MirOS Manual: chroot(2), fchroot(2)

CHROOT(2)                  BSD Programmer's Manual                   CHROOT(2)


     chroot - change root directory


     #include <unistd.h>

     chroot(const char *dirname);

     fchroot(int fd);


     dirname is the address of the pathname of a directory, terminated by an
     ASCII NUL. chroot() causes dirname to become the root directory, that is,
     the starting point for path searches of pathnames beginning with '/'.

     In order for a directory to become the root directory a process must have
     execute (search) access for that directory.

     If the program is not currently running with an altered root directory,
     it should be noted that chroot() has no effect on the process's current

     If the program is already running with an altered root directory, the
     process's current directory is changed to the same new root directory.
     This prevents the current directory from being further up the directory
     tree than the altered root directory.

     This call is restricted to the superuser.

     The fchroot() function performs the same operation on an open directory
     file known by the file descriptor fd.


     Upon successful completion, a value of 0 is returned. Otherwise, a value
     of -1 is returned and errno is set to indicate an error.


     The following example changes the root directory to newroot, sets the
     current directory to the new root, and drops any setuid privileges.

           #include <err.h>
           #include <unistd.h>

           if (chroot(newroot) != 0 || chdir("/") != 0)
                   err(1, "%s", newroot);


     chroot() will fail and the root directory will be unchanged if:

     [ENOTDIR]     A component of the path name is not a directory.

                   A component of a pathname exceeded {NAME_MAX} characters,
                   or an entire path name exceeded {PATH_MAX} characters.

     [ENOENT]      The named directory does not exist.

     [EACCES]      Search permission is denied for any component of the path

     [ELOOP]       Too many symbolic links were encountered in translating the

     [EFAULT]      dirname points outside the process's allocated address

     [EIO]         An I/O error occurred while reading from or writing to the
                   file system.

     [EPERM]       The caller is not the superuser.

     fchroot() will fail and the root directory will be unchanged if:

     [EACCES]      Search permission is denied for the directory referenced by
                   the file descriptor.

     [EBADF]       The argument fd is not a valid file descriptor.

     [EIO]         An I/O error occurred while reading from or writing to the
                   file system.

     [ENOTDIR]     The argument fd does not reference a directory.

     [EPERM]       The effective user ID of the calling process is not the




     The chroot() function conforms to X/Open System Interfaces and Headers
     Issue 5 ("XSH5"), with the restriction that the calling process' working
     directory must be at or under the new root directory. Otherwise, the
     working directory is silently set to the new root directory; this is an
     extension to the standard.

     chroot() was declared a legacy interface, and subsequently removed in
     IEEE Std 1003.1-2001 ("POSIX").


     The chroot() function call appeared in 4.2BSD. The fchroot() function ap-
     peared in NetBSD 1.4 and was ported to MirOS #8.


     There are ways for a root process to escape from the chroot jail.

MirOS BSD #10-current           March 10, 2004                               1

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