CHMOD(2)                   Linux Programmer's Manual                  CHMOD(2)

       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)):

           Since glibc 2.24:
               _POSIX_C_SOURCE >= 199309L
           Glibc 2.19 to 2.23
           Glibc 2.16 to 2.19:
               _BSD_SOURCE || _POSIX_C_SOURCE
           Glibc 2.12 to 2.16:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
                   _POSIX_C_SOURCE >= 200809L
           Glibc 2.11 and earlier:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500

           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:

       The  chmod()  and fchmod() system calls change a files mode bits.  (The
       file mode consists of the file permission bits  plus  the  set-user-ID,
       set-group-ID,  and sticky bits.)  These system calls differ only in how
       the file is specified:

       * chmod() changes the mode of the  file  specified  whose  pathname  is
         given in pathname, which is dereferenced if it is a symbolic link.

       * fchmod()  changes  the  mode of the file referred to by the open file
         descriptor fd.

       The new file mode is 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 ex-

       S_ISGID  (02000)  set-group-ID (set process effective group ID  on  ex-
                         ecve(2); mandatory locking, as described in fcntl(2);
                         take a new file's group from parent directory, as de-
                         scribed 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  direc-
                         tories,  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 er-
       ror 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 in-

       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 en-
       abled 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
       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).

       EPERM  The   file   is   marked   immutable   or   append-only.    (See

       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 ar-

   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.

       chmod(1), chown(2), execve(2), open(2), stat(2), inode(7), path_resolu-
       tion(7), symlink(7)

       This page is part of release 5.05 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at

Linux                             2017-09-15                          CHMOD(2)
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