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

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