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man:fchmod

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():
         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  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
     inode(7).
     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).
     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.

SEE ALSO

     chmod(1), chown(2), execve(2), open(2), stat(2), inode(7), path_resolu-
     tion(7), symlink(7)

COLOPHON

     This  page  is  part of release 4.16 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/.

Linux 2017-09-15 CHMOD(2)

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