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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:
               _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:

       Given  a pathname for a file, open() returns a file descriptor, a small, nonnegative inte‐
       ger 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.  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 unaffected if pathname is subsequently removed or modified  to  refer  to  a  different
       file.  For further details on open file descriptions, see NOTES.

       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,  respec‐

       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

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

              The file is opened in append mode.  Before each write(2), the file offset is  posi‐
              tioned 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.

              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 descrip‐
              tor.  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  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  pro‐
              vide an equivalent of the O_CLOEXEC flag to deal with this problem.)

              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 par‐
              ent  directory;  see  the  mount  options  bsdgroups  and  sysvgroups  described in

              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  permis‐
              sions  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 syn‐
              chronously, but does not give the guarantees of the O_SYNC flag that data and  nec‐
              essary 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

              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.

              Write  operations  on  the file will complete according to the requirements of syn‐
              chronized 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  fdata‐
              sync(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

              (LFS) Allow files whose sizes cannot be represented in an off_t (but can be  repre‐
              sented  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  fea‐

       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.

              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.  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.

              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 call‐
              ing 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

              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.  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.

              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  descrip‐
              tor  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 syn‐
              chronized I/O file integrity completion (by 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",

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

              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  pro‐
              vided  in the ext2, ext3, ext4, UDF, Minix, and shmem filesystems.  XFS support was
              added in Linux 3.15.

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

       The  openat()  system call operates in exactly the same way as open(), except for the dif‐
       ferences 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  inter‐
       preted 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 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_reso‐

       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.


       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

       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.

              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

       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 directory.

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

              The filesystem containing pathname does not support O_TMPFILE.

              pathname refers to a regular file that is too large to be opened.  The  usual  sce‐
              nario   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.1-2001; 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.

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

              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  ver‐
       sion 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-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.

       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 sys‐
       tems 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.

   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 descrip‐
       tors consequently share the file offset and file status  flags.   Such  sharing  can  also
       occur  between  processes:  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(2) of a file creates a new open file description; thus, there  may  be  multiple
       open file descriptions corresponding to a file inode.

   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  syn‐
       chronized  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 guar‐
       antees of file integrity completion.

       To  understand  the difference between 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  time‐
       stamp  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

       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.

       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  com‐
       bination  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), read‐
       linkat(2), renameat(2), symlinkat(2), unlinkat(2), utimensat(2) mkfifoat(3),  and  scandi‐
       rat(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 descrip‐
       tor 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.)

       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.  Since Linux 2.6.0,
       alignment to the logical block size of the underlying storage (typically 512  bytes)  suf‐
       fices.  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 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 restric‐

       O_DIRECT support was added under Linux in kernel version 2.4.10.  Older Linux kernels sim‐
       ply  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 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 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

       Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when call‐
       ing 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.

       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), fifo(7), path_resolution(7), symlink(7)

       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                                       2014-10-02                                    OPEN(2)

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