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



NAME
       open(2,3,n), creat - open(2,3,n) and possibly create a file(1,n) or device

SYNOPSIS
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>

       int open(2,3,n)(const char *pathname, int flags);
       int open(2,3,n)(const char *pathname, int flags, mode_t mode);
       int creat(const char *pathname, mode_t mode);

DESCRIPTION
       The  open(2,3,n)()  system  call  is  used  to  convert a pathname into a file(1,n)
       descriptor (a small, non-negative integer for use in(1,8) subsequent I/O  as
       with  read(2,n,1 builtins),  write(1,2),  etc.).   When  the  call  is  successful, the file(1,n)
       descriptor returned will be the lowest file(1,n)  descriptor  not  currently
       open(2,3,n)  for  the  process.  This call creates a new open(2,3,n) file(1,n), not shared
       with any other process.  (But shared  open(2,3,n)  files  may  arise  via  the
       fork(2)  system  call.)   The new file(1,n) descriptor is set(7,n,1 builtins) to remain open(2,3,n)
       across exec(3,n,1 builtins) functions (see fcntl(2)).  The file(1,n) offset is  set(7,n,1 builtins)  to  the
       beginning of the file.

       The  parameter  flags  is  one  of  O_RDONLY,  O_WRONLY or O_RDWR which
       request opening the file(1,n) read-only, write-only or  read(2,n,1 builtins)/write(1,2),  respec-
       tively, bitwise-or'd with zero or more of the following:

       O_CREAT
              If  the file(1,n) does not exist it will be created.  The owner (user
              ID) of the file(1,n) is set(7,n,1 builtins) to the effective user ID of the  process.
              The  group  ownership  (group ID) is set(7,n,1 builtins) either to the effective
              group ID of the process or to the group ID of the parent  direc-
              tory  (depending  on  filesystem type and mount(2,8) options, and the
              mode of the parent directory, see, e.g., the mount(2,8) options  bsd-
              groups  and  sysvgroups  of the ext2 filesystem, as described in(1,8)
              mount(2,8)(8)).

       O_EXCL When used with O_CREAT, if(3,n) the file(1,n)  already  exists  it  is  an
              error(8,n)  and  the open(2,3,n) will fail. In this context, a symbolic link(1,2)
              exists, regardless of where its points to.  O_EXCL is broken  on
              NFS file(1,n) systems, programs which rely on it for performing lock-
              ing tasks will contain a race condition.  The solution for  per-
              forming  atomic  file(1,n)  locking  using  a lockfile is to create a
              unique file(1,n) on the same fs  (e.g.,  incorporating  hostname  and
              pid),  use  link(1,2)(2)  to  make  a link(1,2) to the lockfile. If link(1,2)()
              returns 0, the lock is successful.  Otherwise,  use  stat(1,2)(2)  on
              the  unique  file(1,n) to check if(3,n) its link(1,2) count has increased to 2,
              in(1,8) which case the lock is also successful.

       O_NOCTTY
              If pathname refers to a terminal device -- see tty(1,4)(4) -- it will
              not  become  the  process's  controlling  terminal  even  if(3,n) the
              process does not have one.

       O_TRUNC
              If the file(1,n) already exists and is a regular file(1,n)  and  the  open(2,3,n)
              mode  allows  writing  (i.e.,  is O_RDWR or O_WRONLY) it will be
              truncated to length 0.  If the file(1,n) is a FIFO or terminal device
              file(1,n),  the  O_TRUNC  flag  is  ignored.  Otherwise the effect of
              O_TRUNC is unspecified.

       O_APPEND
              The file(1,n) is opened in(1,8) append mode. Before each write(1,2),  the  file(1,n)
              pointer  is positioned at the end of the file(1,n), as if(3,n) with lseek.
              O_APPEND may lead to corrupted files on NFS file(1,n) systems if(3,n) more
              than  one  process  appends  data  to  a  file(1,n) at once.  This is
              because NFS does not support appending to a file(1,n), so the  client
              kernel  has  to  simulate it, which can't be done without a race
              condition.

       O_NONBLOCK or O_NDELAY
              When possible, the file(1,n) is opened in(1,8) non-blocking mode.  Neither
              the  open(2,3,n)  nor  any subsequent operations on the file(1,n) descriptor
              which is returned will cause the calling process to  wait.   For
              the  handling  of  FIFOs  (named(5,8) pipes), see also fifo(4).  This
              mode need not have any effect on files other than FIFOs.

       O_SYNC The file(1,n) is opened  for  synchronous  I/O.  Any  write(1,2)s  on  the
              resulting  file(1,n)  descriptor will block the calling process until
              the data has been physically written to the underlying hardware.
              See RESTRICTIONS below, though.

       O_NOFOLLOW
              If  pathname is a symbolic link(1,2), then the open(2,3,n) fails.  This is a
              FreeBSD extension, which was added to Linux in(1,8) version(1,3,5)  2.1.126.
              Symbolic  links in(1,8) earlier components of the pathname will still
              be followed.  The headers from glibc 2.0.100 and later include a
              definition  of  this flag; kernels before 2.1.126 will ignore it
              if(3,n) used.

       O_DIRECTORY
              If pathname is not a directory, cause the open(2,3,n)  to  fail.   This
              flag is Linux-specific, and was added in(1,8) kernel version(1,3,5) 2.1.126,
              to avoid denial-of-service problems if(3,n) opendir(3) is called on a
              FIFO  or  tape  device,  but  should  not be used outside of the
              implementation of opendir.

       O_DIRECT
              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(1,8)
              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, i.e., at the completion of  the  read(2,n,1 builtins)(2)
              or  write(1,2)(2) system call, data is guaranteed to have been trans-
              ferred.  Under Linux 2.4 transfer sizes, and  the  alignment  of
              user buffer and file(1,n) offset must all be multiples of the logical
              block size of the file(1,n) system.  Under  Linux  2.6  alignment  to
              512-byte boundaries suffices.
              A  semantically similar interface for block devices is described
              in(1,8) raw(3x,7,8,3x cbreak)(8).

       O_ASYNC
              Generate a signal(2,7) (SIGIO by default, but this can be changed via
              fcntl(2))  when  input  or  output becomes possible on this file(1,n)
              descriptor.  This  feature  is  only  available  for  terminals,
              pseudo-terminals, and sockets. See fcntl(2) for further details.

       O_LARGEFILE
              (LFS) Allow files whose sizes cannot be represented in(1,8) an  off_t
              (but can be represented in(1,8) an off64_t) to be opened.

       Some  of these optional flags can be altered using fcntl after the file(1,n)
       has been opened.

       The argument mode specifies the permissions to use in(1,8) case a  new  file(1,n)
       is created. It is modified by the process's umask in(1,8) the usual way: the
       permissions of the created file(1,n) are (mode & ~umask).   Note  that  this
       mode  only  applies  to  future accesses of the newly created file(1,n); the
       open(2,3,n) call that creates a read-only file(1,n) may well  return  a  read(2,n,1 builtins)/write(1,2)
       file(1,n) descriptor.

       The following symbolic constants are provided for mode:

       S_IRWXU
              00700 user (file(1,n) owner) has read(2,n,1 builtins), write(1,2) and execute permission

       S_IRUSR (S_IREAD)
              00400 user has read(2,n,1 builtins) permission

       S_IWUSR (S_IWRITE)
              00200 user has write(1,2) permission

       S_IXUSR (S_IEXEC)
              00100 user has execute permission

       S_IRWXG
              00070 group has read(2,n,1 builtins), write(1,2) and execute permission

       S_IRGRP
              00040 group has read(2,n,1 builtins) permission

       S_IWGRP
              00020 group has write(1,2) permission

       S_IXGRP
              00010 group has execute permission

       S_IRWXO
              00007 others have read(2,n,1 builtins), write(1,2) and execute permission

       S_IROTH
              00004 others have read(2,n,1 builtins) permission

       S_IWOTH
              00002 others have write(1,2) permisson

       S_IXOTH
              00001 others have execute permission

       mode  must  be  specified  when O_CREAT is in(1,8) the flags, and is ignored
       otherwise.

       creat    is    equivalent    to    open(2,3,n)    with    flags    equal    to
       O_CREAT|O_WRONLY|O_TRUNC.

RETURN VALUE
       open(2,3,n)  and  creat  return  the  new  file(1,n)  descriptor, or -1 if(3,n) an error(8,n)
       occurred (in(1,8) which case, errno is set(7,n,1 builtins) appropriately).  Note  that  open(2,3,n)
       can  open(2,3,n)  device  special  files,  but  creat cannot create them - use
       mknod(1,2)(2) instead.

       On NFS file(1,n) systems with UID mapping enabled, open(2,3,n) may  return  a  file(1,n)
       descriptor  but  e.g. read(2,n,1 builtins)(2) requests are denied with EACCES.  This is
       because the client performs open(2,3,n) by checking the permissions,  but  UID
       mapping is performed by the server upon read(2,n,1 builtins) and write(1,2) requests.

       If the file(1,n) is newly created, its atime, ctime, mtime fields are set(7,n,1 builtins) to
       the current time(1,2,n), and so are the ctime and mtime fields of  the  parent
       directory.   Otherwise,  if(3,n) the file(1,n) is modified because of the O_TRUNC
       flag, its ctime and mtime fields are set(7,n,1 builtins) to the current time.


ERRORS
       EACCES The requested access(2,5) to the file(1,n) is not allowed, or search  per-
              mission  is denied for one of the directories in(1,8) the path prefix
              of pathname, or the file(1,n) did not exist yet and write(1,2)  access(2,5)  to
              the  parent  directory  is  not allowed.  (See also path_resolu-
              tion(2).)

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.

       EISDIR pathname refers to a directory and the access(2,5) requested involved
              writing (that is, O_WRONLY or O_RDWR is set(7,n,1 builtins)).

       ELOOP  Too  many symbolic links were encountered in(1,8) 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(2,3,n)  files  has  been
              reached.

       ENODEV pathname  refers  to  a device special file(1,n) and no corresponding
              device exists.  (This is a Linux kernel bug - in(1,8) this  situation
              ENXIO must be returned.)

       ENOENT O_CREAT  is  not  set(7,n,1 builtins)  and the named(5,8) file(1,n) does not exist.  Or, a
              directory component in(1,8) 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.

       ENOTDIR
              A component used as a directory in(1,8) pathname is not, in(1,8)  fact,  a
              directory,  or  O_DIRECTORY was specified and pathname was not a
              directory.

       ENXIO  O_NONBLOCK | O_WRONLY is set(7,n,1 builtins), the named(5,8) file(1,n) is a  FIFO  and  no
              process has the file(1,n) open(2,3,n) for reading.  Or, the file(1,n) is a device
              special file(1,n) and no corresponding device exists.

       EOVERFLOW
              pathname refers to a regular file(1,n), too large to be opened -  see
              O_LARGEFILE above.

       EROFS  pathname  refers  to  a file(1,n) on a read-only filesystem and write(1,2)
              access(2,5) was requested.

       ETXTBSY
              pathname refers to an executable image which is currently  being
              executed and write(1,2) access(2,5) was requested.

NOTE
       Under  Linux,  the O_NONBLOCK flag indicates that one wants to open(2,3,n) but
       does not necessarily have the intention to read(2,n,1 builtins) or write.  This is typ-
       ically  used  to open(2,3,n) devices in(1,8) order to get a file(1,n) descriptor for use
       with ioctl(2).

CONFORMING TO
       SVr4, SVID, POSIX, X/OPEN, BSD 4.3.   The  O_NOFOLLOW  and  O_DIRECTORY
       flags are Linux-specific.  One may have to define the _GNU_SOURCE macro
       to get their definitions.

       The (undefined) effect of O_RDONLY | O_TRUNC various among  implementa-
       tions. On many systems the file(1,n) is actually truncated.

       The  O_DIRECT  flag  was introduced in(1,8) 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 same name, but without alignment restrictions.  Support
       was  added  under  Linux in(1,8) kernel version(1,3,5) 2.4.10.  Older Linux kernels
       simply ignore this flag.

BUGS
       "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

RESTRICTIONS
       There are many infelicities in(1,8) the protocol underlying  NFS,  affecting
       amongst others O_SYNC and O_NDELAY.

       POSIX provides for three different variants of synchronised I/O, corre-
       sponding to the flags O_SYNC, O_DSYNC and O_RSYNC.  Currently (2.1.130)
       these are all synonymous under Linux.

SEE ALSO
       close(2,7,n)(2),  fcntl(2),  link(1,2)(2),  mknod(1,2)(2), mount(2,8)(2), path_resolution(2),
       read(2,n,1 builtins)(2), socket(2,7,n)(2), stat(1,2)(2), umask(2), unlink(1,2)(2),  write(1,2)(2),  fopen(3),
       fifo(4)



Linux 2.6.7                       2004-06-23                           OPEN(2)

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