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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, 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_CREAT, O_EXCL,
       O_NOCTTY, 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 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  file
              systems 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  only
              available  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 file system 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
              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 only  applies  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

              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 at an effort to transfer data
              synchronously,  but  does  not give the guarantees of the O_SYNC
              that data and necessary metadata are transferred.  To  guarantee
              synchronous I/O the 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).

              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, but should  not  be  used  outside  of  the
              implementation of opendir(3).

       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 only supported 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 file system (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
              effective  on  all  file systems.  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.

              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_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


       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

       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).

       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
              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

       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 file system  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, and O_NOFOLLOW
       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  only  to  be  used for device-specific
       ioctl(2) operations.

       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 only implements O_SYNC,  but  glibc  maps  O_DSYNC  and
       O_RSYNC to the same numerical value as O_SYNC.  Most Linux file systems
       don't actually implement the POSIX O_SYNC semantics, which require  all
       metadata  updates  of  a write to be on disk on returning to userspace,
       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 file systems 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

       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  userspace  buffers  and the file offset of I/Os.  In Linux
       alignment restrictions vary by file system and kernel version and might
       be   absent   entirely.    However   there   is   currently   no   file
       system-independent interface  for  an  application  to  discover  these
       restrictions  for  a  given  file  or  file  system.  Some file systems
       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  file  system.   Under  Linux 2.6, alignment to 512-byte boundaries

       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 under Linux in kernel version 2.4.10.  Older
       Linux kernels simply ignore this  flag.   Some  file  systems  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  file  system 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 file systems.
       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 only bypass the page cache 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


       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),


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