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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.

       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  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()  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  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) 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(), the file
	      offset is positioned at the end of the file, as if with lseek().
	      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.

       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_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  filesystem type and mount options, and the
	      mode of the parent directory, see, e.g., the mount options  bsd‐
	      groups  and  sysvgroups  of the ext2 filesystem, as described in
	      mount(8)).

       O_DIRECT
	      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, i.e., 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 must  fit  the	 block
	      size of the device.

	      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.

       O_EXCL When used with O_CREAT, if the file  already  exists  it	is  an
	      error and the open() will fail. In this context, a symbolic link
	      exists, regardless of where it points to.	 O_EXCL is  broken  on
	      NFS file systems; programs which rely on it for performing lock‐
	      ing tasks will contain a race condition.	The solution for  per‐
	      forming  atomic  file  locking  using  a lockfile is to create a
	      unique file on the same file system (e.g.,  incorporating	 host‐
	      name  and	 pid),	use link(2) to make a link to the lockfile. If
	      link() 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.

       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.

       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()s  on  the
	      resulting	 file  descriptor will block the calling process until
	      the data has been physically written to the underlying hardware.
	      But see RESTRICTIONS 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() 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:

       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

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

       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).

NOTES
       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 e.g. 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.

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(2).)

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.

       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, too large to be opened; see
	      O_LARGEFILE above.

       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.

       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)).

NOTE
       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).

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001.  The O_NOATIME, O_NOFOLLOW, and O_DIRECTORY
       flags are Linux-specific.  One may have to define the _GNU_SOURCE macro
       to get their definitions.

       The (undefined) effect of O_RDONLY | O_TRUNC varies  among  implementa‐
       tions. 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 intro‐
       duced 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.

BUGS
       "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.

RESTRICTIONS
       There are many infelicities in the protocol underlying  NFS,  affecting
       amongst others O_SYNC and O_NDELAY.

       POSIX provides for three different variants of synchronised I/O, corre‐
       sponding to the flags O_SYNC, O_DSYNC and O_RSYNC.  Currently (2.1.130)
       these are all synonymous under Linux.

SEE ALSO
       close(2),  dup(2),  fcntl(2),  link(2),	lseek(2),  mknod(2), mount(2),
       mmap(2), openat(2), path_resolution(2),	read(2),  socket(2),  stat(2),
       umask(2),     unlink(2),	    write(2),	 fopen(3),    fifo(7),	  fea‐
       ture_test_macros(7)

Linux 2.6.12			  2005-06-22			       OPEN(2)
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