DESCRIPTION

Given a file descriptor referring to a namespace, reassociate the calling thread with that namespace.

The fd argument is a file descriptor referring to one of the namespace entries in a /proc/[pid]/ns/ directory; see proc(5) for further information on /proc/[pid]/ns/. The calling thread will be reassociated with the corresponding namespace, subject to any constraints imposed by the nstype argument.

The nstype argument specifies which type of namespace the calling thread may be reassociated with. This argument can have one of the following values:

Specifying nstype as 0 suffices if the caller knows (or does not care) what type of namespace is referred to by fd. Specifying a nonzero value for nstype is useful if the caller does not know what type of namespace is referred to by fd and wants to ensure that the namespace is of a particular type. (The caller might not know the type of the namespace referred to by fd if the file descriptor was opened by another process and, for example, passed to the caller via a UNIX domain socket.)

RETURN VALUE

On success, setns() returns 0. On failure, −1 is returned and errno is set to indicate the error.

ERRORS

VERSIONS

The setns() system call first appeared in Linux in kernel 3.0; library support was added to glibc in version 2.14.

CONFORMING TO

The setns() system call is Linux-specific.

NOTES

Not all of the attributes that can be shared when a new thread is created using clone(2) can be changed using setns().

EXAMPLE

The program below takes two or more arguments. The first argument specifies the pathname of a namespace file in an existing /proc/[pid]/ns/ directory. The remaining arguments specify a command and its arguments. The program opens the namespace file, joins that namespace using setns(), and executes the specified command inside that namespace.

The following shell session demonstrates the use of this program (compiled as a binary named ns_exec) in conjunction with the CLONE_NEWUTS example program in the clone(2) man page (complied as a binary named newuts).

We begin by executing the example program in clone(2) in the background. That program creates a child in a separate UTS namespace. The child changes the hostname in its namespace, and then both processes display the hostnames in their UTS namespaces, so that we can see that they are different.

We then run the program shown below, using it to execute a shell. Inside that shell, we verify that the hostname is the one set by the child created by the first program:

#define _GNU_SOURCE
#include <fcntl.h>
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

int
main(int argc, char *argv[])
{
    int fd;

    if (argc < 3) {
        fprintf(stderr, "%s /proc/PID/ns/FILE cmd args...\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    fd = open(argv[1], O_RDONLY);   /* Get descriptor for namespace */
    if (fd == −1)
        errExit("open");

    if (setns(fd, 0) == −1)         /* Join that namespace */
        errExit("setns");

    execvp(argv[2], &argv[2]);      /* Execute a command in namespace */
    errExit("execvp");
}

SEE ALSO

clone(2), fork(2), vfork(2), proc(5), unix(7)