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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 filesystem type and mount options, and
              the mode of the parent directory, see, for  example,  the  mount
              options  bsdgroups  and  sysvgroups  of  the ext2 filesystem, as
              described in mount(8)).

       O_DIRECT (Since Linux 2.6.10)
              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.  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.

              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 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.

              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.

       The  argument  mode specifies the permissions to use in case a new file
       is created.  It is 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 descriptor.

       The following symbolic constants are provided for mode:

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

              00400 user has read permission

              00200 user has write permission

              00100 user has execute permission

              00070 group has read, write and execute permission

              00040 group has read permission

              00020 group has write permission

              00010 group has execute permission

              00007 others have read, write and execute permission

              00004 others have read permission

              00002 others have write permission

              00001 others have execute permission

       mode must be specified when O_CREAT is in the  flags,  and  is  ignored

       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 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_CLOEXEC, O_DIRECTORY, O_NOATIME, and
       O_NOFOLLOW  flags  are  Linux-specific.   One  may  have  to define the
       _GNU_SOURCE macro to get their 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).

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

       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 same name,  but  without  alignment  restrictions.
       Support  was  added  under Linux in kernel version 2.4.10.  Older Linux
       kernels  simply  ignore  this  flag.   One  may  have  to  define   the
       _GNU_SOURCE macro to get its definition.

       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 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.


       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)


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