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       chmod, fchmod, fchmodat - change permissions of a file


       #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
                         directory, 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_IXUSR  (00100)  execute/search   by   owner   ("search"  applies  for
                         directories,  and  means  that  entries  within   the
                         directory can be accessed)

       S_IRGRP  (00040)  read by group

       S_IWGRP  (00020)  write by group

       S_IXGRP  (00010)  execute/search by group

       S_IROTH  (00004)  read by others

       S_IWOTH  (00002)  write by others

       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
       influence already open files, because the access control is done on the
       server,  but  open  files  are  maintained by the client.  Widening the
       permissions 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

       See openat(2) for an explanation of the need for fchmodat().


       On  success,  zero is returned.  On error, -1 is returned, and errno is
       set appropriately.


       Depending on the filesystem, errors other than those listed  below  can
       be returned.

       The more general errors for chmod() are listed below:

       EACCES Search  permission  is denied on a component of the path prefix.
              (See also path_resolution(7).)

       EFAULT pathname points outside your accessible address space.

       EIO    An I/O error occurred.

       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.


   C library/kernel differences
       The  GNU  C  library  fchmodat() wrapper function implements the POSIX-
       specified interface described in this  page.   This  interface  differs
       from  the  underlying  Linux  system  call, which does not have a flags

   Glibc notes
       On older kernels where fchmodat() is  unavailable,  the  glibc  wrapper
       function falls back to the use of chmod().  When pathname is a relative
       pathname, glibc constructs a pathname based on  the  symbolic  link  in
       /proc/self/fd that corresponds to the dirfd argument.


       chown(2), execve(2), open(2), stat(2), path_resolution(7), symlink(7)


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