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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:
               _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.  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,
       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, and 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 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).  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
              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  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 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 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 filesystems, 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 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

              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 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 can be employed only if one of the following conditions is true:

              *  The effective UID of the process matches the owner UID of the file.

              *  The  calling process has the CAP_FOWNER capability in its user namespace and the
                 owner UID of the file has a mapping in the namespace.

              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,  with  the  error  ELOOP.
              Symbolic links in earlier components of the pathname 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 POSIX.1-2008.

              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.

              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
              implemented, 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  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), ioctl(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); fstatfs(2)
                 (since Linux 3.12).

              *  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() 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
              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.

              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 directory 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).

       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)  (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 (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  initial  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 4.9)

       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()
       A  call  to  creat()   is   equivalent   to   calling   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 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.

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 per-process limit on the number of open file descriptors has been reached  (see
              the description of RLIMIT_NOFILE in getrlimit(2)).

       ENAMETOOLONG
              pathname was too long.

       ENFILE The system-wide 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 The named file is a FIFO, but memory for the FIFO buffer can't be allocated because
              the per-user hard limit on memory allocation for pipes has  been  reached  and  the
              caller is not privileged; 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.

       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 FIFO
              open for reading.

       ENXIO  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 (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.

       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 a relative pathname 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.

       The  files  in  the /proc/[pid]/fd directory show the open file descriptors 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 directories.

   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 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() 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  processes)  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  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 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.

   FIFOs
       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 details.

   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) 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.,  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),  mkfifoat(3),  and  scandirat(3))  address  two  problems  with  the   older
       interfaces  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 path/to/xxx.dep if the file path/to/xxx exists.  The problem is
       that  between  the  existence check and the file creation step, path or to (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 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.  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 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 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

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.

       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).

SEE ALSO

       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)

COLOPHON

       This page is part of release 4.13 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.