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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  Linux-specific 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, pseudo-terminals, 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.   The  behavior  of O_EXCL is
              undefined if O_CREAT is not specified.

              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.

              O_EXCL is only supported on NFS when using  NFSv3  or  later  on
              kernel  2.6  or later.  In environments where NFS 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  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  obtaining  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 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 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  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  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 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),  feature_test_macros(7),
       fifo(7), path_resolution(7), symlink(7)


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