that a bug or injected code has far less room to do harm. They can only ever be tightened, never loosened. The catalog and rules are Process mitigations.
• Who may operate on it. Every process has its own security descriptor — the rules for who is allowed to act on the process itself: inspect it, signal it, and so on. It lives on the PSB alongside the rest.

A few more specialised settings live here too — a process can be marked so that it may no longer create children, for instance — but those four are the core.

Inspecting and managing the PSB #

Because the PSB is where a process's trust level and hardening live, there is a dedicated tool for working with it: the psb command, which inspects a process's PSB and manages its mitigations and PIP mode.

Where to go next #

The two largest parts of the PSB each have a topic of their own: Process integrity protection, for the trust label that governs which processes may interfere with which, and Process mitigations, for the self-hardening flags and how they are applied.

This is the detailed counterpart to Creating processes. That page explains the ideas; this one gives the exact contract — what each call does, what a new process inherits, and what each clone sharing flag controls.

fork #

fork() creates a new process by duplicating the caller. The child is a near-identical copy that runs independently from the point of the call.

The child inherits:

• a private copy of the parent's memory, made lazily (copy-on-write), so the duplication is cheap and changes on either side stay invisible to the other
• copies of the parent's open file descriptors — each refers to the same open file description, so file offset, status flags, and signal-driven-I/O settings are shared between parent and child
• open directory streams and open message-queue descriptors
• open-file-description locks (flock, OFD locks) through those shared descriptors
• the parent's signal dispositions (the handler set for each signal)
• the file-creation mask (umask), the current directory, and the root directory
• resource limits and the timer-slack value
• the parent's identity — it begins acting on the same token (see Token lifecycle)
• the parent's mitigations and protection, carried on its PSB

The child does not inherit:

• the parent's PID — it receives a new PID and a new Process GUID
• memory locks (mlock, mlockall)
• memory regions marked MADV_DONTFORK (absent in the child) or MADV_WIPEONFORK (present but zeroed)
• process resource usage and CPU-time counters — reset to zero
• pending signals — the child's set starts empty
• semaphore adjustments (semadj)
• process-associated record locks (POSIX fcntl locks — distinct from the OFD and flock locks above, which are shared through the inherited descriptors)
• timers (setitimer, alarm, timer_create)
• outstanding asynchronous I/O
• directory-change notifications (dnotify)
• the PR_SET_PDEATHSIG parent-death-signal setting (reset)
• I/O-port access permissions (ioperm)

The child's termination signal — what its parent is notified with when it ends — is SIGCHLD.

The child is created with a single thread — the one that called fork(). If the parent had other threads, they do not exist in the child.

vfork #

vfork() is a lightweight variant of fork() for the one case where the child will immediately replace itself with another program. It differs in two ways:

• the caller is suspended from the call until the child either execs or exits;
• the child runs in the parent's memory — no copy is made — until that point.

Because the two share memory and the parent is frozen, the child must do almost nothing first. The contract: the child must not return from the function that called vfork(), must not modify any variable other than the one holding the return value, and must not call any other function before a successful exec or _exit. Doing otherwise is undefined behaviour.

vfork is an optimisation for the create-then-replace pattern. Ordinary code should use posix_spawn (below) rather than reach for it directly.

clone and clone3 #

clone() is the general creation call. fork and thread creation are both clone with particular flag sets; clone exposes the choice directly. The caller gives a set of flags controlling what the new task shares with the creator, a stack for the new task, and locations for thread-ID and thread-local-storage bookkeeping.

clone3() is the modern form, taking a structure instead of positional arguments so it can grow new fields over time without changing the call:

The structure is size-versioned: the caller passes the size it knows, the kernel reads up to the size it knows, and any unknown trailing fields must be zero. This is how new fields are added without breaking existing programs.

Requesting a specific thread ID (set_tid) is a privileged operation — it exists for checkpoint-and-restore tools that must recreate a process with its original ID. The privilege required follows the capability model.

clone sharing flags #

Each flag makes the new task share something with its creator instead of getting its own copy. The flags that build threads and control creation:

A thread is CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND | CLONE_THREAD (plus TLS and thread-ID bookkeeping) — everything shared, same process. A fork is the opposite end, with almost nothing shared.

Being in one thread group has consequences beyond shared memory. Every thread returns the same PID from getpid (the thread-group ID), and signal actions are process-wide — an unhandled fatal signal delivered to any thread ends them all — though each thread keeps its own signal mask. A thread that ends does not notify the thread that created it and cannot be collected with a wait call; only once every thread in the group has ended is the process's parent sent SIGCHLD. The new thread shares the creator's parent, and CLONE_THREAD requires CLONE_SIGHAND (and hence CLONE_VM).

Two further flag groups select separation rather than sharing, each documented where it belongs:

• The CLONE_NEW* flags create namespaces — separate views of system resources (mounts, process IDs, networking, and more), covered under namespaces.
• CLONE_INTO_CGROUP places the new process into a resource group at creation, covered under resource management.

CLONE_DETACHED is a historical flag with no effect and is ignored.

The exec family #

An exec replaces the program running in the current process with a different one. The process keeps its PID, its Process GUID, and its identity — only the program changes (see Token lifecycle for the identity detail).

What survives an exec #

A successful exec keeps the process itself but replaces the program it runs. Preserved across the call:

• the PID, the parent PID, and the Process GUID
• the process's identity (its token), its process group and session, and its controlling terminal
• open file descriptors — except those marked close-on-exec, which are closed
• the current directory, root directory, file-creation mask, and resource limits
• pending signals, and the dispositions of signals that were ignored or left at their default

Reset or discarded:

• all memory — the program's mappings, stack, heap, and data are replaced; memory locks are dropped and MADV_* region markings are gone
• every thread except the one calling exec — the new program starts single-threaded
• handlers for caught signals — reset to the default; the alternate signal stack is dropped
• attached System V shared memory, POSIX shared-memory mappings, open POSIX message-queue descriptors and named semaphores, and in-process synchronisation objects (mutexes, condition variables)
• POSIX timers, outstanding asynchronous I/O, open directory streams, and registered exit handlers
• the floating-point environment, and the process name (set to the new program)

What an exec does to a process's identity when the program file is marked to run as another principal is not the Linux set-user-ID model — Peios handles that through the token, covered in setuid and uid0.

Whether a program is allowed to run at all is a separate, execution-policy question — covered under binary signing — not part of these calls.

posix_spawn #

posix_spawn() is the standard library function for the create-then-replace pattern: it makes a new process and runs a named program in it with a single call, with controls for arranging the child's file descriptors and a few attributes first. It is built on the calls above and is the recommended way to launch a program, in preference to assembling fork/vfork and exec by hand.

Process handles (the pidfd family) #

A pidfd is a file descriptor referring to one specific process — the reliable handle introduced in Creating processes. CLONE_PIDFD hands one back at creation; a few calls work with them afterwards:

pidfd_getfd reaches into another process, so it is not something any process may do to any other: it is gated by the right to act on the target, governed by that process's security descriptor.

See also #

• Creating processes — the conceptual counterpart to this page.
• Process lifecycle reference — ending a process and collecting its result.
• Thread operations reference — per-thread calls: TIDs, exit notification, TLS.

This is the detailed counterpart to Process lifecycle. That page explains how a process ends and is cleaned up; this one gives the exact calls — how a process terminates, what its exit status encodes, and how a parent collects it.

Ending a process #

A process can end itself in a few ways, differing in how much tidying happens first:

Returning from the program's entry point is equivalent to calling exit() with the returned value.

At the lowest level the raw _exit system call ends only the calling thread; the library _exit and ordinary termination route through exit_group so the whole process ends. Ending one thread while the rest of the process keeps running is the unusual case.

A process can also be ended from outside, by a signal rather than by a call of its own — covered under signals. Either way, it leaves an exit status behind.

The exit status #

When a process ends it leaves a small exit status for its parent to read. It records one of two outcomes:

• Ended on its own — carries an exit code: the low 8 bits of the value passed to exit/_exit (so 0–255, with 0 conventionally meaning success).
• Ended by a signal — carries the number of the signal that ended it, plus a flag for whether the process produced a core dump.

The status is a packed value, read not directly but through a set of macros:

Collecting the result #

A parent collects a finished child's status — and in doing so clears the zombie — with one of the wait calls. They differ in which child they target and how much they report:

waitpid's target is chosen by a single number: a positive value waits for that exact PID; -1 for any child; 0 for any child in the caller's own process group; and a value below -1 for any child in the process group whose ID is its absolute value.

Two further options matter for threads and other clone-created children, which may not notify their parent with SIGCHLD the way an ordinary child does: __WCLONE waits only for such clone children, and __WALL waits for every child regardless of type.

See also #

• Process lifecycle — the conceptual counterpart to this page.
• Process creation reference — fork, clone, exec, and the pidfd calls.

This is the thread-level companion to the process and thread model — the operations that act on an individual thread rather than on the process as a whole.

Thread IDs #

Every thread has its own thread ID (TID), returned by gettid. Within a multi-threaded process all threads share one PID — the value getpid returns, which is really the thread-group ID (TGID) — but each thread's TID is unique. In a single-threaded process the TID, PID, and TGID are all the same number.

The TID is how the system targets one specific thread rather than the whole process: sending a signal to a single thread, setting one thread's CPU affinity, or addressing it under /proc. It names a thread the way a PID names a process — and, like a PID, it is a reference, not an identity. What a thread is acting as is its token, never its TID.

The TID is also distinct from the opaque handle a threading library returns when it creates a thread; that is a library-level identifier, not the kernel's TID.

Thread-exit notification #

A thread can have the kernel notify waiters when it exits — the mechanism threading libraries build thread join on. set_tid_address sets a clear-tid address for the calling thread (and returns its TID). When that thread later terminates, if it shares memory with other threads, the kernel writes 0 to the integer at that address and wakes one thread waiting on it (a single futex wake).

A thread waiting for another to finish blocks on that address; the kernel clears it and wakes the waiter at the exact moment the other thread ends. This is the same machinery as the CLONE_CHILD_CLEARTID flag in process creation — that flag arranges the clear-tid address at thread creation, where set_tid_address sets it afterwards.

Thread-local storage #

Thread-local storage (TLS) gives each thread its own private copy of a variable — a value that is per-thread rather than shared across the process, even though the threads share the same memory. The per-thread data lives in an ordinary block of memory; what makes it per-thread is a register pointing at this thread's block, so the same code reaches a different copy depending on which thread is running.

The kernel's part is maintaining that per-thread pointer. It is set when the thread is created (the CLONE_SETTLS flag carries the address) and, on x86-64, a running thread can change it through arch_prctl with ARCH_SET_FS (the older set_thread_area serves the same purpose on 32-bit). Everything above that — how thread-local variables are laid out and reached in code — is handled by the compiler and the threading library.

See also #

• The process and thread model — the conceptual counterpart to this page.
• Process creation reference — the clone flags that create threads and arrange TLS and clear-tid addresses.