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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,
       non-negative 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 parameter 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 an additional
              fcntl(2)  F_SETFD  operation  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 I/O is synchronous, that is, at the completion of a  read(2)
              or  write(2),  data is guaranteed to have been transferred.  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 not supported on NFSv2 or on Linux before kernel  2.6;
              it is supported on Linux 2.6 and later, with NFSv3 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 non-blocking 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.

       EFBIG  pathname refers to a regular file, too large to be  opened;  see
              O_LARGEFILE  above.  (POSIX.1-2001 specifies the error EOVERFLOW
              for this case.)

       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.

       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 planned for
       inclusion in the next revision of the standard; one has to  define  the
       _GNU_SOURCE macro to get its definitions.

       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,  non-standard  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  non-standard  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
       (2.1.130) these are all synonymous under Linux.

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

       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),  mount(2),  mmap(2),  openat(2), read(2), socket(2), stat(2),
       umask(2),      unlink(2),      write(2),       fopen(3),       fifo(7),
       feature_test_macros(7), path_resolution(7), symlink(7)


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