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NAME

       open, openat, creat - open and possibly create a file

SYNOPSIS

       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>

       int open(const char *pathname, int flags);
       int open(const char *pathname, int flags, mode_t mode);

       int creat(const char *pathname, mode_t mode);

       int openat(int dirfd, const char *pathname, int flags);
       int openat(int dirfd, const char *pathname, int flags, mode_t mode);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       openat():
           Since glibc 2.10:
               _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE

DESCRIPTION

       Given a pathname for a file, open() returns a file descriptor, a small,
       nonnegative integer  for  use  in  subsequent  system  calls  (read(2),
       write(2), lseek(2), fcntl(2), etc.).  The file descriptor returned by a
       successful  call  will  be  the  lowest-numbered  file  descriptor  not
       currently open for the process.

       By  default,  the  new  file descriptor is set to remain open across an
       execve(2) (i.e., the  FD_CLOEXEC  file  descriptor  flag  described  in
       fcntl(2)  is  initially  disabled; the O_CLOEXEC flag, described below,
       can be used to change this default).  The file offset  is  set  to  the
       beginning of the file (see lseek(2)).

       A  call  to open() creates a new open file description, an entry in the
       system-wide table of open files.  This entry records  the  file  offset
       and  the  file  status  flags  (modifiable  via  the  fcntl(2)  F_SETFL
       operation).  A file descriptor is a reference to one of these  entries;
       this  reference  is  unaffected  if pathname is subsequently removed or
       modified to refer to a different file.  The new open  file  description
       is  initially  not shared with any other process, but sharing may arise
       via fork(2).

       The argument flags must include one  of  the  following  access  modes:
       O_RDONLY,  O_WRONLY,  or  O_RDWR.  These request opening the file read-
       only, write-only, or read/write, respectively.

       In addition, zero or more file creation flags and file status flags can
       be  bitwise-or'd  in  flags.   The  file  creation flags are O_CLOEXEC,
       O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TMPFILE, O_TRUNC,
       and  O_TTY_INIT.   The file status flags are all of the remaining flags
       listed below.  The distinction between these two  groups  of  flags  is
       that  the  file  status  flags  can  be  retrieved  and (in some cases)
       modified; see fcntl(2) for details.

       The full list of file creation  flags  and  file  status  flags  is  as
       follows:

       O_APPEND
              The  file  is  opened in append mode.  Before each write(2), the
              file offset is positioned at the end of the  file,  as  if  with
              lseek(2).    O_APPEND   may  lead  to  corrupted  files  on  NFS
              filesystems if more than one process appends data to a  file  at
              once.  This is because NFS does not support appending to a file,
              so the client kernel has to simulate it,  which  can't  be  done
              without a race condition.

       O_ASYNC
              Enable  signal-driven  I/O: generate a signal (SIGIO by default,
              but this can be changed  via  fcntl(2))  when  input  or  output
              becomes  possible  on  this  file  descriptor.   This feature is
              available only  for  terminals,  pseudoterminals,  sockets,  and
              (since  Linux  2.6)  pipes  and FIFOs.  See fcntl(2) for further
              details.  See also BUGS, below.

       O_CLOEXEC (since Linux 2.6.23)
              Enable the close-on-exec  flag  for  the  new  file  descriptor.
              Specifying  this  flag  permits  a  program  to avoid additional
              fcntl(2) F_SETFD operations to set the FD_CLOEXEC flag.

              Note  that  the  use  of  this  flag  is   essential   in   some
              multithreaded   programs,  because  using  a  separate  fcntl(2)
              F_SETFD operation to set the FD_CLOEXEC flag does not suffice to
              avoid  race  conditions where one thread opens a file descriptor
              and attempts to set its close-on-exec flag using fcntl(2) at the
              same  time  as  another  thread  does  a fork(2) plus execve(2).
              Depending on the order of execution, the race may  lead  to  the
              file  desriptor  returned by open() being unintentionally leaked
              to the program executed by the child process created by fork(2).
              (This  kind of race is in principle possible for any system call
              that creates a file descriptor whose close-on-exec  flag  should
              be  set,  and  various  other  Linux  system  calls  provide  an
              equivalent of the O_CLOEXEC flag to deal with this problem.)

       O_CREAT
              If the file does not exist, it will be created.  The owner (user
              ID)  of the file is set to the effective user ID of the process.
              The group ownership (group ID) is set either  to  the  effective
              group  ID  of  the  process  or  to  the  group ID of the parent
              directory (depending on filesystem type and mount  options,  and
              the  mode  of  the  parent  directory;  see  the  mount  options
              bsdgroups and sysvgroups described in mount(8)).

              mode specifies the permissions to use in  case  a  new  file  is
              created.   This  argument  must  be  supplied  when  O_CREAT  or
              O_TMPFILE  is  specified  in  flags;  if  neither  O_CREAT   nor
              O_TMPFILE  is  specified,  then  mode is ignored.  The effective
              permissions are modified by the process's  umask  in  the  usual
              way:  The  permissions  of the created file are (mode & ~umask).
              Note that this mode applies only to future accesses of the newly
              created  file; the open() call that creates a read-only file may
              well return a read/write file descriptor.

              The following symbolic constants are provided for mode:

              S_IRWXU  00700 user (file owner) has  read,  write  and  execute
                       permission

              S_IRUSR  00400 user has read permission

              S_IWUSR  00200 user has write permission

              S_IXUSR  00100 user has execute permission

              S_IRWXG  00070 group has read, write and execute permission

              S_IRGRP  00040 group has read permission

              S_IWGRP  00020 group has write permission

              S_IXGRP  00010 group has execute permission

              S_IRWXO  00007 others have read, write and execute permission

              S_IROTH  00004 others have read permission

              S_IWOTH  00002 others have write permission

              S_IXOTH  00001 others have execute permission

       O_DIRECT (since Linux 2.4.10)
              Try  to minimize cache effects of the I/O to and from this file.
              In general this will degrade performance, but it  is  useful  in
              special  situations,  such  as  when  applications  do their own
              caching.  File I/O is done directly to/from user-space  buffers.
              The  O_DIRECT  flag  on its own makes an effort to transfer data
              synchronously, but does not give the guarantees  of  the  O_SYNC
              flag  that  data  and  necessary  metadata  are transferred.  To
              guarantee synchronous I/O, O_SYNC must be used  in  addition  to
              O_DIRECT.  See NOTES below for further discussion.

              A  semantically  similar  (but  deprecated)  interface for block
              devices is described in raw(8).

       O_DIRECTORY
              If pathname is not a directory, cause the open  to  fail.   This
              flag  was  added  in kernel version 2.1.126, to avoid denial-of-
              service problems if opendir(3) is  called  on  a  FIFO  or  tape
              device.

       O_DSYNC
              Write  operations  on  the  file  will complete according to the
              requirements of synchronized I/O data integrity completion.

              By the time write(2) (and similar) return, the output  data  has
              been transferred to the underlying hardware, along with any file
              metadata that would be required to retrieve that data (i.e.,  as
              though  each  write(2)  was followed by a call to fdatasync(2)).
              See NOTES below.

       O_EXCL Ensure that  this  call  creates  the  file:  if  this  flag  is
              specified  in  conjunction  with  O_CREAT,  and pathname already
              exists, then open() will fail.

              When these two flags  are  specified,  symbolic  links  are  not
              followed:  if  pathname  is  a  symbolic link, then open() fails
              regardless of where the symbolic link points to.

              In general, the behavior of O_EXCL is undefined if  it  is  used
              without  O_CREAT.   There  is  one  exception:  on Linux 2.6 and
              later, O_EXCL can be used without O_CREAT if pathname refers  to
              a  block  device.   If  the block device is in use by the system
              (e.g., mounted), open() fails with the error EBUSY.

              On NFS, O_EXCL is supported only when using NFSv3  or  later  on
              kernel  2.6  or later.  In NFS environments where O_EXCL support
              is not provided, programs that rely on it for performing locking
              tasks  will  contain  a  race condition.  Portable programs that
              want to perform atomic file locking using a lockfile,  and  need
              to avoid reliance on NFS support for O_EXCL, can create a unique
              file on the same filesystem (e.g.,  incorporating  hostname  and
              PID),  and  use  link(2)  to  make  a  link to the lockfile.  If
              link(2) returns 0,  the  lock  is  successful.   Otherwise,  use
              stat(2)  on  the  unique  file  to  check  if its link count has
              increased to 2, in which case the lock is also successful.

       O_LARGEFILE
              (LFS) Allow files whose sizes cannot be represented in an  off_t
              (but  can  be  represented  in  an  off64_t)  to be opened.  The
              _LARGEFILE64_SOURCE macro must be defined (before including  any
              header  files)  in order to obtain this definition.  Setting the
              _FILE_OFFSET_BITS feature test macro to 64  (rather  than  using
              O_LARGEFILE) is the preferred method of accessing large files on
              32-bit systems (see feature_test_macros(7)).

       O_NOATIME (since Linux 2.6.8)
              Do not update the file last access time (st_atime in the  inode)
              when  the  file  is  read(2).   This flag is intended for use by
              indexing or backup programs, where  its  use  can  significantly
              reduce  the  amount  of  disk  activity.   This  flag may not be
              effective on all filesystems.  One example  is  NFS,  where  the
              server maintains the access time.

       O_NOCTTY
              If  pathname  refers to a terminal device—see tty(4)—it will not
              become the process's controlling terminal even  if  the  process
              does not have one.

       O_NOFOLLOW
              If  pathname is a symbolic link, then the open fails.  This is a
              FreeBSD extension, which was added to Linux in version  2.1.126.
              Symbolic  links in earlier components of the pathname will still
              be followed.  See also O_PATH below.

       O_NONBLOCK or O_NDELAY
              When possible, the file is opened in nonblocking mode.   Neither
              the  open() nor any subsequent operations on the file descriptor
              which is returned will cause the calling process to  wait.   For
              the  handling  of  FIFOs (named pipes), see also fifo(7).  For a
              discussion of the  effect  of  O_NONBLOCK  in  conjunction  with
              mandatory file locks and with file leases, see fcntl(2).

       O_PATH (since Linux 2.6.39)
              Obtain  a  file descriptor that can be used for two purposes: to
              indicate a location  in  the  filesystem  tree  and  to  perform
              operations  that  act  purely at the file descriptor level.  The
              file itself is not opened,  and  other  file  operations  (e.g.,
              read(2),  write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2))
              fail with the error EBADF.

              The following operations can be performed on the resulting  file
              descriptor:

              *  close(2);  fchdir(2) (since Linux 3.5); fstat(2) (since Linux
                 3.6).

              *  Duplicating the file descriptor  (dup(2),  fcntl(2)  F_DUPFD,
                 etc.).

              *  Getting  and  setting file descriptor flags (fcntl(2) F_GETFD
                 and F_SETFD).

              *  Retrieving open file status flags using the fcntl(2)  F_GETFL
                 operation: the returned flags will include the bit O_PATH.

              *  Passing   the  file  descriptor  as  the  dirfd  argument  of
                 openat(2) and the other "*at()" system calls.

              *  Passing the file descriptor to another  process  via  a  UNIX
                 domain socket (see SCM_RIGHTS in unix(7)).

              When  O_PATH  is  specified  in  flags,  flag  bits  other  than
              O_DIRECTORY and O_NOFOLLOW are ignored.

              If pathname is a symbolic link and the O_NOFOLLOW flag  is  also
              specified,  then the call returns a file descriptor referring to
              the symbolic link.  This file descriptor  can  be  used  as  the
              dirfd  argument  in calls to fchownat(2), fstatat(2), linkat(2),
              and readlinkat(2) with an  empty  pathname  to  have  the  calls
              operate on the symbolic link.

       O_SYNC Write  operations  on  the  file  will complete according to the
              requirements of synchronized I/O file integrity  completion  (by
              contrast  with contrast with the synchronized I/O data integrity
              completion provided by O_DSYNC.)

              By the time write(2) (and similar) return, the output  data  and
              associated file metadata have been transferred to the underlying
              hardware (i.e., as though each write(2) was followed by  a  call
              to fsync(2)).  See NOTES below.

       O_TMPFILE (since Linux 3.11)
              Create   an  unnamed  temporary  file.   The  pathname  argument
              specifies a directory; an unnamed inode will be created in  that
              directory's  filesystem.  Anything written to the resulting file
              will be lost when the last file descriptor is closed, unless the
              file is given a name.

              O_TMPFILE  must be specified with one of O_RDWR or O_WRONLY and,
              optionally, O_EXCL.  If O_EXCL is not specified, then  linkat(2)
              can  be  used  to  link  the temporary file into the filesystem,
              making it permanent, using code like the following:

                  char path[PATH_MAX];
                  fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
                                          S_IRUSR | S_IWUSR);

                  /* File I/O on 'fd'... */

                  snprintf(path, PATH_MAX,  "/proc/self/fd/%d", fd);
                  linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
                                          AT_SYMLINK_FOLLOW);

              In this case, the  open()  mode  argument  determines  the  file
              permission mode, as with O_CREAT.

              Specifying  O_EXCL  in  conjunction  with  O_TMPFILE  prevents a
              temporary file from being linked  into  the  filesystem  in  the
              above  manner.  (Note that the meaning of O_EXCL in this case is
              different from the meaning of O_EXCL otherwise.)

              There are two main use cases for O_TMPFILE:

              *  Improved  tmpfile(3)  functionality:  race-free  creation  of
                 temporary  files  that  (1)  are  automatically  deleted when
                 closed; (2) can never be reached via any  pathname;  (3)  are
                 not  subject  to  symlink attacks; and (4) do not require the
                 caller to devise unique names.

              *  Creating a file that is initially invisible,  which  is  then
                 populated   with   data  and  adjusted  to  have  appropriate
                 filesystem  attributes  (chown(2),  chmod(2),   fsetxattr(2),
                 etc.)   before being atomically linked into the filesystem in
                 a fully formed state (using linkat(2) as described above).

              O_TMPFILE requires support by the underlying filesystem; only  a
              subset  of  Linux  filesystems  provide  that  support.   In the
              initial implementation, support was provided in the  ex2,  ext3,
              ext4,  UDF, Minix, and shmem filesystems.  XFS support was added
              in Linux 3.15.

       O_TRUNC
              If the file already exists and is a regular file and the  access
              mode  allows  writing  (i.e.,  is O_RDWR or O_WRONLY) it will be
              truncated to length 0.  If the file is a FIFO or terminal device
              file,  the  O_TRUNC  flag  is  ignored.  Otherwise the effect of
              O_TRUNC is unspecified.

   creat()
       creat()   is   equivalent   to   open()    with    flags    equal    to
       O_CREAT|O_WRONLY|O_TRUNC.

   openat()
       The  openat()  system  call operates in exactly the same way as open(),
       except for the differences described here.

       If the pathname given in pathname is relative, then it  is  interpreted
       relative  to  the  directory  relative  to by the file descriptor dirfd
       (rather than relative to the current working directory of  the  calling
       process, as is done by open() 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 open()).

       If pathname is absolute, then dirfd is ignored.

RETURN VALUE

       open(),  openat(), and creat() return the new file descriptor, or -1 if
       an error occurred (in which case, errno is set appropriately).

ERRORS

       open(), openat(), and creat() can fail with the following errors:

       EACCES The requested access to the  file  is  not  allowed,  or  search
              permission  is  denied  for  one  of the directories in the path
              prefix of pathname, or the file did  not  exist  yet  and  write
              access  to  the  parent  directory  is  not  allowed.  (See also
              path_resolution(7).)

       EDQUOT Where O_CREAT is specified, the file does  not  exist,  and  the
              user's quota of disk blocks or inodes on the filesystem has been
              exhausted.

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.

       EFBIG  See EOVERFLOW.

       EINTR  While blocked waiting to complete  an  open  of  a  slow  device
              (e.g.,  a  FIFO;  see  fifo(7)),  the  call was interrupted by a
              signal handler; see signal(7).

       EINVAL The filesystem does not support the O_DIRECT  flag.   See  NOTES
              for more information.

       EINVAL Invalid value in flags.

       EINVAL O_TMPFILE  was  specified  in  flags,  but  neither O_WRONLY nor
              O_RDWR was specified.

       EISDIR pathname refers to a directory and the access requested involved
              writing (that is, O_WRONLY or O_RDWR is set).

       EISDIR pathname  refers  to an existing directory, O_TMPFILE and one of
              O_WRONLY or O_RDWR were specified  in  flags,  but  this  kernel
              version does not provide the O_TMPFILE functionality.

       ELOOP  Too many symbolic links were encountered in resolving pathname.

       ELOOP  pathname was a symbolic link, and flags specified O_NOFOLLOW but
              not O_PATH.

       EMFILE The process already has the maximum number of files open.

       ENAMETOOLONG
              pathname was too long.

       ENFILE The system limit on the total number  of  open  files  has  been
              reached.

       ENODEV pathname  refers  to  a device special file and no corresponding
              device exists.  (This is a Linux kernel bug; in  this  situation
              ENXIO must be returned.)

       ENOENT O_CREAT  is  not  set  and the named file does not exist.  Or, a
              directory component in pathname does not exist or is a  dangling
              symbolic link.

       ENOENT pathname refers to a nonexistent directory, O_TMPFILE and one of
              O_WRONLY or O_RDWR were specified  in  flags,  but  this  kernel
              version does not provide the O_TMPFILE functionality.

       ENOMEM Insufficient kernel memory was available.

       ENOSPC pathname  was  to  be created but the device containing pathname
              has no room for the new file.

       ENOTDIR
              A component used as a directory in pathname is not, in  fact,  a
              directory,  or  O_DIRECTORY was specified and pathname was not a
              directory.

       ENXIO  O_NONBLOCK | O_WRONLY is set, the named file is a  FIFO  and  no
              process has the file open for reading.  Or, the file is a device
              special file and no corresponding device exists.

       EOPNOTSUPP
              The filesystem containing pathname does not support O_TMPFILE.

       EOVERFLOW
              pathname refers to a regular  file  that  is  too  large  to  be
              opened.  The usual scenario here is that an application compiled
              on a 32-bit platform  without  -D_FILE_OFFSET_BITS=64  tried  to
              open  a  file  whose  size  exceeds  (2<<31)-1  bits;  see  also
              O_LARGEFILE above.  This is the error specified by POSIX.1-2001;
              in  kernels  before  2.6.24, Linux gave the error EFBIG for this
              case.

       EPERM  The O_NOATIME flag was specified, but the effective user  ID  of
              the  caller  did  not match the owner of the file and the caller
              was not privileged (CAP_FOWNER).

       EROFS  pathname refers to a file on a read-only  filesystem  and  write
              access was requested.

       ETXTBSY
              pathname  refers to an executable image which is currently being
              executed and write access was requested.

       EWOULDBLOCK
              The O_NONBLOCK flag was specified, and an incompatible lease was
              held on the file (see fcntl(2)).

       The following additional errors can occur for openat():

       EBADF  dirfd is not a valid file descriptor.

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

VERSIONS

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

CONFORMING TO

       open(), creat() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

       openat(): POSIX.1-2008.

       The  O_DIRECT,  O_NOATIME,  O_PATH,  and  O_TMPFILE  flags  are  Linux-
       specific.  One must define _GNU_SOURCE to obtain their definitions.

       The O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW flags are not  specified  in
       POSIX.1-2001, but are specified in POSIX.1-2008.  Since glibc 2.12, one
       can obtain their definitions by defining either _POSIX_C_SOURCE with  a
       value  greater  than  or equal to 200809L or _XOPEN_SOURCE with a value
       greater than or equal to 700.  In glibc 2.11 and earlier,  one  obtains
       the definitions by defining _GNU_SOURCE.

       As  noted  in  feature_test_macros(7),  feature  test  macros  such  as
       _POSIX_C_SOURCE, _XOPEN_SOURCE, and _GNU_SOURCE must be defined  before
       including any header files.

NOTES

       Under  Linux,  the O_NONBLOCK flag indicates that one wants to open but
       does not necessarily have the intention to  read  or  write.   This  is
       typically  used  to  open devices in order to get a file descriptor for
       use with ioctl(2).

       The  (undefined)  effect   of   O_RDONLY   |   O_TRUNC   varies   among
       implementations.  On many systems the file is actually truncated.

       Note  that  open()  can  open  device special files, but creat() cannot
       create them; use mknod(2) instead.

       If the file is newly created, its st_atime, st_ctime,  st_mtime  fields
       (respectively,  time  of  last  access, time of last status change, and
       time of last modification; see stat(2)) are set to  the  current  time,
       and  so  are  the st_ctime and st_mtime fields of the parent directory.
       Otherwise, if the file is modified because of  the  O_TRUNC  flag,  its
       st_ctime and st_mtime fields are set to the current time.

   Synchronized I/O
       The POSIX.1-2008 "synchronized I/O" option specifies different variants
       of synchronized I/O, and specifies the open()  flags  O_SYNC,  O_DSYNC,
       and  O_RSYNC  for  controlling  the behavior.  Regardless of whether an
       implementation supports this option, it must at least support  the  use
       of O_SYNC for regular files.

       Linux  implements  O_SYNC  and  O_DSYNC,  but  not  O_RSYNC.  (Somewhat
       incorrectly, glibc defines O_RSYNC to have the same value as O_SYNC.)

       O_SYNC provides synchronized I/O  file  integrity  completion,  meaning
       write  operations  will  flush  data and all associated metadata to the
       underlying hardware.  O_DSYNC provides synchronized I/O data  integrity
       completion,  meaning write operations will flush data to the underlying
       hardware, but will only flush metadata updates  that  are  required  to
       allow  a  subsequent  read  operation  to  complete successfully.  Data
       integrity completion can reduce the number of disk operations that  are
       required  for  applications  that  don't  need  the  guarantees of file
       integrity completion.

       To understand the difference between the the two types  of  completion,
       consider  two  pieces  of  file  metadata:  the  file last modification
       timestamp (st_mtime) and the file length.  All  write  operations  will
       update  the  last file modification timestamp, but only writes that add
       data to the end of the file will change  the  file  length.   The  last
       modification  timestamp  is  not needed to ensure that a read completes
       successfully, but  the  file  length  is.   Thus,  O_DSYNC  would  only
       guarantee  to flush updates to the file length metadata (whereas O_SYNC
       would also always flush the last modification timestamp metadata).

       Before Linux 2.6.33, Linux implemented only the O_SYNC flag for open().
       However,  when  that  flag  was  specified,  most  filesystems actually
       provided the equivalent of synchronized I/O data  integrity  completion
       (i.e., O_SYNC was actually implemented as the equivalent of O_DSYNC).

       Since  Linux  2.6.33,  proper  O_SYNC support is provided.  However, to
       ensure backward binary compatibility, O_DSYNC was defined with the same
       value  as  the historical O_SYNC, and O_SYNC was defined as a new (two-
       bit) flag value that includes the O_DSYNC  flag  value.   This  ensures
       that  applications  compiled  against  new headers get at least O_DSYNC
       semantics on pre-2.6.33 kernels.

   NFS
       There are many infelicities in the protocol underlying  NFS,  affecting
       amongst others O_SYNC and O_NDELAY.

       On  NFS  filesystems with UID mapping enabled, open() may return a file
       descriptor but, for example, read(2) requests are denied  with  EACCES.
       This is because the client performs open() by checking the permissions,
       but UID mapping  is  performed  by  the  server  upon  read  and  write
       requests.

   File access mode
       Unlike the other values that can be specified in flags, the access mode
       values O_RDONLY, O_WRONLY, and O_RDWR do not specify  individual  bits.
       Rather,  they  define  the low order two bits of flags, and are defined
       respectively as 0, 1, and 2.  In other words, the combination  O_RDONLY
       |  O_WRONLY  is  a  logical error, and certainly does not have the same
       meaning as O_RDWR.

       Linux reserves the special, nonstandard access mode 3  (binary  11)  in
       flags  to  mean:  check  for  read and write permission on the file and
       return a descriptor that can't be used for reading  or  writing.   This
       nonstandard  access  mode  is  used  by  some Linux drivers to return a
       descriptor that  is  to  be  used  only  for  device-specific  ioctl(2)
       operations.

   Rationale for openat() and other directory file descriptor APIs
       openat()  and  the other system calls and library functions that take a
       directory    file    descriptor    argument    (i.e.,     faccessat(2),
       fanotify_mark(2),  fchmodat(2),  fchownat(2), fstatat(2), futimesat(2),
       linkat(2), mkdirat(2), mknodat(2), name_to_handle_at(2), readlinkat(2),
       renameat(2),  symlinkat(2),  unlinkat(2), utimensat(2) mkfifoat(3), and
       scandirat(3)) are supported for two reasons.  Here, the explanation  is
       in  terms  of the openat() call, but the rationale is analogous for the
       other interfaces.

       First, openat() allows an application to  avoid  race  conditions  that
       could  occur  when using open() to open files in directories other than
       the current working directory.  These race conditions result  from  the
       fact  that some component of the directory prefix given to open() could
       be changed in parallel with the call to  open().   Such  races  can  be
       avoided by opening a file descriptor for the target directory, and then
       specifying that file descriptor as the dirfd argument of openat().

       Second, openat() allows the implementation  of  a  per-thread  "current
       working   directory",   via   file   descriptor(s)  maintained  by  the
       application.  (This functionality can also be obtained by tricks  based
       on the use of /proc/self/fd/dirfd, but less efficiently.)

   O_DIRECT
       The  O_DIRECT  flag may impose alignment restrictions on the length and
       address of user-space buffers and the file offset of  I/Os.   In  Linux
       alignment  restrictions vary by filesystem and kernel version and might
       be    absent    entirely.     However    there    is    currently    no
       filesystem-independent  interface  for an application to discover these
       restrictions for a given file or filesystem.  Some filesystems  provide
       their  own  interfaces  for  doing  so, for example the XFS_IOC_DIOINFO
       operation in xfsctl(3).

       Under Linux 2.4, transfer sizes, and the alignment of the  user  buffer
       and  the file offset must all be multiples of the logical block size of
       the filesystem.  Under Linux  2.6,  alignment  to  512-byte  boundaries
       suffices.

       O_DIRECT  I/Os should never be run concurrently with the fork(2) system
       call, if the memory buffer is a  private  mapping  (i.e.,  any  mapping
       created  with  the  mmap(2)  MAP_PRIVATE  flag;  this  includes  memory
       allocated on the heap and  statically  allocated  buffers).   Any  such
       I/Os,  whether  submitted  via  an  asynchronous  I/O interface or from
       another thread in the process, should be completed  before  fork(2)  is
       called.   Failure  to do so can result in data corruption and undefined
       behavior in parent and child  processes.   This  restriction  does  not
       apply  when  the  memory buffer for the O_DIRECT I/Os was created using
       shmat(2)  or  mmap(2)  with  the  MAP_SHARED  flag.   Nor   does   this
       restriction   apply   when  the  memory  buffer  has  been  advised  as
       MADV_DONTFORK with madvise(2), ensuring that it will not  be  available
       to the child after fork(2).

       The  O_DIRECT  flag  was introduced in SGI IRIX, where it has alignment
       restrictions similar to those of Linux 2.4.  IRIX has also  a  fcntl(2)
       call   to   query  appropriate  alignments,  and  sizes.   FreeBSD  4.x
       introduced a flag of the same name, but without alignment restrictions.

       O_DIRECT support was added under Linux in kernel version 2.4.10.  Older
       Linux  kernels  simply  ignore  this  flag.   Some  filesystems may not
       implement the flag and open() will fail with EINVAL if it is used.

       Applications should avoid mixing O_DIRECT and normal I/O  to  the  same
       file,  and  especially  to  overlapping  byte regions in the same file.
       Even when the filesystem correctly handles the coherency issues in this
       situation,  overall  I/O  throughput  is likely to be slower than using
       either mode alone.  Likewise, applications should avoid mixing  mmap(2)
       of files with direct I/O to the same files.

       The  behaviour of O_DIRECT with NFS will differ from local filesystems.
       Older kernels, or kernels configured in certain ways, may  not  support
       this  combination.   The NFS protocol does not support passing the flag
       to the server, so O_DIRECT I/O will bypass the page cache only  on  the
       client; the server may still cache the I/O.  The client asks the server
       to make the I/O synchronous to preserve the  synchronous  semantics  of
       O_DIRECT.   Some servers will perform poorly under these circumstances,
       especially if the  I/O  size  is  small.   Some  servers  may  also  be
       configured  to  lie  to  clients  about  the  I/O having reached stable
       storage; this will avoid the performance penalty at some risk  to  data
       integrity  in  the event of server power failure.  The Linux NFS client
       places no alignment restrictions on O_DIRECT I/O.

       In summary, O_DIRECT is a potentially powerful tool that should be used
       with  caution.   It  is  recommended  that  applications  treat  use of
       O_DIRECT as a performance option which is disabled by default.

              "The thing that has always disturbed me about O_DIRECT  is  that
              the whole interface is just stupid, and was probably designed by
              a   deranged   monkey   on   some    serious    mind-controlling
              substances."—Linus

BUGS

       Currently, it is not possible to enable signal-driven I/O by specifying
       O_ASYNC when calling open(); use fcntl(2) to enable this flag.

       One must check for two different error codes, EISDIR and  ENOENT,  when
       trying   to   determine   whether   the   kernel   supports   O_TMPFILE
       functionality.

SEE ALSO

       chmod(2), chown(2),  close(2),  dup(2),  fcntl(2),  link(2),  lseek(2),
       mknod(2),  mmap(2),  mount(2),  open_by_name_at(2), read(2), socket(2),
       stat(2),   umask(2),   unlink(2),    write(2),    fopen(3),    fifo(7),
       path_resolution(7), symlink(7)

COLOPHON

       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



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