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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, non-nega-
       tive 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 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 dis-
       abled; 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 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, 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) 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
              positioned  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
              descriptor.  This feature is only available for terminals, pseudo-terminals,
              sockets, and (since Linux 2.6) pipes and FIFOs.  See  fcntl(2)  for  further

       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 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 directory (depending on file system  type  and  mount
              options,  and  the  mode of the parent directory, see the mount options bsd-
              groups and sysvgroups described in mount(8)).

              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 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  caching.   File  I/O  is  done  directly
              to/from user space buffers.  The I/O is synchronous, that is, at the comple-
              tion 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-ser-
              vice 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 specified in con-
              junction 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 followed: if
              pathname is a symbolic link, then open() fails regardless of where the  sym-
              bolic 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_OFF-
              SET_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 sys-
              tems (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 pro-
              grams, 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.

              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  conjunc-
              tion 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

       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_resolution(7).)

       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_NOFOL-
              LOW 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 com-
              ponent 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
              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_LARGEFILE  above.   This  is  the  error  specified  by
              POSIX.1-2001; in kernels before 2.6.24, Linux gave the error EFBIG for  this

       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

       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, 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

       O_DIRECT  is not specified in POSIX; one has to define _GNU_SOURCE to get its defi-

       Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not nec-
       essarily  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 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
       permission 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 drivers 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 implementations.  On many
       systems the file is actually truncated.

       There  are many infelicities in the protocol underlying NFS, affecting amongst oth-
       ers 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 (2.1.130) these are all synony-
       mous under Linux.

       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  (respec-
       tively, time of last access, time of last status change, and time of last modifica-
       tion; 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  cur-
       rent 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-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

       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 allo-
       cated 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 restrictions.

       O_DIRECT support was added under Linux in kernel version 2.4.10.  Older Linux  ker-
       nels  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 ker-
       nels, 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 configured to lie to
       clients  about  the  I/O having reached stable storage; this will avoid the perfor-
       mance 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 cau-
       tion.  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 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2008-12-03                           OPEN(2)

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