man_2_open.txt

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

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
       The  open()  system  call  opens the file specified by pathname.  If the specified file
       does not exist, it may optionally (if O_CREAT is specified  in  flags)  be  created  by
       open().

       The  return  value of open() is a file descriptor, a small, nonnegative integer that is
       used in subsequent system calls (read(2), write(2), lseek(2), fcntl(2), etc.) to  refer
       to  the  open file.  The file descriptor returned by a successful call will be the low‐
       est-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,  re‐
       spectively.

       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 re‐
       maining 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 po‐
              sitioned 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 de‐
              scriptor.  This feature is available only for terminals, pseudoterminals,  sock‐
              ets,  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,  be‐
              cause  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 descrip‐
              tor  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 exe‐
              cution,  the race may lead to the file descriptor returned by open() being unin‐
              tentionally 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 pathname does not exist, create it as a regular file.

              The owner (user ID) of the new file is set to  the  effective  user  ID  of  the
              process.

              The  group  ownership  (group ID) of the new file is set either to the effective
              group ID of the process (System V semantics) or to the group ID  of  the  parent
              directory  (BSD  semantics).  On Linux, the behavior depends on whether the set-
              group-ID mode bit is set on the parent directory: if that bit is set,  then  BSD
              semantics apply; otherwise, System V semantics apply.  For some 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 de‐
              scriptor.

              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  syn‐
              chronously,  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 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
              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() fails with the error EEX‐
              IST.

              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.

              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 rep‐
              resented in an off64_t) to be opened.  The _LARGEFILE64_SOURCE macro must be de‐
              fined  (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 effec‐
              tive on all filesystems.  One example is NFS, where the server maintains the ac‐
              cess 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.  Sym‐
              bolic 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 I/O operations on the file  descriptor  which  is  returned  will
              cause the calling process to wait.

              Note  that  the  setting of this flag has no effect on the operation of poll(2),
              select(2), epoll(7), and similar,  since  those  interfaces  merely  inform  the
              caller about whether a file descriptor is "ready", meaning that an I/O operation
              performed on the file descriptor with the O_NONBLOCK flag clear would not block.

              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 specify‐
              ing 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  loca‐
              tion  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), if the file descriptor refers to a directory (since Linux 3.5).

              *  fstat(2) (since Linux 3.6).

              *  fstatfs(2) (since Linux 3.12).

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

              Opening  a file or directory with the O_PATH flag requires no permissions on the
              object itself (but does require execute permission on  the  directories  in  the
              path  prefix).  Depending on the subsequent operation, a check for suitable file
              permissions may be performed (e.g., fchdir(2) requires execute permission on the
              directory  referred to by its file descriptor argument).  By contrast, obtaining
              a reference to a filesystem object by opening it with the O_RDONLY flag requires
              that the caller have read permission on the object, even when the subsequent op‐
              eration (e.g., fchdir(2), fstat(2)) does not require read permission on the  ob‐
              ject.

              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), fs‐
              tatat(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  re‐
              ferring  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).

              One  use  of  O_PATH for regular files is to provide the equivalent of POSIX.1's
              O_EXEC functionality.  This permits us to open a file for which we have  execute
              permission but not read permission, and then execute that file, with steps some‐
              thing like the following:

                  char buf[PATH_MAX];
                  fd = open("some_prog", O_PATH);
                  snprintf(buf, PATH_MAX, "/proc/self/fd/%d", fd);
                  execl(buf, "some_prog", (char *) NULL);

              An O_PATH file descriptor can also be passed as the argument of fexecve(3).

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

       O_TMPFILE (since Linux 3.11)
              Create an unnamed temporary regular file.  The pathname argument specifies a di‐
              rectory;  an unnamed inode will be created in that directory's filesystem.  Any‐
              thing 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 tem‐
              porary  file  into the filesystem, making it permanent, using code like the fol‐
              lowing:

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

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

                  linkat(fd, NULL, AT_FDCWD, "/path/for/file", AT_EMPTY_PATH);

                  /* If the caller doesn't have the CAP_DAC_READ_SEARCH
                     capability (needed to use AT_EMPTY_PATH with linkat(2)),
                     and there is a proc(5) filesystem mounted, then the
                     linkat(2) call above can be replaced with:

                  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), fch‐
                 mod(2), fsetxattr(2), etc.)  before being atomically linked into the filesys‐
                 tem 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  path‐
       name).

       If  pathname  is relative and dirfd is the special value AT_FDCWD, then pathname is in‐
       terpreted 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 oc‐
       curred (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 informa‐
              tion.

       EINVAL Invalid value in flags.

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

       EINVAL O_CREAT was specified in flags and the final component ("basename") of  the  new
              file's  pathname  is  invalid (e.g., it contains characters not permitted by the
              underlying filesystem).

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

       EISDIR pathname  refers  to  an  existing  directory,  O_TMPFILE and one of O_WRONLY or
              O_RDWR were specified in flags, but this kernel 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.

       ENOENT 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 be‐
              cause 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.

       ENXIO  The file is a UNIX domain socket.

       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 re‐
              quested.

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

       ETXTBSY
              pathname  refers  to  a  file  that  is currently in use as a swap file, and the
              O_TRUNC flag was specified.

       ETXTBSY
              pathname refers to a file that is currently being read by the kernel  (e.g.  for
              module/firmware loading), 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  de‐
       fine _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 is sometimes used in cases where one wants to open but
       does  not  necessarily  have  the intention to read or write.  For example, this may be
       used to open a device 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 infor‐
       mation about these file descriptors.  See proc(5) for further details of both of  these
       directories.

       The  Linux  header  file <asm/fcntl.h> doesn't define O_ASYNC; the (BSD-derived) FASYNC
       synonym is defined instead.

   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  ker‐
       nel-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  de‐
       scriptors  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 de‐
       scriptions.

       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 descrip‐
       tors (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  de‐
       fines  O_RSYNC  to  have  the  same  value as O_SYNC.  (O_RSYNC is defined in the Linux
       header file <asm/fcntl.h> on HP PA-RISC, but it is not used.)

       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 pro‐
       vides 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  comple‐
       tion  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 modifica‐
       tion 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  meta‐
       data).

       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  synchro‐
       nized  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 bi‐
       nary  compatibility,  O_DSYNC was defined with the same value as the historical O_SYNC,
       and O_SYNC was defined as a new (two-bit) flag value that  includes  the  O_DSYNC  flag
       value.   This  ensures  that  applications  compiled  against  new headers get at least
       O_DSYNC semantics on pre-2.6.33 kernels.

   C library/kernel differences
       Since version 2.26, the glibc wrapper function for open() employs the  openat()  system
       call,  rather than the kernel's open() system call.  For certain architectures, this is
       also true in glibc versions before 2.26.

   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  per‐
       forms  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_han‐
       dle_at(2), readlinkat(2), renameat(2),  statx(2),  symlinkat(2),  unlinkat(2),  utimen‐
       sat(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 ex‐
       ample,  that we wish to create the file dir1/dir2/xxx.dep if the file dir1/dir2/xxx ex‐
       ists.  The problem is that between the existence check and the file-creation step, dir1
       or dir2 (which might be symbolic links) could be modified to point to a different loca‐
       tion.  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  do‐
       ing 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 off‐
       set 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_PRI‐
       VATE  flag;  this  includes  memory allocated on the heap and statically allocated buf‐
       fers).  Any such I/Os, whether submitted via an asynchronous I/O interface or from  an‐
       other  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  pro‐
       cesses.   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 restric‐
       tion  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 simi‐
       lar  to  those of Linux 2.4.  IRIX has also a fcntl(2) call to query appropriate align‐
       ments, and sizes.  FreeBSD 4.x introduced a flag of the same name, but  without  align‐
       ment 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,  in  which  case
       open() fails with the error EINVAL if it is used.

       Applications  should  avoid  mixing O_DIRECT and normal I/O to the same file, and espe‐
       cially to overlapping byte regions in the same file.  Even  when  the  filesystem  cor‐
       rectly 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  pro‐
       tocol  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 align‐
       ment 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.

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 deter‐
       mine 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 ig‐
       nored).

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 5.05 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/.

Linux                                     2020-02-09                                   OPEN(2)