open


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.

              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
              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_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

       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-
       O_RSYNC to the same numerical value as O_SYNC  Most  Linux  filesystems
       don't  actually implement the POSIX O_SYNC semantics, which require all
       metadata updates of a write to be on disk on  returning  to  userspace,
       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
       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.

       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.


       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 3.23 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                             2009-09-20                           OPEN(2)
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