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           #include <sys/stat.h>
           int chmod(const char *pathname, mode_t mode);
           int fchmod(int fd, mode_t mode);
           #include <fcntl.h>           /* Definition of AT_* constants */
           #include <sys/stat.h>
           int fchmodat(int dirfd, const char *pathname, mode_t mode, int flags);
       Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
               _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
               || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
               Since glibc 2.10:
                   _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
               Before glibc 2.10:


           The chmod() and fchmod() system calls change the permissions of a file.
           They differ only in how the file is specified:
           * chmod()  changes the permissions of the file specified whose pathname
             is given in pathname, which is dereferenced if it is a symbolic link.
           * fchmod()  changes the permissions of the file referred to by the open
             file descriptor fd.
           The new file permissions are specified in mode, which  is  a  bit  mask
           created by ORing together zero or more of the following:
           S_ISUID  (04000)  set-user-ID   (set   process  effective  user  ID  on
           S_ISGID  (02000)  set-group-ID  (set  process  effective  group  ID  on
                             execve(2);   mandatory   locking,   as  described  in
                             fcntl(2); take a new file's group from parent  direc-
                             tory, as described in chown(2) and mkdir(2))
           S_ISVTX  (01000)  sticky bit (restricted deletion flag, as described in
           S_IRUSR  (00400)  read by owner
           S_IWUSR  (00200)  write by owner
           S_IXOTH  (00001)  execute/search by others
           The effective UID of the calling process must match the  owner  of  the
           file,  or  the  process  must  be  privileged  (Linux: it must have the
           CAP_FOWNER capability).
           If the calling process is not privileged  (Linux:  does  not  have  the
           CAP_FSETID  capability),  and  the group of the file does not match the
           effective group ID of the process or one  of  its  supplementary  group
           IDs,  the  S_ISGID  bit  will be turned off, but this will not cause an
           error to be returned.
           As a security measure, depending on the filesystem, the set-user-ID and
           set-group-ID  execution  bits  may  be turned off if a file is written.
           (On Linux this  occurs  if  the  writing  process  does  not  have  the
           CAP_FSETID  capability.)   On  some filesystems, only the superuser can
           set the sticky bit, which may have a special meaning.  For  the  sticky
           bit,  and  for  set-user-ID  and  set-group-ID bits on directories, see
           On NFS filesystems, restricting the permissions will immediately influ-
           ence  already  open  files,  because  the access control is done on the
           server, but open files are maintained by the client.  Widening the per-
           missions  may  be  delayed  for  other  clients if attribute caching is
           enabled on them.
           The fchmodat() system call operates in exactly the same way as chmod(),
           except for the differences described here.
           If  the  pathname given in pathname is relative, then it is interpreted
           relative to the directory referred to  by  the  file  descriptor  dirfd
           (rather  than  relative to the current working directory of the calling
           process, as is done by chmod() for a relative pathname).
           If pathname is relative and dirfd is the special value  AT_FDCWD,  then
           pathname  is  interpreted  relative to the current working directory of
           the calling process (like chmod()).
           If pathname is absolute, then dirfd is ignored.
           flags can either be 0, or include the following flag:
                  If pathname is a symbolic link, do not dereference  it:  instead
                  operate  on  the link itself.  This flag is not currently imple-
           See openat(2) for an explanation of the need for fchmodat().


           On success, zero is returned.  On error, -1 is returned, and  errno  is
           ELOOP  Too  many symbolic links were encountered in resolving pathname.
                  pathname is too long.
           ENOENT The file does not exist.
           ENOMEM Insufficient kernel memory was available.
                  A component of the path prefix is not a directory.
           EPERM  The effective UID does not match the owner of the file, and  the
                  process   is  not  privileged  (Linux:  it  does  not  have  the
                  CAP_FOWNER capability).
           EROFS  The named file resides on a read-only filesystem.
           The general errors for fchmod() are listed below:
           EBADF  The file descriptor fd is not valid.
           EIO    See above.
           EPERM  See above.
           EROFS  See above.
           The same errors that occur for chmod() can also occur  for  fchmodat().
           The following additional errors can occur for fchmodat():
           EBADF  dirfd is not a valid file descriptor.
           EINVAL Invalid flag specified in flags.
                  pathname is relative and dirfd is a file descriptor referring to
                  a file other than a directory.
                  flags specified AT_SYMLINK_NOFOLLOW, which is not supported.


           fchmodat() was added to Linux in kernel  2.6.16;  library  support  was
           added to glibc in version 2.4.


           chmod(), fchmod(): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.
           fchmodat(): POSIX.1-2008.

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