Ubuntu Manpage: chmod, fchmod, fchmodat - change permissions of a file

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
                         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
       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 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
       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 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_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 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.

COLOPHON

       This  page  is  part of release 4.04 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/.