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,
       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
       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.

              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 only supported 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.

       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 (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.

       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

       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.

       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.


       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  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
       (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.

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
       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 only implements 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 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.
       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-
              stances."--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), fifo(7), path_resolution(7),
       symlink(7)

COLOPHON
       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



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