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

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

       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/stat.h>&gt;
       #include <&lt;fcntl.h>&gt;

       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  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

       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:

              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  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.   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 &amp; ~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

              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).

              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.

              (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.

              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.

       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-

       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.)

       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.

       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, 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 planned for
       inclusion in the next revision of the standard; one has to  define  the
       _GNU_SOURCE macro to get its 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 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

       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 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

       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.

       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

       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),  feature_test_macros(7),
       fifo(7), path_resolution(7), symlink(7)

       This page is part of release 3.05 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                             2008-07-22                           OPEN(2)