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():
           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
       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 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
       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).

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
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 4.15 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/.