chmod(2)



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():
           _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
           || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

       fchmodat():
           Since glibc 2.10:
               _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE

DESCRIPTION
       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
                         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 direc-
                         tory, 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 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
       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
       stat(2).

       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.

   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 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:

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

       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_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.

       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).

       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.

       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 version 2.4.

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

       fchmodat(): POSIX.1-2008.

NOTES
   C library/kernel ABI 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
       chown(2), execve(2), open(2), stat(2), path_resolution(7), symlink(7)

COLOPHON
       This  page  is  part of release 3.71 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2014-08-19                          CHMOD(2)

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