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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 inte-
       ger 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 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 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, respec-

       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_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY,
       O_NOFOLLOW, O_TRUNC, and 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) 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 posi-
              tioned 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  descrip-
              tor.   This  feature is available only for terminals, pseudoterminals, 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.  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

              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 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

              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  O_DIRECT  flag  on  its  own  makes an effort to transfer data syn-
              chronously, but does not give the guarantees of the O_SYNC flag that data and  nec-
              essary metadata are transferred.  To guarantee 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

              If  pathname  is  not a directory, cause the open to fail.  This flag is Linux-spe-
              cific, 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 specified in conjunction
              with O_CREAT, and pathname already exists, then open() will fail.

              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.

              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

              (LFS) Allow files whose sizes cannot be represented in an off_t (but can be  repre-
              sented  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  fea-

       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.  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 call-
              ing 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 operations 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 descriptor:

              *  close(2); fchdir(2) (since Linux 3.5); fstat(2) (since Linux 3.6).

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

              *  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  openat(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_DIRECTORY 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 fchownat(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 writ-
              ten 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 O_CREAT|O_WRONLY|O_TRUNC.

       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  path_reso-

       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 signal 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 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.

              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.

              pathname  refers  to a regular file that is too large to be opened.  The usual sce-
              nario  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_LARGEFILE 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

              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 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 (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 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  com-
       bination  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)

       The (undefined) effect of O_RDONLY | O_TRUNC varies among implementations.  On  many  sys-
       tems 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 (2.6.31), 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  seman-
       tics,  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  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 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.

       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-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 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 created 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 introduced a flag of the same name,  but  without  alignment  restric-

       O_DIRECT support was added under Linux in kernel version 2.4.10.  Older Linux kernels sim-
       ply 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 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 call-
       ing 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), symlink(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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