clone(2) Example

clone_uts_ns.c

#define _GNU_SOURCE
#include <sys/wait.h>
#include <sys/utsname.h>
#include <sched.h>
#include <string.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>

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

static int   /* start function for cloned child */
childFunc(void *arg)
{
    struct utsname uts;

    /* Change hostname in UTS namespace of child */
    if (sethostname(arg, strlen(arg)) == -1)
        errExit("sethostname");
    
    /* Retrieve and display hostname */
    if (uname(&uts) == -1)
        errExit("uname");

    printf("uts.nodename in child:  %s\n", uts.nodename);

    /* Keep the namespace open for a while, by sleeping.
        This allows some experimentation -- for example,
        another process might join the namespace. */

    sleep(200);

    return 0;    /* Child terminates now */
}

#define STACK_SIZE (1024 * 1024)    /* Stack size for cloned child */

int 
main(int argc, char *argv[])
{
    char *stack;                    /* Start of stack buffer */
    char *stackTop;                 /* End of stack buffer */
    pid_t pid;
    struct utsname uts;

    if (argc < 2) {
        fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]);
        exit(EXIT_SUCCESS);
    }

    /* Allocate memory to be used for the stack of the child */

    stack = mmap(NULL, STACK_SIZE, PROT_READ | PROT_WRITE,
                 MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
    if (stack == MAP_FAILED)
        errExit("mmap");

    stackTop = stack + STACK_SIZE;  /* Assume stack grows downward */

    /* Create child that has its own UTS namespace;
       child commences execution in childFunc() */
    
    pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
    if (pid == -1)
        errExit("clone");
    
    printf("clone() returned %jd\n", (intmax_t) pid);

    /* Parent falls through to here */
    sleep(1);     /* Give child time to change its hostname */

    /* Display hostname in parent's UTS namespace.
       This will be different from hostname in child's UTS namespace.  */

    if (uname(&uts) == -1)
        errExit("uname");

    printf("uts.nodename in parent: %s\n", uts.nodename);

    if (waitpid(pid, NULL, 0) == -1)    /* Wait for child */
        errExit("waitpid");

    printf("child has terminated\n");

    exit(EXIT_SUCCESS);    /* Child has already called exit() */
}