#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  cur-
       rently 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 opera-
       tion).  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  ini-
       tially  not  shared  with  any other process, but sharing may arise via

       The argument flags must include one  of  the  following  access  modes:
       O_RDONLY,  O_WRONLY,  or  O_RDWR.  These request opening the file read-
       only, write-only, or read/write, respectively.

       In addition, zero or more file creation flags and file status flags can
       be  bitwise-or'd  in  flags.   The  file  creation flags are O_CLOEXEC,
       O_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) modi-
       fied 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 filesys-
              tems 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.
              fcntl(2) F_SETFD operations to set the FD_CLOEXEC  flag.   Addi-
              tionally,  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 condi-
              tions 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  direc-
              tory  (depending  on  filesystem type and mount options, and the
              mode of the parent directory, see the  mount  options  bsdgroups
              and sysvgroups described in mount(8)).

              mode specifies the permissions to use in case a new file is cre-
              ated.  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 descriptor.

              The following symbolic constants are provided for mode:

              S_IRWXU  00700 user (file owner) has  read,  write  and  execute

              S_IRUSR  00400 user has read permission

              S_IWUSR  00200 user has write permission

              S_IXUSR  00100 user has execute permission

              S_IRWXG  00070 group has read, write and execute permission

              S_IRGRP  00040 group has read permission

              S_IWGRP  00020 group has write permission

              S_IXGRP  00010 group has execute permission

              S_IRWXO  00007 others have read, write and execute permission

              S_IROTH  00004 others have read permission

              S_IWOTH  00002 others have write permission

              S_IXOTH  00001 others have execute permission

       O_DIRECT (Since Linux 2.4.10)
              Try  to minimize cache effects of the I/O to and from this file.

              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 speci-
              fied in conjunction with O_CREAT, and pathname  already  exists,
              then open() will fail.

              When  these two flags are specified, symbolic links are not fol-
              lowed: if pathname is a symbolic link, then open() fails regard-
              less of where the symbolic link points to.

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

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

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

       O_NOATIME (Since Linux 2.6.8)
              Do  not update the file last access time (st_atime in the inode)
              when the file is read(2).  This flag  is  intended  for  use  by
              indexing  or  backup  programs,  where its use can significantly
              reduce the amount of disk activity.  This flag may not be effec-
              tive  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.

              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 opera-
              tions that act purely at the file descriptor  level.   The  file
              itself  is not opened, and other file operations (e.g., read(2),
              write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2)) fail with
              the error EBADF.

              The  following operations can be performed on the resulting file

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

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

              *  Getting and setting file descriptor flags  (fcntl(2)  F_GETFD
                 and F_SETFD).

              *  Retrieving  open file status flags using the fcntl(2) F_GETFL
                 operation: the returned flags will include the bit O_PATH.

              *  Passing the file descriptor as the  dirfd  argument  of  ope-
                 nat(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_DIREC-
              TORY 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 fchow-
              nat(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
              of  pathname,  or the file did not exist yet and write access to
              the parent directory is not  allowed.   (See  also  path_resolu-

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

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.


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

       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

       ENXIO  O_NONBLOCK | O_WRONLY is set, the named file is a  FIFO  and  no
              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 defini-

       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 typ-
       ically  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) operations.

       The (undefined) effect of O_RDONLY | O_TRUNC varies  among  implementa-
       tions.  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, corre-
       sponding   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
       but UID mapping  is  performed  by  the  server  upon  read  and  write

       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-indepen-
       dent interface for an application to discover these restrictions for  a
       given  file  or  filesystem.  Some filesystems provide their own inter-
       faces for doing  so,  for  example  the  XFS_IOC_DIOINFO  operation  in

       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 suf-

       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 cre-
       ated 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 intro-
       duced a flag of the same name, but without alignment restrictions.

       O_DIRECT support was added under Linux in kernel version 2.4.10.  Older
       Linux kernels simply ignore this flag.  Some filesystems may not imple-
       ment 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.

       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   sub-

       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),

       This  page  is  part of release 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

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