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


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


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


     The  open()  system  call opens the file specified by pathname.  If the
     specified file does not exist, it may optionally (if O_CREAT is  speci-
     fied in flags) be created by open().
     The  return  value of open() is a file descriptor, a small, nonnegative
     integer that is used in subsequent  system  calls  (read(2),  write(2),
     lseek(2), fcntl(2), etc.) to refer to the open file.  The file descrip-
     tor 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.  The open file description records the
     file  offset  and the file status flags (see below).  A file descriptor
     is a reference to an open file description;  this  reference  is  unaf-
     fected  if  pathname  is subsequently removed or modified to refer to a
     different file.  For further details on  open  file  descriptions,  see
     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_TRUNC.  The file status flags are all of the remaining  flags  listed
     below.   The  distinction between these two groups of flags is that the
     file creation flags affect the semantics of the open operation  itself,
     while  the  file  status  flags  affect the semantics of subsequent I/O
     operations.  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 fol-
            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).  The modification of the file  offset  and  the  write
            operation are performed as a single atomic step.
            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
            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  multi-
            threaded  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  descriptor 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 equiva-
            lent of the O_CLOEXEC flag to deal with this problem.)
            If pathname does not exist, create it as a regular file.
            The owner (user ID) of the new file is set to the effective user
            ID of the process.
            The  group ownership (group ID) of the new file is set either to
            the effective group ID of the process (System V semantics) or to
            the group ID of the parent directory (BSD semantics).  On Linux,
            the behavior depends on whether the set-group-ID mode bit is set
            on  the parent directory: if that bit is set, then BSD semantics
            apply; otherwise, System V semantics apply.  For  some  filesys-
            tems,  the behavior also depends on the bsdgroups and sysvgroups
            mount options described in mount(8)).
            The mode argument specifies the file mode bits be applied when 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
            mode is modified by the process's umask in the usual way: in the
            absence  of  a  default  ACL,  the  mode  of the created file is
            (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
            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
            According  to  POSIX, the effect when other bits are set in mode
            is unspecified.  On Linux, the following bits are  also  honored
            in mode:
            S_ISUID  0004000 set-user-ID bit
            S_ISGID  0002000 set-group-ID bit (see inode(7)).
            S_ISVTX  0001000 sticky bit (see inode(7)).
     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 guar-
            antee 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).
            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
            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  speci-
            fied  in  conjunction with O_CREAT, and pathname already exists,
            then open() fails with the error EEXIST.
            When these two flags are specified, symbolic links are not  fol-
            lowed: if pathname is a symbolic link, then open() fails regard-
            less of where the symbolic link points.
            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.
            (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  can  be employed only if one of the following condi-
            tions is true:
  • The effective UID of the process matches the owner UID of the


  • The calling process has the CAP_FOWNER capability in its user

namespace and the owner UID of the file has a mapping in the

            This  flag  is  intended for use by indexing or backup programs,
            where its use can significantly reduce the amount of disk activ-
            ity.   This  flag  may not be effective on all filesystems.  One
            example is NFS, where the server maintains the access time.
            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.
            If pathname is a symbolic link, then the open  fails,  with  the
            error  ELOOP.  Symbolic links in earlier components of the path-
            name will still be followed.  (Note that the  ELOOP  error  that
            can  occur in this case is indistinguishable from the case where
            an open fails because there are too many  symbolic  links  found
            while  resolving components in the prefix part of the pathname.)
            This flag is a FreeBSD extension, which was added  to  Linux  in
            version  2.1.126,  and  has  subsequently  been  standardized in
            See also O_PATH below.
            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.
            Note that this flag has no effect for regular  files  and  block
            devices;  that  is,  I/O  operations  will  (briefly) block when
            device activity is required, regardless of whether O_NONBLOCK is
            set.   Since  O_NONBLOCK  semantics  might  eventually be imple-
            mented, applications should not depend  upon  blocking  behavior
            when specifying this flag for regular files and block devices.
            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 opera-
            tions  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), ioctl(2), mmap(2))
            fail with the error EBADF.
            The following operations can be performed on the resulting  file
  • close(2).
  • fchdir(2), if the file descriptor refers to a directory

(since Linux 3.5).

  • fstat(2) (since Linux 3.6).
  • fstatfs(2) (since Linux 3.12).
  • Duplicating the file descriptor (dup(2), fcntl(2) F_DUPFD,


  • 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()

and the other "*at()" system calls. This includes linkat(2)

               with  AT_EMPTY_PATH  (or  via procfs using AT_SYMLINK_FOLLOW)
               even if the file is not a directory.
  • 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_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.
            Opening a file or directory with the  O_PATH  flag  requires  no
            permissions  on the object itself (but does require execute per-
            mission on the directories in the path  prefix).   Depending  on
            the  subsequent operation, a check for suitable file permissions
            may be performed (e.g., fchdir(2) requires execute permission on
            the  directory referred to by its file descriptor argument).  By
            contrast, obtaining a reference to a filesystem object by  open-
            ing it with the O_RDONLY flag requires that the caller have read
            permission on the object, even  when  the  subsequent  operation
            (e.g.,  fchdir(2), fstat(2)) does not require read permission on
            the object.
            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 oper-
            ate on the symbolic link.
            If  pathname  refers to an automount point that has not yet been
            triggered, so no other filesystem is mounted  on  it,  then  the
            call returns a file descriptor referring to the automount direc-
            tory without triggering a mount.  fstatfs(2) can then be used to
            determine  if  it  is,  in  fact, an untriggered automount point
            (.f_type == AUTOFS_SUPER_MAGIC).
            One use of O_PATH for regular files is to provide the equivalent
            of  POSIX.1's  O_EXEC  functionality.  This permits us to open a
            file for which we have execute permission but not  read  permis-
            sion,  and then execute that file, with steps something like the
                char  buf[PATH_MAX];   fd   =   open("some_prog",   O_PATH);
                snprintf(buf,  PATH_MAX, "/proc/self/fd/%d", fd); execl(buf,
                "some_prog", (char *) NULL);
            An O_PATH file descriptor can also be passed as the argument  of
     O_SYNC Write  operations  on  the  file  will complete according to the
            requirements of synchronized I/O file integrity  completion  (by
            contrast  with  the  synchronized  I/O data integrity completion
            provided by O_DSYNC.)
            By the time write(2) (or similar) returns, 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 regular 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, mak-
            ing it permanent, using code like the following:
                char path[PATH_MAX]; fd = open("/path/to/dir",  O_TMPFILE  |
                                        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",
            In this case, the open() mode argument determines the file  per-
            mission mode, as with O_CREAT.
            Specifying  O_EXCL in conjunction with O_TMPFILE prevents a tem-
            porary file from being linked into the filesystem in  the  above
            manner.   (Note  that the meaning of O_EXCL in this case is dif-
            ferent from the meaning of O_EXCL otherwise.)
            There are two main use cases for O_TMPFILE:
  • Improved tmpfile(3) functionality: race-free creation of tem-

porary files that (1) are automatically deleted when closed;

               (2) can never be reached via any pathname; (3) are  not  sub-
               ject 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 filesys-

               tem attributes  (fchown(2),  fchmod(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 ini-
            tial implementation, support was provided  in  the  ext2,  ext3,
            ext4,  UDF,  Minix,  and  shmem  filesystems.  Support for other
            filesystems has subsequently been added as follows:  XFS  (Linux
            3.15);  Btrfs  (Linux 3.16); F2FS (Linux 3.16); and ubifs (Linux
            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.
     A call to creat() is equivalent to calling open() with flags  equal  to
     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  referred  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.


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


     open(), openat(), and creat() can fail with the following errors:
     EACCES The requested access to the file is not allowed, or search  per-
            mission  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_resolu-
     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
     EEXIST pathname already exists and O_CREAT and O_EXCL were used.
     EFAULT pathname points outside your accessible address space.
     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 sig-
            nal 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.
     EINVAL O_CREAT was specified in flags and the final  component  ("base-
            name")  of the new file's pathname is invalid (e.g., it contains
            characters not permitted by the underlying filesystem).
     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 ver-
            sion 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 per-process limit on the number of open file descriptors has
            been reached (see the  description  of  RLIMIT_NOFILE  in  getr-
            pathname was too long.
     ENFILE The system-wide limit on the total number of open files has been
     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 ver-
            sion does not provide the O_TMPFILE functionality.
     ENOMEM The named file is a FIFO, but memory for the FIFO  buffer  can't
            be  allocated  because the per-user hard limit on memory alloca-
            tion for pipes has been reached and the  caller  is  not  privi-
            leged; see pipe(7).
     ENOMEM Insufficient kernel memory was available.
     ENOSPC pathname  was  to  be created but the device containing pathname
            has no room for the new file.
            A component used as a directory in pathname is not, in  fact,  a
            directory,  or  O_DIRECTORY was specified and pathname was not a
     ENXIO  O_NONBLOCK | O_WRONLY is set, the named file is a FIFO,  and  no
            process has the FIFO open for reading.
     ENXIO  The  file  is  a device special file and no corresponding device
            The filesystem containing pathname does not support O_TMPFILE.
            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  (1<<31)-1  bytes;  see also
            O_LARGEFILE above.  This is the error specified by  POSIX.1;  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.
     EPERM  The operation was prevented by a file seal; see fcntl(2).
     EROFS  pathname refers to a file on a read-only  filesystem  and  write
            access was requested.
            pathname  refers to an executable image which is currently being
            executed and write access was requested.
            pathname refers to a file that is currently in  use  as  a  swap
            file, and the O_TRUNC flag was specified.
            pathname  refers  to  a file that is currently being read by the
            kernel (e.g. for module/firmware loading), and write access  was
            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.
            pathname is a relative pathname and dirfd is a  file  descriptor
            referring to a file other than a directory.


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


     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-spe-
     cific.  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.


     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 typ-
     ically 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 implementa-
     tions.  On many systems the file is actually truncated.
     Note that open() can open device special files, but creat() cannot cre-
     ate 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.
     The files in the /proc/[pid]/fd directory show the open  file  descrip-
     tors   of   the   process   with   the  PID  pid.   The  files  in  the
     /proc/[pid]/fdinfo directory show even  more  information  about  these
     files  descriptors.   See  proc(5) for further details of both of these
 Open file descriptions
     The term open file description is the one used by POSIX to refer to the
     entries  in  the  system-wide  table of open files.  In other contexts,
     this object is variously also called an "open  file  object",  a  "file
     handle", an "open file table entry", or--in kernel-developer parlance--
     a struct file.
     When a file descriptor is duplicated (using  dup(2)  or  similar),  the
     duplicate refers to the same open file description as the original file
     descriptor, and the two file descriptors consequently  share  the  file
     offset and file status flags.  Such sharing can also occur between pro-
     cesses: a child process created via fork(2) inherits duplicates of  its
     parent's  file descriptors, and those duplicates refer to the same open
     file descriptions.
     Each open() of a file creates a new open file description; thus,  there
     may be multiple open file descriptions corresponding to a file inode.
     On  Linux,  one can use the kcmp(2) KCMP_FILE operation to test whether
     two file descriptors (in the same process  or  in  two  different  pro-
     cesses) refer to the same open file description.
 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 incor-
     rectly, 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 two types of completion,  con-
     sider 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 pro-
     vided 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.
 C library/kernel differences
     Since version 2.26, the glibc wrapper function for open()  employs  the
     openat() system call, rather than the kernel's open() system call.  For
     certain architectures, this is also true in glibc versions before 2.26.
     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
     Opening the read or write end of a FIFO blocks until the other  end  is
     also  opened  (by  another process or thread).  See fifo(7) for further
 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  file  descriptor  that can't be used for reading or writing.
     This nonstandard access mode is used by some Linux drivers to return  a
     file  descriptor  that  is to be used only for device-specific ioctl(2)
 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., execveat(2), 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), statx(2), symlinkat(2), unlinkat(2),  utimensat(2),  mkfi-
     foat(3),  and  scandirat(3)) address two problems with the older inter-
     faces that preceded them.  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().  Suppose, for  example,
     that  we  wish  to  create  the  file  dir1/dir2/xxx.dep  if  the  file
     dir1/dir2/xxx exists.  The problem is that between the existence  check
     and  the  file-creation  step,  dir1  or  dir2 (which might be symbolic
     links) could be modified to point to a different location.  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 (say)
     fstatat(2) and openat().  The use of the dirfd file descriptor also has
     other benefits:
  • the file descriptor is a stable reference to the directory, even if

the directory is renamed; and

  • the open file descriptor prevents the underlying filesystem from

being dismounted, just as when a process has a current working

        directory on a filesystem.
     Second,  openat()  allows  the  implementation of a per-thread "current
     working directory", via file descriptor(s) maintained by  the  applica-
     tion.   (This functionality can also be obtained by tricks based on the
     use of /proc/self/fd/dirfd, but less efficiently.)
     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-indepen-
     dent interface for an application to discover these restrictions for  a
     given  file  or  filesystem.  Some filesystems provide their own inter-
     faces for doing  so,  for  example  the  XFS_IOC_DIOINFO  operation  in
     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.  Since Linux 2.6.0, alignment to the logical block size
     of the underlying storage (typically 512 bytes) suffices.  The  logical
     block  size can be determined using the ioctl(2) BLKSSZGET operation or
     from the shell using the command:
         blockdev --getss
     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 cre-
     ated 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 intro-
     duced 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 imple-
     ment the flag, in which case open() fails with the error 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 behavior 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 config-
     ured 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  sub-


     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  functional-
     When  both  O_CREAT and O_DIRECTORY are specified in flags and the file
     specified by pathname does not exist, open() will create a regular file
     (i.e., O_DIRECTORY is ignored).


     chmod(2),  chown(2),  close(2),  dup(2),  fcntl(2),  link(2), lseek(2),
     mknod(2), mmap(2), mount(2), open_by_handle_at(2), read(2),  socket(2),
     stat(2),  umask(2),  unlink(2),  write(2),  fopen(3),  acl(5), fifo(7),
     inode(7), path_resolution(7), symlink(7)


     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

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