PID_NAMESPACES(7) Linux Programmer's Manual PID_NAMESPACES(7)
pid_namespaces - overview of Linux PID namespaces
For an overview of namespaces, see namespaces(7).
PID namespaces isolate the process ID number space, meaning that pro‐
cesses in different PID namespaces can have the same PID. PID names‐
paces allow containers to provide functionality such as suspend‐
ing/resuming the set of processes in the container and migrating the
container to a new host while the processes inside the container main‐
tain the same PIDs.
PIDs in a new PID namespace start at 1, somewhat like a standalone sys‐
tem, and calls to fork(2), vfork(2), or clone(2) will produce processes
with PIDs that are unique within the namespace.
Use of PID namespaces requires a kernel that is configured with the
The namespace init process
The first process created in a new namespace (i.e., the process created
using clone(2) with the CLONE_NEWPID flag, or the first child created
by a process after a call to unshare(2) using the CLONE_NEWPID flag)
has the PID 1, and is the "init" process for the namespace (see
init(1)). This process becomes the parent of any child processes that
are orphaned because a process that resides in this PID namespace ter‐
minated (see below for further details).
If the "init" process of a PID namespace terminates, the kernel termi‐
nates all of the processes in the namespace via a SIGKILL signal. This
behavior reflects the fact that the "init" process is essential for the
correct operation of a PID namespace. In this case, a subsequent
fork(2) into this PID namespace fail with the error ENOMEM; it is not
possible to create a new process in a PID namespace whose "init"
process has terminated. Such scenarios can occur when, for example, a
process uses an open file descriptor for a /proc/[pid]/ns/pid file cor‐
responding to a process that was in a namespace to setns(2) into that
namespace after the "init" process has terminated. Another possible
scenario can occur after a call to unshare(2): if the first child sub‐
sequently created by a fork(2) terminates, then subsequent calls to
fork(2) fail with ENOMEM.
Only signals for which the "init" process has established a signal han‐
dler can be sent to the "init" process by other members of the PID
namespace. This restriction applies even to privileged processes, and
prevents other members of the PID namespace from accidentally killing
the "init" process.
Likewise, a process in an ancestor namespace can—subject to the usual
permission checks described in kill(2)—send signals to the "init"
process of a child PID namespace only if the "init" process has estab‐
lished a handler for that signal. (Within the handler, the siginfo_t
si_pid field described in sigaction(2) will be zero.) SIGKILL or
SIGSTOP are treated exceptionally: these signals are forcibly delivered
when sent from an ancestor PID namespace. Neither of these signals can
be caught by the "init" process, and so will result in the usual
actions associated with those signals (respectively, terminating and
stopping the process).
Starting with Linux 3.4, the reboot(2) system call causes a signal to
be sent to the namespace "init" process. See reboot(2) for more
Nesting PID namespaces
PID namespaces can be nested: each PID namespace has a parent, except
for the initial ("root") PID namespace. The parent of a PID namespace
is the PID namespace of the process that created the namespace using
clone(2) or unshare(2). PID namespaces thus form a tree, with all
namespaces ultimately tracing their ancestry to the root namespace.
Since Linux 3.7, the kernel limits the maximum nesting depth for PID
namespaces to 32.
A process is visible to other processes in its PID namespace, and to
the processes in each direct ancestor PID namespace going back to the
root PID namespace. In this context, "visible" means that one process
can be the target of operations by another process using system calls
that specify a process ID. Conversely, the processes in a child PID
namespace can't see processes in the parent and further removed ances‐
tor namespaces. More succinctly: a process can see (e.g., send signals
with kill(2), set nice values with setpriority(2), etc.) only processes
contained in its own PID namespace and in descendants of that names‐
A process has one process ID in each of the layers of the PID namespace
hierarchy in which is visible, and walking back though each direct
ancestor namespace through to the root PID namespace. System calls
that operate on process IDs always operate using the process ID that is
visible in the PID namespace of the caller. A call to getpid(2) always
returns the PID associated with the namespace in which the process was
Some processes in a PID namespace may have parents that are outside of
the namespace. For example, the parent of the initial process in the
namespace (i.e., the init(1) process with PID 1) is necessarily in
another namespace. Likewise, the direct children of a process that
uses setns(2) to cause its children to join a PID namespace are in a
different PID namespace from the caller of setns(2). Calls to getp‐
pid(2) for such processes return 0.
While processes may freely descend into child PID namespaces (e.g.,
using setns(2) with a PID namespace file descriptor), they may not move
in the other direction. That is to say, processes may not enter any
ancestor namespaces (parent, grandparent, etc.). Changing PID names‐
paces is a one-way operation.
The NS_GET_PARENT ioctl(2) operation can be used to discover the
parental relationship between PID namespaces; see ioctl_ns(2).
setns(2) and unshare(2) semantics
Calls to setns(2) that specify a PID namespace file descriptor and
calls to unshare(2) with the CLONE_NEWPID flag cause children subse‐
quently created by the caller to be placed in a different PID namespace
from the caller. (Since Linux 4.12, that PID namespace is shown via
the /proc/[pid]/ns/pid_for_children file, as described in names‐
paces(7).) These calls do not, however, change the PID namespace of
the calling process, because doing so would change the caller's idea of
its own PID (as reported by getpid()), which would break many applica‐
tions and libraries.
To put things another way: a process's PID namespace membership is
determined when the process is created and cannot be changed there‐
after. Among other things, this means that the parental relationship
between processes mirrors the parental relationship between PID names‐
paces: the parent of a process is either in the same namespace or
resides in the immediate parent PID namespace.
A process may call unshare(2) with the CLONE_NEWPID flag only once.
After it has performed this operation, its /proc/PID/ns/pid_for_chil‐
dren symbolic link will be empty until the first child is created in
Adoption of orphaned children
When a child process becomes orphaned, it is reparented to the "init"
process in the PID namespace of its parent (unless one of the nearer
ancestors of the parent employed the prctl(2) PR_SET_CHILD_SUBREAPER
command to mark itself as the reaper of orphaned descendant processes).
Note that because of the setns(2) and unshare(2) semantics described
above, this may be the "init" process in the PID namespace that is the
parent of the child's PID namespace, rather than the "init" process in
the child's own PID namespace.
Compatibility of CLONE_NEWPID with other CLONE_* flags
In current versions of Linux, CLONE_NEWPID can't be combined with
CLONE_THREAD. Threads are required to be in the same PID namespace
such that the threads in a process can send signals to each other.
Similarly, it must be possible to see all of the threads of a processes
in the proc(5) filesystem. Additionally, if two threads were in dif‐
ferent PID namespaces, the process ID of the process sending a signal
could not be meaningfully encoded when a signal is sent (see the
description of the siginfo_t type in sigaction(2)). Since this is com‐
puted when a signal is enqueued, a signal queue shared by processes in
multiple PID namespaces would defeat that.
In earlier versions of Linux, CLONE_NEWPID was additionally disallowed
(failing with the error EINVAL) in combination with CLONE_SIGHAND
(before Linux 4.3) as well as CLONE_VM (before Linux 3.12). The
changes that lifted these restrictions have also been ported to earlier
/proc and PID namespaces
A /proc filesystem shows (in the /proc/[pid] directories) only pro‐
cesses visible in the PID namespace of the process that performed the
mount, even if the /proc filesystem is viewed from processes in other
After creating a new PID namespace, it is useful for the child to
change its root directory and mount a new procfs instance at /proc so
that tools such as ps(1) work correctly. If a new mount namespace is
simultaneously created by including CLONE_NEWNS in the flags argument
of clone(2) or unshare(2), then it isn't necessary to change the root
directory: a new procfs instance can be mounted directly over /proc.
From a shell, the command to mount /proc is:
$ mount -t proc proc /proc
Calling readlink(2) on the path /proc/self yields the process ID of the
caller in the PID namespace of the procfs mount (i.e., the PID names‐
pace of the process that mounted the procfs). This can be useful for
introspection purposes, when a process wants to discover its PID in
/proc/sys/kernel/ns_last_pid (since Linux 3.3)
This file displays the last PID that was allocated in this PID
namespace. When the next PID is allocated, the kernel will
search for the lowest unallocated PID that is greater than this
value, and when this file is subsequently read it will show that
This file is writable by a process that has the CAP_SYS_ADMIN
capability inside its user namespace. This makes it possible to
determine the PID that is allocated to the next process that is
created inside this PID namespace.
When a process ID is passed over a UNIX domain socket to a process in a
different PID namespace (see the description of SCM_CREDENTIALS in
unix(7)), it is translated into the corresponding PID value in the
receiving process's PID namespace.
Namespaces are a Linux-specific feature.
clone(2), reboot(2), setns(2), unshare(2), proc(5), capabilities(7),
credentials(7), mount_namespaces(7), namespaces(7), user_namespaces(7),
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Linux 2019-03-06 PID_NAMESPACES(7)