Provided by: manpages-dev_6.03-1_all bug


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


       Standard C library (libc, -lc)


       #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);

       /* Documented separately, in openat2(2): */
       int openat2(int dirfd, const char *pathname,
                   const struct open_how *how, size_t size);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:


       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  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 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,

       In addition, zero or more file creation flags and file status flags can be bitwise-or'd in
       flags.  The file creation flags are O_CLOEXEC,  O_CREAT,  O_DIRECTORY,  O_EXCL,  O_NOCTTY,
       O_NOFOLLOW,  O_TMPFILE, and O_TRUNC.  The file status flags are all of the remaining flags
       listed below.  The distinction between these two groups of flags is that the file creation
       flags  affect  the  semantics  of  the  open operation itself, while the file status flags
       affect the semantics of subsequent I/O operations.  The file status flags can be retrieved
       and (in some cases) modified; see fcntl(2) for details.

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

              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

              Enable signal-driven I/O: generate a signal (SIGIO by  default,  but  this  can  be
              changed  via  fcntl(2))  when  input  or  output  becomes  possible  on  this  file
              descriptor.   This  feature  is  available  only  for  terminals,  pseudoterminals,
              sockets,  and (since Linux 2.6) pipes and FIFOs.  See fcntl(2) for further details.
              See also BUGS, below.

       O_CLOEXEC (since Linux 2.6.23)
              Enable the close-on-exec flag for the new file descriptor.   Specifying  this  flag
              permits  a  program  to  avoid  additional  fcntl(2)  F_SETFD operations to set the
              FD_CLOEXEC flag.

              Note that the use of this flag is essential in some multithreaded programs, because
              using  a  separate  fcntl(2)  F_SETFD operation to set the FD_CLOEXEC flag does not
              suffice to avoid race conditions where one  thread  opens  a  file  descriptor  and
              attempts  to  set its close-on-exec flag using fcntl(2) at the same time as another
              thread does a fork(2) plus execve(2).  Depending on the  order  of  execution,  the
              race  may  lead  to  the  file  descriptor returned by open() being unintentionally
              leaked to the program executed by the child process created by fork(2).  (This kind
              of race is in principle possible for any system call that creates a file descriptor
              whose close-on-exec flag should be  set,  and  various  other  Linux  system  calls
              provide an equivalent of the O_CLOEXEC flag to deal with this problem.)

              If 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 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

              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

              If  pathname  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.

              Write  operations  on  the  file  will  complete  according  to the requirements of
              synchronized I/O data integrity completion.

              By the time write(2) (and similar) return, the output data has been transferred  to
              the  underlying  hardware,  along  with any file metadata that would be required to
              retrieve that data (i.e., as though  each  write(2)  was  followed  by  a  call  to
              fdatasync(2)).  See NOTES below.

       O_EXCL Ensure  that  this  call creates the file: if this flag is specified in conjunction
              with O_CREAT, and pathname already exists, then open() fails with the error EEXIST.

              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

              (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

              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

              If pathname refers to a terminal device—see tty(4)—it will not become the process's
              controlling terminal even if the process does not have one.

              If the trailing component (i.e., basename) of pathname is a symbolic link, then the
              open fails, with the error ELOOP.  Symbolic links  in  earlier  components  of  the
              pathname will still be followed.  (Note that the ELOOP error that can occur in this
              case is indistinguishable from the case where an open fails because there  are  too
              many  symbolic  links  found  while  resolving components in the prefix part of the

              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.

              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 pathname is a symbolic link and the O_NOFOLLOW flag is also specified, then  the
              call  returns  a  file  descriptor  referring  to  the  symbolic  link.   This file
              descriptor can be used as the dirfd argument in calls to  fchownat(2),  fstatat(2),
              linkat(2),  and  readlinkat(2)  with an empty pathname to have the calls operate on
              the symbolic link.

              If pathname refers to an automount point that has not yet  been  triggered,  so  no
              other  filesystem  is  mounted  on  it,  then  the  call  returns a file descriptor
              referring to the automount directory without triggering a  mount.   fstatfs(2)  can
              then  be  used  to  determine  if  it  is,  in fact, an untriggered automount point
              (.f_type == AUTOFS_SUPER_MAGIC).

              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 NOTES below.

       O_TMPFILE (since Linux 3.11)
              Create  an  unnamed  temporary  regular  file.   The  pathname argument specifies a
              directory; an unnamed  inode  will  be  created  in  that  directory's  filesystem.
              Anything  written  to the resulting file will be lost when the last file descriptor
              is closed, unless the file is given a name.

              O_TMPFILE must be specified with one of O_RDWR or O_WRONLY and, optionally, O_EXCL.
              If  O_EXCL  is not specified, then linkat(2) can be used to link the temporary file
              into the filesystem, making it permanent, using code like the following:

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

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

                  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",

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

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

              There are two main use cases for O_TMPFILE:

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

              •  Creating a file that is initially invisible, which is then populated  with  data
                 and  adjusted  to  have appropriate filesystem attributes (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)

              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.

       A  call  to  creat()   is   equivalent   to   calling   open()   with   flags   equal   to

       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 pathname argument as follows:

       •  If the pathname given in pathname is absolute, then dirfd is ignored.

       •  If the pathname given in pathname is relative and dirfd is the special value  AT_FDCWD,
          then  pathname  is interpreted relative to the current working directory of the calling
          process (like open()).

       •  If the pathname given in pathname is relative, then it is interpreted relative  to  the
          directory referred to by the file descriptor dirfd (rather than relative to the current
          working directory of the  calling  process,  as  is  done  by  open()  for  a  relative
          pathname).   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 pathname 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 pathname is absolute.)

       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).


       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.


       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

       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(5).

       EBADF  (openat()) pathname is relative but dirfd is neither  AT_FDCWD  nor  a  valid  file

       EBUSY  O_EXCL  was specified in flags and pathname 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 pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.


       EINTR  While  blocked  waiting  to  complete  an  open of a slow device (e.g., a FIFO; see
              fifo(7)), the call was interrupted by a signal handler; see signal(7).

       EINVAL The filesystem does not support the O_DIRECT flag.  See NOTES for more information.

       EINVAL Invalid value in flags.

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

       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).

       EINVAL The final  component  ("basename")  of  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

       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)).

              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

       ENOMEM The named file is a FIFO, but memory for the FIFO buffer can't be allocated because
              the  per-user  hard  limit  on memory allocation for pipes has been reached and the
              caller is not privileged; see pipe(7).

       ENOMEM Insufficient kernel memory was available.

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

              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.

              (openat()) pathname 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.

              The filesystem containing pathname does not support O_TMPFILE.

              pathname refers to a regular file that is  too  large  to  be  opened.   The  usual
              scenario  here  is  that  an  application  compiled  on  a  32-bit platform without
              -D_FILE_OFFSET_BITS=64 tried to open a file whose size exceeds (1<<31)-1 bytes; see
              also  O_LARGEFILE  above.   This  is  the  error specified by POSIX.1; 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  pathname refers to a file on a read-only filesystem and write access was requested.

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

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

              pathname  refers  to  a  file that is currently being read by the kernel (e.g., for
              module/firmware loading), and write access was requested.

              The O_NONBLOCK flag was specified, and an incompatible lease was held on  the  file
              (see fcntl(2)).


       openat() was added in Linux 2.6.16; library support was added in glibc 2.4.


       open(), creat() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

       openat(): POSIX.1-2008.

       openat2(2) is Linux-specific.

       The  O_DIRECT, O_NOATIME, O_PATH, and O_TMPFILE flags are Linux-specific.  One must define
       _GNU_SOURCE to obtain their definitions.

       The O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW flags are not specified  in  POSIX.1-2001,  but
       are  specified  in  POSIX.1-2008.   Since  glibc 2.12, one can obtain their definitions by
       defining either _POSIX_C_SOURCE  with  a  value  greater  than  or  equal  to  200809L  or
       _XOPEN_SOURCE  with  a value greater than or equal to 700.  In glibc 2.11 and earlier, one
       obtains the definitions by defining _GNU_SOURCE.

       As  noted  in  feature_test_macros(7),  feature  test  macros  such  as   _POSIX_C_SOURCE,
       _XOPEN_SOURCE, and _GNU_SOURCE must be defined before including any header files.


       Under  Linux,  the  O_NONBLOCK flag 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
       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

       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

   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

       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.

       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.

       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

       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

       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

       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.


       Currently, it is not possible to enable  signal-driven  I/O  by  specifying  O_ASYNC  when
       calling open(); use fcntl(2) to enable this flag.

       One  must check for two different error codes, EISDIR and ENOENT, when trying to determine
       whether the kernel supports O_TMPFILE functionality.

       When both O_CREAT and O_DIRECTORY are  specified  in  flags  and  the  file  specified  by
       pathname does not exist, open() will create a regular file (i.e., O_DIRECTORY is ignored).


       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),  openat2(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)