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

NAME
       chmod, fchmod, fchmodat - change permissions of a file

SYNOPSIS
       #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)):

       fchmod():
           Since glibc 2.24:
               _POSIX_C_SOURCE >= 199309L
           Glibc 2.19 to 2.23
               _POSIX_C_SOURCE
           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

       fchmodat():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE

DESCRIPTION
       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 execve(2))

       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 unlink(2))

       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  exe-
       cution 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 inode(7).

       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.

   fchmodat()
       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  inter-
       preted 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:

       AT_SYMLINK_NOFOLLOW
              If  pathname is a symbolic link, do not dereference it: instead operate on the link
              itself.  This flag is not currently implemented.

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

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

ERRORS
       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_res-
              olution(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.

       ENAMETOOLONG
              pathname is too long.

       ENOENT The file does not exist.

       ENOMEM Insufficient kernel memory was available.

       ENOTDIR
              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 ioctl_iflags(2).)

       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 addi-
       tional errors can occur for fchmodat():

       EBADF  dirfd is not a valid file descriptor.

       EINVAL Invalid flag specified in flags.

       ENOTDIR
              pathname  is relative and dirfd is a file descriptor referring to a file other than
              a directory.

       ENOTSUP
              flags specified AT_SYMLINK_NOFOLLOW, which is not supported.

VERSIONS
       fchmodat() was added to Linux in kernel 2.6.16; library support was added to glibc in ver-
       sion 2.4.

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

       fchmodat(): POSIX.1-2008.

NOTES
   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 argument.

   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.

SEE ALSO
       chmod(1), chown(2), execve(2), open(2), stat(2), inode(7), path_resolution(7), symlink(7)

COLOPHON
       This  page  is  part of release 4.15 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 https://www.kernel.org/doc/man-pages/.

Linux                                       2017-09-15                                   CHMOD(2)