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OPEN(2)			   Linux Programmer's Manual		       OPEN(2)

NAME
       open, creat - open and possibly create a file or device

SYNOPSIS
       #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);

DESCRIPTION
       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  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	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 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
       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) mod‐
       ified  using  fcntl(2).	 The full list of file creation flags and file
       status flags is as follows:

       O_APPEND
	      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 sys‐
	      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.

       O_ASYNC
	      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.	 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).

       O_CREAT
	      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 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 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  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:

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

	      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).

       O_DIRECTORY
	      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	speci‐
	      fied  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 fol‐
	      lowed: if pathname is a symbolic link, then open() fails regard‐
	      less 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.

       O_LARGEFILE
	      (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 effec‐
	      tive  on all file systems.  One example is NFS, where the server
	      maintains the access time.

       O_NOCTTY
	      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.

       O_NOFOLLOW
	      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.

       O_NONBLOCK or O_NDELAY
	      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.

       O_TRUNC
	      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
       O_CREAT|O_WRONLY|O_TRUNC.

RETURN VALUE
       open()  and  creat()  return the new file descriptor, or -1 if an error
       occurred (in which case, errno is set appropriately).

ERRORS
       EACCES The requested access to the file is not allowed, or search  per‐
	      mission  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 path_resolu‐
	      tion(7).)

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.

       EFBIG  See EOVERFLOW.

       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).

       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.

       ENAMETOOLONG
	      pathname was too long.

       ENFILE The system limit on the total number  of	open  files  has  been
	      reached.

       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.

       ENOTDIR
	      A component used as a directory in pathname is not, in  fact,  a
	      directory,  or  O_DIRECTORY was specified and pathname was not a
	      directory.

       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.

       EOVERFLOW
	      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_LARGE‐
	      FILE above.  This is the error  specified	 by  POSIX.1-2001;  in
	      kernels before 2.6.24, Linux gave the error EFBIG for this case.

       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.

       ETXTBSY
	      pathname refers to an executable image which is currently	 being
	      executed and write access was requested.

       EWOULDBLOCK
	      The O_NONBLOCK flag was specified, and an incompatible lease was
	      held on the file (see fcntl(2)).

CONFORMING TO
       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.

NOTES
       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,  non-standard	access
       mode  3	(binary 11) in flags to mean: check for read and write permis‐
       sion 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 driv‐
       ers 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 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 (2.1.130)
       these are all synonymous under Linux.

       Note that open() can open device special files, but creat() cannot cre‐
       ate 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
       requests.

       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.

   O_DIRECT
       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-indepen‐
       dent  interface for an application to discover these restrictions for a
       given file or file system.  Some file systems provide their own	inter‐
       faces  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
       suffices.

       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  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 config‐
       ured 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

BUGS
       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.

SEE ALSO
       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)

COLOPHON
       This page is part of release 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2008-12-03			       OPEN(2)
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