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       open, creat - open and possibly create a file or device


       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>

       int open(const char *pathname, int flags);
       int open(const char *pathname, int flags, mode_t mode);

       int creat(const char *pathname, mode_t mode);


       Given  a  pathname  for  a  file,  open()  returns a file descriptor, a small, nonnegative
       integer for use in subsequent system calls (read(2), write(2), lseek(2), fcntl(2),  etc.).
       The  file  descriptor  returned  by  a  successful  call  will be the lowest-numbered file
       descriptor not currently open for the process.

       By default, the new file descriptor is set to remain open across an execve(2)  (i.e.,  the
       FD_CLOEXEC file descriptor flag described in fcntl(2) is initially disabled; the O_CLOEXEC
       flag, described below, can be used to change this default).  The file offset is set to the
       beginning of the file (see lseek(2)).

       A call to open() creates a new open file description, an entry in the system-wide table of
       open files.  This entry records the file offset and the file status flags (modifiable  via
       the  fcntl(2)  F_SETFL  operation).   A  file  descriptor  is  a reference to one of these
       entries; this reference is unaffected if pathname is subsequently removed or  modified  to
       refer to a different file.  The new open file description is initially not shared with any
       other process, but sharing may arise via fork(2).

       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_TRUNC, and O_TTY_INIT.  The file status flags are all of the remaining flags
       listed  below.   The distinction between these two groups of flags is that the file status
       flags can be retrieved and (in some cases) modified using fcntl(2).  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).   O_APPEND  may  lead  to
              corrupted  files on NFS filesystems if more than one process appends data to a file
              at once.  This is because NFS does not support appending to a file, so  the  client
              kernel has to simulate it, which can't be done without a race condition.

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

       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.  Additionally, use of this flag is essential in some multithreaded
              programs  since  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 at the same time as another thread does a fork(2) plus execve(2).

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

              mode specifies the permissions to use in case a new file is created.  This argument
              must  be  supplied when O_CREAT is specified in flags; if O_CREAT is not specified,
              then mode is ignored.  The effective permissions  are  modified  by  the  process's
              umask  in  the  usual way: The permissions of the created file are (mode & ~umask).
              Note that this mode applies only to future accesses of the newly created file;  the
              open()  call  that  creates  a  read-only  file  may  well return a read/write file

              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

       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 is Linux-
              specific, and was added in  kernel  version  2.1.126,  to  avoid  denial-of-service
              problems if opendir(3) is called on a FIFO or tape device.

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

              When these two flags are specified, symbolic links are not followed: if pathname is
              a symbolic link, then open() fails regardless of where the symbolic link points to.

              In  general,  the  behavior  of  O_EXCL is undefined if it is used without O_CREAT.
              There is one exception: on Linux 2.6 and later, O_EXCL can be used without  O_CREAT
              if  pathname refers to a block device.  If the block device is in use by the system
              (e.g., mounted), open() fails with the error EBUSY.

              On NFS, O_EXCL is supported only when using NFSv3 or later on kernel 2.6 or  later.
              In  NFS environments where O_EXCL support is not provided, programs that rely on it
              for performing locking tasks will contain a race condition.  Portable programs that
              want to perform atomic file locking using a lockfile, and need to avoid reliance on
              NFS support for O_EXCL, can create a unique file  on  the  same  filesystem  (e.g.,
              incorporating  hostname  and  PID), and use link(2) to make a link to the lockfile.
              If link(2) returns 0, the lock is successful.  Otherwise, use stat(2) on the unique
              file  to check if its link count has increased to 2, in which case the lock is also

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

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

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

              If pathname is a symbolic link, then the open fails.  This is a FreeBSD  extension,
              which  was added to Linux in version 2.1.126.  Symbolic links in earlier components
              of the pathname will still be followed.  See also O_NOPATH below.

              When possible, the file is opened in nonblocking mode.  Neither the open() nor  any
              subsequent  operations  on  the  file  descriptor  which is returned will cause the
              calling process to wait.  For  the  handling  of  FIFOs  (named  pipes),  see  also
              fifo(7).   For  a  discussion  of  the  effect  of  O_NONBLOCK  in conjunction with
              mandatory file locks and with file leases, see fcntl(2).

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

              The following operations can be performed on the resulting file descriptor:

              *  close(2); fchdir(2) (since Linux 3.5); fstat(2) (since Linux 3.6).

              *  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(2) and the other
                 "*at()" system calls.

              *  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_DIRECTORY and O_NOFOLLOW
              are ignored.

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

       O_SYNC The  file  is  opened  for  synchronous  I/O.   Any write(2)s on the resulting file
              descriptor will block the calling  process  until  the  data  has  been  physically
              written to the underlying hardware.  But see NOTES below.

              If  the  file already exists and is a regular file and the open 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.

       Some of these optional flags can be altered using fcntl(2) after the file has been opened.

       creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.


       open() and creat() return the new file descriptor, or -1 if an error  occurred  (in  which
       case, errno is set appropriately).


       EACCES The requested access to the file is not allowed, or search permission is denied for
              one of the directories in the path prefix of pathname, or the file  did  not  exist
              yet  and  write  access  to  the  parent  directory  is  not  allowed.   (See  also

       EDQUOT Where O_CREAT is specified, the file does not exist, and the user's quota  of  disk
              blocks or inodes on the filesystem has been 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.

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

       ELOOP  Too  many  symbolic links were encountered in resolving pathname, or O_NOFOLLOW was
              specified but pathname was a symbolic link.

       EMFILE The process already has the maximum number of files open.

              pathname was too long.

       ENFILE The system limit on the total number of open files has been reached.

       ENODEV pathname refers to a device special file and no corresponding device exists.  (This
              is a Linux kernel bug; in this situation ENXIO must be returned.)

       ENOENT O_CREAT is not set and the named file does not exist.  Or, a directory component in
              pathname does not exist or is a dangling symbolic link.

       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.

       ENXIO  O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no process has the  file
              open  for  reading.   Or,  the  file  is a device special file and no corresponding
              device exists.

              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 (2<<31)-1 bits;  see
              also  O_LARGEFILE  above.   This is the error specified by POSIX.1-2001; in kernels
              before 2.6.24, Linux gave the error EFBIG for this case.

       EPERM  The O_NOATIME flag was specified, but the effective user ID of the caller  did  not
              match the owner of the file and the caller was not privileged (CAP_FOWNER).

       EROFS  pathname refers to a file on a read-only filesystem and write access was requested.

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

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


       SVr4,  4.3BSD, POSIX.1-2001.  The O_DIRECTORY, O_NOATIME, O_NOFOLLOW, and O_PATH flags are
       Linux-specific, and one may need to define _GNU_SOURCE (before including any header files)
       to obtain their definitions.

       The O_CLOEXEC flag is not specified in POSIX.1-2001, but is specified in POSIX.1-2008.

       O_DIRECT  is  not  specified in POSIX; one has to define _GNU_SOURCE (before including any
       header files) to get its definition.


       Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not necessarily
       have  the  intention to read or write.  This is typically used to open devices in order to
       get a file descriptor for use with ioctl(2).

       Unlike the other values that can be specified in flags, the access mode  values  O_RDONLY,
       O_WRONLY,  and  O_RDWR, do not specify individual bits.  Rather, they define the low order
       two bits of flags, and are defined respectively as 0, 1,  and  2.   In  other  words,  the
       combination  O_RDONLY  | O_WRONLY is a logical error, and certainly does not have the same
       meaning as O_RDWR.  Linux reserves the special, nonstandard access mode 3 (binary  11)  in
       flags  to  mean:  check  for read and write permission on the file and return a descriptor
       that can't be used for reading or writing.  This nonstandard access mode is used  by  some
       Linux  drivers to return a descriptor that is to be used only for device-specific ioctl(2)

       The (undefined) effect of O_RDONLY  |  O_TRUNC  varies  among  implementations.   On  many
       systems the file is actually truncated.

       There  are  many  infelicities  in  the  protocol underlying NFS, affecting amongst others
       O_SYNC and O_NDELAY.

       POSIX provides for three different variants of  synchronized  I/O,  corresponding  to  the
       flags O_SYNC, O_DSYNC, and O_RSYNC.  Currently (2.6.31), Linux implements only O_SYNC, but
       glibc maps O_DSYNC and O_RSYNC  to  the  same  numerical  value  as  O_SYNC.   Most  Linux
       filesystems  don't  actually  implement  the  POSIX  O_SYNC  semantics,  which require all
       metadata updates of a write to be on disk on returning to user space, but only the O_DSYNC
       semantics, which require only actual file data and metadata necessary to retrieve it to be
       on disk by the time the system call returns.

       Note that open() can open device special  files,  but  creat()  cannot  create  them;  use
       mknod(2) instead.

       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.

       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 O_DIRECT flag may impose alignment restrictions on the length  and  address  of  user-
       space  buffers  and  the  file  offset  of  I/Os.  In Linux alignment restrictions vary by
       filesystem and kernel version and might be absent entirely.  However there is currently no
       filesystem-independent  interface  for an application to discover these restrictions for a
       given file or filesystem.  Some filesystems provide their own interfaces for doing so, for
       example the XFS_IOC_DIOINFO operation in xfsctl(3).

       Under  Linux 2.4, transfer sizes, and the alignment of the user buffer and the file offset
       must all be multiples of the logical block size  of  the  filesystem.   Under  Linux  2.6,
       alignment to 512-byte boundaries suffices.

       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  under  Linux in kernel version 2.4.10.  Older Linux kernels
       simply ignore this flag.  Some filesystems may not implement the flag and open() will fail
       with EINVAL if it is used.

       Applications  should avoid mixing O_DIRECT and normal I/O to the same file, and especially
       to overlapping byte regions in the same file.  Even when the filesystem correctly  handles
       the coherency issues in this situation, overall I/O throughput is likely to be slower than
       using either mode alone.  Likewise, applications should avoid mixing mmap(2) of files with
       direct I/O to the same files.

       The  behaviour of O_DIRECT with NFS will differ from local filesystems.  Older kernels, or
       kernels configured in certain ways, may not support this combination.   The  NFS  protocol
       does  not  support  passing  the  flag to the server, so O_DIRECT I/O will bypass the page
       cache only on the client; the server may still cache the I/O.  The client asks the  server
       to  make  the  I/O  synchronous  to  preserve the synchronous semantics of O_DIRECT.  Some
       servers will perform poorly under these circumstances,  especially  if  the  I/O  size  is
       small.  Some servers may also be configured to lie to clients about the I/O having reached
       stable storage; this will avoid the performance penalty at some risk to data integrity  in
       the  event of server power failure.  The Linux NFS client places no alignment restrictions
       on O_DIRECT I/O.

       In summary, O_DIRECT is a potentially powerful tool that should be used with caution.   It
       is  recommended  that  applications treat use of O_DIRECT as a performance option which is
       disabled by default.

              "The thing that has always disturbed me about O_DIRECT is that the whole  interface
              is  just  stupid,  and  was  probably designed by a deranged monkey on some serious
              mind-controlling substances."—Linus


       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.


       chmod(2),  chown(2),  close(2),  dup(2),  fcntl(2),  link(2), lseek(2), mknod(2), mmap(2),
       mount(2), openat(2), read(2), socket(2), stat(2), umask(2), unlink(2), write(2), fopen(3),
       fifo(7), path_resolution(7), symlink(7)


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