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       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 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

       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 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

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

              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.)

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

              mode  specifies  the  mode 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 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 stat(2))

              S_ISVTX  0001000 sticky bit (see stat(2))

       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).

              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
              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.

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

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

              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 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

              For  the handling of FIFOs (named pipes), see also fifo(7).  For
              a discussion of the effect of  O_NONBLOCK  in  conjunction  with
              mandatory file locks and with file leases, see fcntl(2).

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

              The following operations can be performed on the resulting  file

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

              *  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(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 descriptor  can  be  used  as  the
              dirfd  argument  in calls to fchownat(2), fstatat(2), linkat(2),
              and readlinkat(2) with an  empty  pathname  to  have  the  calls
              operate on the symbolic link.

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

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

              There are two main use cases for O_TMPFILE:

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

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

              O_TMPFILE requires support by the underlying filesystem; only  a
              subset  of  Linux  filesystems  provide  that  support.   In the
              initial implementation, support was provided in the 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

       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
              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

       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
              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

              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
              version does not provide the O_TMPFILE functionality.

       ENOMEM Insufficient kernel memory was available.

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

              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.  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 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 (CAP_FOWNER).

       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.

              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-
       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 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.

   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(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 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.

       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 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)

   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),  symlinkat(2), unlinkat(2), utimensat(2), mkfifoat(3), and
       scandirat(3)) are supported for two reasons.  Here, the explanation  is
       in  terms  of the openat() call, but the rationale is analogous for the
       other interfaces.

       First, openat() allows an application to  avoid  race  conditions  that
       could  occur  when using open() to open files in directories other than
       the current working directory.  These race conditions result  from  the
       fact  that some component of the directory prefix given to open() could
       be changed in parallel with the call to open().  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().

       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) 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


       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

       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),
       path_resolution(7), symlink(7)


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