fchmodat(2) — Linux manual page


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

NAME         top

       chmod, fchmod, fchmodat - change permissions of a file

SYNOPSIS         top

       #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)):

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

           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:

DESCRIPTION         top

       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

       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.

       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:

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

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

RETURN VALUE         top

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

ERRORS         top

       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 is too long.

       ENOENT The file does not exist.

       ENOMEM Insufficient kernel memory was available.

              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

       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.

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

              flags specified AT_SYMLINK_NOFOLLOW, which is not supported.

VERSIONS         top

       fchmodat() was added to Linux in kernel 2.6.16; library support was
       added to glibc in version 2.4.

CONFORMING TO         top

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

       fchmodat(): POSIX.1-2008.

NOTES         top

   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

   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         top

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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2017-09-15                         CHMOD(2)

Pages that refer to this page: creat(2)open(2)openat(2)syscalls(2)signal-safety(7)symlink(7)