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       setns - reassociate thread with a namespace


       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sched.h>

       int setns(int fd, int nstype);


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

       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:

       0      Allow any type of namespace to be joined.

              fd must refer to an IPC namespace.

              fd must refer to a network namespace.

              fd must refer to a UTS namespace.

       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.)


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


       EBADF  fd is not a valid file descriptor.

       EINVAL fd refers to a namespace whose type does not match that  specified  in  nstype,  or
              there is problem with reassociating the the thread with the specified namespace.

       ENOMEM Cannot allocate sufficient memory to change the specified namespace.

       EPERM  The  calling  thread  did  not have the required privilege (CAP_SYS_ADMIN) for this


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


       The setns() system call is Linux-specific.


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


       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.

           $ su                   # Need privilege for namespace operations
           # ./newuts bizarro &
           [1] 3549
           clone() returned 3550
           uts.nodename in child:  bizarro
           uts.nodename in parent: antero
           # uname -n             # Verify hostname in the shell

       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:

           # ./ns_exec /proc/3550/ns/uts /bin/bash
           # uname -n             # Executed in shell started by ns_exec

   Program source
       #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)

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

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

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

           if (setns(fd, 0) == -1)         /* Join that namespace */

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


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


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