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 processes in different PID namespaces can have the same PID. PID names-
paces allow containers to provide functionality such as suspending/resuming the set of processes in the container and migrating the con-
tainer to a new host while the processes inside the container maintain the same PIDs.
PIDs in a new PID namespace start at 1, somewhat like a standalone system, and calls to fork(2), vfork(2), or clone(2) will produce pro-
cesses with PIDs that are unique within the namespace.
Use of PID namespaces requires a kernel that is configured with the CONFIG_PID_NS option.
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 cre-
ated 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)). A child process that is orphaned within the namespace will be reparented to this process rather than init(1) (unless one of the
ancestors of the child in the same PID namespace employed the prctl(2) PR_SET_CHILD_SUBREAPER command to mark itself as the reaper of
orphaned descendant processes).
If the "init" process of a PID namespace terminates, the kernel terminates 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 subse-
quent fork(2) into this PID namespace fail with the error ENOMEM; it is not possible to create a new processes 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 corresponding 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 subsequently 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 handler 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 established 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 names-
pace 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
ancestor 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 namespace.
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
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 namespaces(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 thereafter.
Among other things, this means that the parental relationship between processes mirrors the parental relationship between PID namespaces:
the parent of a process is either in the same namespace or resides in the immediate parent 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 different 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 processes visible in the PID namespace of the process that performed the
mount, even if the /proc filesystem is viewed from processes in other namespaces.
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
namespace of the process that mounted the procfs). This can be useful for introspection purposes, when a process wants to discover its PID
in other namespaces.
/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 PID.
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_names-
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Linux 2017-11-26 PID_NAMESPACES(7)