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

       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  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 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

       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_CLOEXEC,
       O_TTY_INIT.   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) modi‐
       fied 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 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.

	      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
	      available only  for  terminals,  pseudoterminals,	 sockets,  and
	      (since  Linux  2.6)  pipes  and FIFOs.  See fcntl(2) for further

       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).

	      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  applies  only  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

	      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 an effort to transfer data
	      synchronously, but does not give the guarantees  of  the	O_SYNC
	      flag that data and necessary metadata are transferred.  To guar‐
	      antee synchronous I/O,  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.

       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.

	      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.

	      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 supported only 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 effec‐
	      tive  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.  See also O_NOPATH below.

	      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_PATH (since Linux 2.6.39)
	      Obtain a file descriptor that can be used for two	 purposes:  to
	      indicate a location in the file-system tree and to perform oper‐
	      ations that act purely at the file descriptor level.   The  file
	      itself  is not opened, and other file operations (e.g., read(2),
	      write(2), fchmod(2),  fchown(2),	fgetxattr(2))  fail  with  the
	      error EBADF.

	      The  following operations can be performed on the resulting file

	      *	 close(2); fchdir(2) (since Linux 3.5); fstat(2) (since	 Linux

	      *	 Duplicating  the  file	 descriptor (dup(2), fcntl(2) F_DUPFD,

	      *	 Getting and setting file descriptor flags  (fcntl(2)  F_GETFD
		 and F_SETFD).

	      *	 Retrieving  open file status flags using the fcntl(2) F_GETFL
		 operation: the returned flags will include the bit O_PATH.

	      *	 Passing the file descriptor as the  dirfd  argument  of  ope‐
		 nat(2) and the other "*at()" system calls.

	      *	 Passing  the  file  descriptor	 to another process via a UNIX
		 domain socket (see SCM_RIGHTS in unix(7)).

	      When O_PATH is specified in flags, flag bits other than O_DIREC‐
	      TORY and O_NOFOLLOW are ignored.

	      If  the O_NOFOLLOW flag is also specified, then the call returns
	      a file descriptor referring to the  symbolic  link.   This  file
	      descriptor  can be used as the dirfd argument in calls to fchow‐
	      nat(2), fstatat(2), linkat(2), and readlinkat(2) with  an	 empty
	      pathname to have the calls operate on the symbolic link.

       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 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‐

       EDQUOT Where  O_CREAT  is  specified,  the file does not exist, and the
	      user's quota of disk blocks or inodes on	the  file  system  has
	      been exhausted.

       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  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.

	      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_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.

	      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, O_NOFOLLOW,
       and O_PATH flags	 are  Linux-specific,  and  one	 may  need  to	define
       _GNU_SOURCE (before including any header files) to obtain their defini‐

       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 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, 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  to  be  used  only  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
       (2.6.31),  Linux	 implements  only  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  user	space,
       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 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

       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 user-space 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

       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

       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 bypass the page cache only 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  sub‐

       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),

       This page is part of release 3.53 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2013-07-21			       OPEN(2)

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