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():
           _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  path‐
         name, 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 exe‐
       cution 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 inter‐
       preted 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_res‐
              olution(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 addi‐
       tional 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 ver‐
       sion 2.4.

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

       fchmodat(): POSIX.1-2008.

NOTES
   C library/kernel ABI 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
       chown(2), execve(2), open(2), stat(2), path_resolution(7), symlink(7)

COLOPHON
       This page is part of release 3.74 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/.



Linux                                       2014-08-19                                   CHMOD(2)