# open

## NAME



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


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <fcntl.h>**
    
           **int open(const char ***_path_**, int** _flags_**, ...**
                      /* **mode_t** _mode_ */ **);**
    
           **int creat(const char ***_path_**, mode_t** _mode_**);**
    
           **int openat(int** _dirfd_**, const char ***_path_**, int** _flags_**, ...**
                      /* **mode_t** _mode_ */ **);**
    
           /* Documented separately, in openat2(2): */
           **int openat2(int** _dirfd_**, const char ***_path_**,**
                      **const struct open_how ***_how_**, size_t** _size_**);**
    
       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 _path_.  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 an index to an entry in the process's
           table of open file descriptors.  The file descriptor 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 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 _path_ 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 ORed 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 _path_ 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 to be
                  applied when a new file is created.  If neither **O_CREAT** nor
                  **O_TMPFILE** is specified in _flags_ , then _mode_ is ignored (and
                  can thus be specified as 0, or simply omitted).  The _mode_
                  argument **must** be supplied if **O_CREAT** or **O_TMPFILE** is
                  specified in _flags_ ; if it is not supplied, some arbitrary
                  bytes from the stack will be applied as the file mode.
    
                  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 _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 _path_ is not a directory, cause the open to fail.  This
                  flag was added in Linux 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 VERSIONS.
    
           **O_EXCL** Ensure that this call creates the file: if this flag is
                  specified in conjunction with **O_CREAT** , and _path_ already
                  exists, then **open**() fails with the error **EEXIST**.
    
                  When these two flags are specified, symbolic links are not
                  followed: if _path_ 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 _path_
                  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 _path_ 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 the trailing component (i.e., basename) of _path_ 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 path prefix of the
                  pathname.)
    
                  This flag is a FreeBSD extension, which was added in Linux
                  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 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), 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 operation (e.g.,
                  fchdir(2), fstat(2)) does not require read permission on
                  the object.
    
                  If _path_ 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 _path_ 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**).
    
                  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
                  something 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 synchronized I/O _file_ integrity completion
                  (by contrast with the synchronized I/O _data_ integrity
                  completion provided by **O_DSYNC**.)
    
                  By the time write(2) (or similar) returns, the output data
                  and associated file metadata have been transferred to the
                  underlying hardware (i.e., as though each write(2) was
                  followed by a call to fsync(2)).  See VERSIONS.
    
           **O_TMPFILE**(since Linux 3.11)
                  Create an unnamed temporary regular file.  The _path_
                  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'... */
    
                      linkat(fd, "", 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), 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 tmpfs 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.
    
           The _dirfd_ argument is used in conjunction with the _path_ argument
           as follows:
    
           •  If the pathname given in _path_ is absolute, then _dirfd_ is
              ignored.
    
           •  If the pathname given in _path_ is relative and _dirfd_ is the
              special value **AT_FDCWD** , then _path_ is interpreted relative to
              the current working directory of the calling process (like
              **open**()).
    
           •  If the pathname given in _path_ 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).  In this case, _dirfd_ must be a directory
              that was opened for reading (**O_RDONLY**) or using the **O_PATH**
              flag.
    
           If the pathname given in _path_ is relative, and _dirfd_ is not a
           valid file descriptor, an error (**EBADF**) results.  (Specifying an
           invalid file descriptor number in _dirfd_ can be used as a means to
           ensure that _path_ is absolute.)
    
       **openat2(2)**
           The openat2(2) system call is an extension of **openat**(), and
           provides a superset of the features of **openat**().  It is documented
           separately, in openat2(2).
    


## RETURN VALUE



           On success, **open**(), **openat**(), and **creat**() return the new file
           descriptor (a nonnegative integer).  On error, -1 is returned and
           _errno_ is set to indicate the error.
    


## 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 _path_ , or the file did not exist yet and write
                  access to the parent directory is not allowed.  (See also
                  path_resolution(7).)
    
           **EACCES** Where **O_CREAT** is specified, the _protected_fifos_ or
                  _protected_regular_ sysctl is enabled, the file already
                  exists and is a FIFO or regular file, the owner of the file
                  is neither the current user nor the owner of the containing
                  directory, and the containing directory is both world- or
                  group-writable and sticky.  For details, see the
                  descriptions of _/proc/sys/fs/protected_fifos_ and
                  _/proc/sys/fs/protected_regular_ in proc_sys_fs(5).
    
           **EBADF**(**openat**()) _path_ is relative but _dirfd_ is neither **AT_FDCWD**
                  nor a valid file descriptor.
    
           **EBUSY  O_EXCL** was specified in _flags_ and _path_ refers to a block
                  device that is in use by the system (e.g., it is mounted).
    
           **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** _path_ already exists and **O_CREAT** and **O_EXCL** were used.
    
           **EFAULT** _path_ 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.
    
           **EINVAL O_CREAT** and **O_DIRECTORY** were both specified in _flags ,_ and
                  the Linux kernel version is 6.4 or later.  (Earlier kernels
                  were inconsistent in this area, and POSIX does not specify
                  the behavior.)
    
           **EINVAL O_CREAT** was specified in _flags_ and the final component
                  ("basename") of the new file's _path_ is invalid (e.g., it
                  contains characters not permitted by the underlying
                  filesystem).
    
           **EINVAL** The final component ("basename") of _path_ is invalid (e.g.,
                  it contains characters not permitted by the underlying
                  filesystem).
    
           **EISDIR** _path_ refers to a directory and the access requested
                  involved writing (that is, **O_WRONLY** or **O_RDWR** is set).
    
           **EISDIR** _path_ 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 _path_.
    
           **ELOOP** _path_ 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**
                  _path_ was too long.
    
           **ENFILE** The system-wide limit on the total number of open files has
                  been reached.
    
           **ENODEV** _path_ 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 _path_ does not exist or is a
                  dangling symbolic link.
    
           **ENOENT** _path_ 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** _path_ was to be created but the device containing _path_ has
                  no room for the new file.
    
           **ENOTDIR**
                  A component used as a directory in _path_ is not, in fact, a
                  directory, or **O_DIRECTORY** was specified and _path_ was not a
                  directory.
    
           **ENOTDIR**
                  (**openat**()) _path_ is a relative pathname and _dirfd_ is a file
                  descriptor referring to a file other than 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 _path_ does not support **O_TMPFILE**.
    
           **EOVERFLOW**
                  _path_ 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; before Linux 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** _path_ refers to a file on a read-only filesystem and write
                  access was requested.
    
           **ETXTBSY**
                  _path_ refers to an executable image which is currently being
                  executed and write access was requested.
    
           **ETXTBSY**
                  _path_ refers to a file that is currently in use as a swap
                  file, and the **O_TRUNC** flag was specified.
    
           **ETXTBSY**
                  _path_ 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)).
    


## VERSIONS



           The (undefined) effect of **O_RDONLY | O_TRUNC** varies among
           implementations.  On many systems the file is actually truncated.
    
       **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_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** 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 before Linux 2.6.33.
    
       **C library/kernel differences**
           Since glibc 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 before glibc
           2.26.
    
       **POSIX**
           POSIX.1-2024 specifies **O_CLOFORK** , but Linux doesn't support it.
    


## STANDARDS



           **open**()
           **creat**()
           **openat**()
                  POSIX.1-2008.
    
           openat2(2) Linux.
    
           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**.
    


## 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).
    
           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
           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
           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.
    
       **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),
           **fspick**(2), fstatat(2), futimesat(2), linkat(2), mkdirat(2),
           mknodat(2), mount_setattr(2), **move_mount**(2), name_to_handle_at(2),
           **open_tree**(2), openat2(2), readlinkat(2), renameat(2),
           renameat2(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
           _dir1/dir2/xxx.dep_ if the file _dir1/dir2/xxx_ exists.  The problem
           is that between the existence check and the file-creation step,
           _dir1_ or _dir2_ (which might be symbolic links) could be modified to
           point to a different location.  Such races can be avoided by
           opening a file descriptor for the target directory, and then
           specifying that file descriptor as the _dirfd_ argument of (say)
           fstatat(2) and **openat**().  The use of the _dirfd_ file descriptor
           also has other benefits:
    
           •  the file descriptor is a stable reference to the directory,
              even if the directory is renamed; and
    
           •  the open file descriptor prevents the underlying filesystem
              from being dismounted, just as when a process has a current
              working directory on a filesystem.
    
           Second, **openat**() allows the implementation of a per-thread
           "current working directory", via file descriptor(s) maintained by
           the application.  (This functionality can also be obtained by
           tricks based on the use of _/proc/self/fd/_ dirfd, but less
           efficiently.)
    
           The _dirfd_ argument for these APIs can be obtained by using **open**()
           or **openat**() to open a directory (with either the **O_RDONLY** or the
           **O_PATH** flag).  Alternatively, such a file descriptor can be
           obtained by applying dirfd(3) to a directory stream created using
           opendir(3).
    
           When these APIs are given a _dirfd_ argument of **AT_FDCWD** or the
           specified pathname is absolute, then they handle their pathname
           argument in the same way as the corresponding conventional APIs.
           However, in this case, several of the APIs have a _flags_ argument
           that provides access to functionality that is not available with
           the corresponding conventional APIs.
    
       **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.  The handling of misaligned **O_DIRECT**
           I/Os also varies; they can either fail with **EINVAL** or fall back to
           buffered I/O.
    
           Since Linux 6.1, **O_DIRECT** support and alignment restrictions for a
           file can be queried using statx(2), using the **STATX_DIOALIGN** flag.
           Support for **STATX_DIOALIGN** varies by filesystem; see statx(2).
    
           Some filesystems provide their own interfaces for querying
           **O_DIRECT** alignment restrictions, for example the **XFS_IOC_DIOINFO**
           operation in xfsctl(3).  **STATX_DIOALIGN** should be used instead
           when it is available.
    
           If none of the above is available, then direct I/O support and
           alignment restrictions can only be assumed from known
           characteristics of the filesystem, the individual file, the
           underlying storage device(s), and the kernel version.  In Linux
           2.4, most filesystems based on block devices require that the file
           offset and the length and memory address of all I/O segments be
           multiples of the filesystem block size (typically 4096 bytes).  In
           Linux 2.6.0, this was relaxed to the logical block size of the
           block device (typically 512 bytes).  A block device's 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 in Linux 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 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.
    


## 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.
    


* * *