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개요

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open

Name
     open, openat - open a file

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

     int open(const char *path, int oflag, /* mode_t mode */);


     int openat(int fildes, const char *path, int oflag,
          /* mode_t mode */);

Description
     The open() function establishes  the  connection  between  a
     file and a file descriptor. It creates an open file descrip-
     tion that refers to a file and a file descriptor that refers
     to  that  open file description. The file descriptor is used
     by other I/O functions to refer to that file. The path argu-
     ment points to a pathname naming the file.


     The openat() function is identical to  the  open()  function
     except that the path argument is interpreted relative to the
     starting point implied by the fildes argument. If the fildes
     argument  has  the  special  value AT_FDCWD, a relative path
     argument will be resolved relative to  the  current  working
     directory.  If  the  path  argument  is absolute, the fildes
     argument is ignored.


     The open() function returns a file descriptor for the  named
     file  that  is the lowest file descriptor not currently open
     for that process. The open  file  description  is  new,  and
     therefore  the  file  descriptor  does not share it with any
     other process in the system. The FD_CLOEXEC  and  FD_CLOFORK
     flags associated with the new file descriptor are cleared.


     The file offset used to mark the current position within the
     file is set to the beginning of the file.


     The file status flags and file access modes of the open file
     description  are  set  according  to the value of oflag. The
     mode argument is used only when O_CREAT  is  specified  (see
     below.)

     Values for oflag are constructed by  a  bitwise-inclusive-OR
     of  flags  from  the  following  list, defined in <fcntl.h>.
     Applications must specify exactly  one  of  the  first  five
     values (file access modes) below in the value of oflag:

     O_RDONLY
                 Open for reading only.


     O_WRONLY
                 Open for writing only.


     O_RDWR
                 Open for reading  and  writing.  The  result  is
                 undefined if this flag is applied to a FIFO.


     O_EXEC
                 Open ordinary file for execute only.


     O_SEARCH
                 Open directory for search only.



     Any combination of the following may be used:

     O_APPEND
         If set, the file offset is set to the end  of  the  file
         prior to each write.


     O_CLOEXEC
         If set, the FD_CLOEXEC flag is  set  for  the  new  file
         descriptor.


     O_CLOFORK
         If set, the FD_CLOFORK flag is  set  for  the  new  file
         descriptor.


     O_CREAT
         Create the file if it does not exist. This flag requires
         that the mode argument be specified.

         If the file exists, this flag has no  effect  except  as
         noted  under  O_EXCL  below.   Otherwise,  the  file  is
         created with the user ID of the file set to  the  effec-
         tive user ID of the process. The group ID of the file is
         set to the effective group IDs of the process, or if the
         S_ISGID bit is set in the directory in which the file is
         being created, the file's group ID is set to  the  group

         ID  of its parent directory.  If the group ID of the new
         file does not match the effective group ID or one of the
         supplementary  groups  IDs,  the S_ISGID bit is cleared.
         The access permission bits  (see  <sys/stat.h>)  of  the
         file mode are set to the value of mode, modified as fol-
         lows (see creat(2)): a bitwise-AND is performed  on  the
         file-mode bits and the corresponding bits in the comple-
         ment of the process's file mode creation mask. Thus, all
         bits  set  in the process's file mode creation mask (see
         umask(2)) are correspondingly cleared in the file's per-
         mission  mask. The "save text image after execution bit"
         of the mode is cleared (see chmod(2)). O_SYNC Write  I/O
         operations on the file descriptor complete as defined by
         synchronized  I/O  file   integrity   completion    (see
         fcntl.h(3HEAD)  definition  of  O_SYNC.) When bits other
         than the file permission bits are  set,  the  effect  is
         unspecified.  The  mode argument does not affect whether
         the file is open for reading, writing or for both.


     O_DIRECTORY
         If path does not specify a directory, fail and set errno
         to ENOTDIR.


     O_DSYNC
         Write I/O operations on the file descriptor complete  as
         defined by synchronized I/O data integrity completion.


     O_EXCL
         If O_CREAT and O_EXCL are set, open() fails if the  file
         exists.  The check for the existence of the file and the
         creation of the file if it does not exist is atomic with
         respect  to  other  threads  executing open() naming the
         same filename in the  same  directory  with  O_EXCL  and
         O_CREAT  set.  If  O_EXCL  and O_CREAT are set, and path
         names a symbolic link, open() fails and  sets  errno  to
         EEXIST, regardless of the contents of the symbolic link.
         If O_EXCL is set and O_CREAT is not set, the  result  is
         undefined.


     O_LARGEFILE
         If set, the offset maximum in the open file  description
         is  the  largest value that can be represented correctly
         in an object of type off64_t.

     O_NOCTTY
         If set and path identifies  a  terminal  device,  open()
         does  not  cause  the terminal device to become the con-
         trolling terminal for the process.


     O_NOFOLLOW
         If the path names a symbolic link, open() fails and sets
         errno to ELOOP.


     O_NOLINKS
         If the link count of the named file is greater  than  1,
         open() fails and sets errno to EMLINK.


     O_NONBLOCK or O_NDELAY
         These flags can affect subsequent reads and writes  (see
         read(2)  and  write(2)). If both O_NDELAY and O_NONBLOCK
         are set, O_NONBLOCK takes precedence.

         When opening a FIFO with O_RDONLY or O_WRONLY set:

             o    If O_NONBLOCK or O_NDELAY is set, an open() for
                  reading  only returns without delay.  An open()
                  for writing only returns an error if no process
                  currently has the file open for reading.

             o    If O_NONBLOCK and O_NDELAY are clear, an open()
                  for  reading  only  blocks until a thread opens
                  the file for writing.  An  open()  for  writing
                  only  blocks  the calling thread until a thread
                  opens the file for reading.
         After both ends of a FIFO have been opened, there is  no
         guarantee   that   further   calls  to  open()  O_RDONLY
         (O_WRONLY) will synchronize with later calls  to  open()
         O_WRONLY  (O_RDONLY)  until  both  ends of the FIFO have
         been closed by all readers and writers.  Any data  writ-
         ten  into  a  FIFO will be lost if both ends of the FIFO
         are closed before the data is read.

         When opening a block special or character  special  file
         that supports non-blocking opens:

             o    If O_NONBLOCK or O_NDELAY is  set,  the  open()
                  function  returns without blocking for the dev-
                  ice  to  be  ready  or  available.   Subsequent
                  behavior of the device is device-specific.

             o    If  O_NONBLOCK  and  O_NDELAY  are  clear,  the

                  open() function blocks the calling thread until
                  the device is ready or available before return-
                  ing.
         Otherwise, the behavior of O_NONBLOCK  and  O_NDELAY  is
         unspecified.


     O_RSYNC
         Read I/O operations on the file descriptor  complete  at
         the  same level of integrity as specified by the O_DSYNC
         and O_SYNC flags. If both O_DSYNC and O_RSYNC are set in
         oflag,  all  I/O  operations on the file descriptor com-
         plete as defined by synchronized I/O data integrity com-
         pletion.   If  both O_SYNC and O_RSYNC are set in oflag,
         all I/O operations on the file  descriptor  complete  as
         defined by synchronized I/O file integrity completion.


     O_SYNC
         Write I/O operations on the file descriptor complete  as
         defined by synchronized I/O file integrity completion.


     O_TPDSAFE
         Allow opening a file  for  reading  that  is  not  MWAC-
         protected even if the process is part of the TPD.

         Allow opening STREAM devices or fifo when  the  peer  is
         not a TPD process.


     O_TRUNC
         If the file exists and is a regular file, and  the  file
         is successfully opened O_RDWR or O_WRONLY, its length is
         truncated to 0 and the mode and owner are unchanged.  It
         has  no  effect on FIFO special files or terminal device
         files.   Its   effect   on   other   file    types    is
         implementation-dependent.  The  result  of using O_TRUNC
         with O_RDONLY is undefined.


     O_TTY_INIT
         The O_TTY_INIT flag is  ignored.  Terminal  devices  are
         always opened in a state providing conforming behavior.


     O_XATTR
         If set in openat(), a relative path argument  is  inter-
         preted  as  a  reference to an extended attribute of the
         file associated with the supplied file descriptor.  This
         flag  therefore  requires the presence of a legal fildes
         argument. If set in open(), the implied file  descriptor
         is  that  for  the  current  working directory. Extended
         attributes must be referenced with a relative path; pro-
         viding  an absolute path results in a normal file refer-
         ence.



     If O_CREAT is set and the file  did  not  previously  exist,
     upon  successful  completion,  open()  marks  for update the
     st_atime, st_ctime, and st_mtime fields of the file and  the
     st_ctime and st_mtime fields of the parent directory.


     If O_TRUNC is set and the file did  previously  exist,  upon
     successful  completion, open() marks for update the st_ctime
     and st_mtime fields of the file.


     If both the O_SYNC and O_DSYNC flags are set, the effect  is
     as if only the O_SYNC flag was set.


     If path refers to a STREAMS file, oflag may  be  constructed
     from  O_NONBLOCK  or  O_NODELAY  OR-ed with either O_RDONLY,
     O_WRONLY, or O_RDWR. Other flag values are not applicable to
     STREAMS  devices  and  have  no  effect on them.  The values
     O_NONBLOCK and O_NODELAY affect  the  operation  of  STREAMS
     drivers  and  certain  functions  (see  read(2),  getmsg(2),
     putmsg(2), and write(2)) applied to file descriptors associ-
     ated with STREAMS files.  For STREAMS drivers, the implemen-
     tation of O_NONBLOCK and O_NODELAY is device-specific.


     When open() is invoked to  open  a  named  stream,  and  the
     connld  module (see connld(7M)) has been pushed on the pipe,
     open()  blocks  until  the  server  process  has  issued  an
     I_RECVFD  ioctl()  (see  streamio(7I))  to  receive the file
     descriptor.


     If path names the master side of a  pseudo-terminal  device,
     then  it  is unspecified whether open() locks the slave side
     so that it cannot be  opened.   Portable  applications  must
     call unlockpt(3C) before opening the slave side.


     If the file is a regular file and the local file  system  is
     mounted with the nbmand mount option, then a mandatory share
     reservation is automatically obtained on the file. The share
     reservation  is obtained as if fcntl(2) were called with cmd
     F_SHARE_NBMAND and the fshare_t values set as follows:

     f_access
                 Set to the type of read/write access  for  which
                 the file is opened.


     f_deny
                 F_NODNY


     f_id
                 The file descriptor value returned from open().



     If path is a symbolic link and O_CREAT and O_EXCL  are  set,
     the link is not followed.


     Certain flag values can be set following open() as described
     in fcntl(2).


     The largest value that can be represented  correctly  in  an
     object of type off_t is established as the offset maximum in
     the open file description.

Return Values
     Upon successful completion, the open()  function  opens  the
     file  and  return  a  non-negative  integer representing the
     lowest numbered unused file  descriptor.  Otherwise,  -1  is
     returned,  errno  is set to indicate the error, and no files
     are created or modified.

Errors
     The open() and openat() functions will fail if:

     EACCES
                     Search permission is denied on  a  component
                     of the path prefix.

                     The file exists and the  permissions  speci-
                     fied by oflag are denied.

                     The file does not exist and write permission
                     is  denied  for  the parent directory of the
                     file to be created.

                     The file is being opened  for  reading,  the
                     process  is  a TPD process, the process flag
                     PRIV_TPD_UNSAFE is not set and the file  can
                     be  modified by non-TPD processes inside the
                     immutable zone.

                     O_TRUNC is specified and write permission is
                     denied.

                     The {PRIV_FILE_DAC_SEARCH} privilege  allows
                     processes  to  search directories regardless
                     of        permission        bits.     The
                     {PRIV_FILE_DAC_WRITE}    privilege    allows
                     processes to open files for writing  regard-
                     less  of  permission bits. See privileges(5)
                     for  special  considerations  when   opening
                     files  owned  by  UID  0  for  writing.  The
                     {PRIV_FILE_DAC_READ}    privilege     allows
                     processes  to open files for reading regard-
                     less of permission bits.

                     To open a file for reading or  writing,  the
                     basic    privileges   {PRIV_FILE_READ}   and
                     {PRIV_FILE_WRITE}, respectively, need to  be
                     asserted in the effective set.


     EAGAIN
                     A mandatory share reservation could  not  be
                     obtained  because  the  desired  access con-
                     flicts with an existing f_deny share  reser-
                     vation.


     EBADF
                     The file descriptor provided to openat()  is
                     invalid.


     EDQUOT
                     The file does not exist, O_CREAT  is  speci-
                     fied, and either the directory where the new
                     file  entry  is  being  placed   cannot   be
                     extended  because  the  user's quota of disk
                     blocks  on  that  file   system   has   been
                     exhausted,  or the user's quota of inodes on
                     the file system  where  the  file  is  being
                     created has been exhausted.


     EEXIST
                     The O_CREAT and O_EXCL flags are set and the
                     named file exists.


     EILSEQ
                     The path argument includes non-UTF8  charac-
                     ters  and  the file system accepts only file
                     names where all characters are part  of  the
                     UTF-8 character codeset.

     EINTR
                     A signal was caught during open().


     EFAULT
                     The  path  argument  points  to  an  illegal
                     address.


     EINVAL
                     The system does not support synchronized I/O
                     for  this file, or the O_XATTR flag was sup-
                     plied and the underlying  file  system  does
                     not support extended file attributes.


     EIO
                     The path argument names a STREAMS file and a
                     hangup or error occurred during the open().


     EISDIR
                     The named file  is  a  directory  and  oflag
                     includes O_WRONLY or O_RDWR.


     ELOOP
                     Too many symbolic links were encountered  in
                     resolving path.

                     A loop exists in symbolic links  encountered
                     during resolution of the path argument.

                     The O_NOFOLLOW flag is  set  and  the  final
                     component of path is a symbolic link.


     EMFILE
                     There are currently {OPEN_MAX} file descrip-
                     tors open in the calling process.


     EMLINK
                     The O_NOLINKS flag is set and the named file
                     has a link count greater than 1.


     EMULTIHOP
                     Components of path require hopping to multi-
                     ple remote machines and the file system does
                     not allow it.


     ENAMETOOLONG
                     The length  of  the  path  argument  exceeds
                     {PATH_MAX} or a pathname component is longer
                     than {NAME_MAX}.


     ENFILE
                     The maximum allowable  number  of  files  is
                     currently open in the system.

     ENOENT
                     The O_CREAT flag is not set  and  the  named
                     file  does not exist; or the O_CREAT flag is
                     set and either  the  path  prefix  does  not
                     exist  or  the  path  argument  points to an
                     empty string.


     ENOEXEC
                     The O_EXEC access mode was specified and the
                     file to be opened is not an ordinary file.


     ENOLINK
                     The  path  argument  points  to   a   remote
                     machine,  and the link to that machine is no
                     longer active.


     ENOSR
                     The path argument names a STREAMS-based file
                     and  the  system  is  unable  to  allocate a
                     STREAM.


     ENOSPC
                     The directory or file system that would con-
                     tain  the  new  file cannot be expanded, the
                     file does not exist, and O_CREAT  is  speci-
                     fied.


     ENOSYS
                     The device specified by path does  not  sup-
                     port the open operation.


     ENOTDIR
                     A component of the  path  prefix  is  not  a
                     directory,  a  relative path was supplied to
                     openat(), the O_XATTR flag was not supplied,
                     and  the file descriptor does not refer to a
                     directory,  the  O_SEARCH  access  mode  was
                     specified and the file to be opened is not a
                     directory, or O_DIRECTORY was specified  and
                     the  path argument does not specify a direc-
                     tory.


     ENXIO
                     The O_NONBLOCK flag is set, the  named  file
                     is  a FIFO, the O_WRONLY flag is set, and no
                     process has the file open  for  reading;  or
                     the  named  file  is  a character special or
                     block special file and the device associated
                     with this special file does not exist or has
                     been retired by the fault management  frame-
                     work .

     EOPNOTSUPP
                     An attempt was made  to  open  a  path  that
                     corresponds to a AF_UNIX socket.


     EOVERFLOW
                     The named file is a regular file and  either
                     O_LARGEFILE  is  not set and the size of the
                     file cannot be represented correctly  in  an
                     object  of  type off_t or O_LARGEFILE is set
                     and  the  size  of  the   file   cannot   be
                     represented  correctly  in an object of type
                     off64_t.


     EROFS
                     The named file resides on a  read-only  file
                     system  and either O_WRONLY, O_RDWR, O_CREAT
                     (if file does not exist), or O_TRUNC is  set
                     in the oflag argument.



     The openat() function will fail if:

     EACCES
               The permissions of the directory underlying fildes
               do not permit directory searches.


     EBADF
               The path argument does  not  specify  an  absolute
               path  and  the fildes argument is neither AT_FDCWD
               nor a valid file descriptor open  for  reading  or
               searching.



     The open() function may fail if:

     EAGAIN
                     The path argument names the slave side of  a
                     pseudo-terminal device that is locked.


     EINVAL
                     The value  of  the  oflag  argument  is  not
                     valid.


     ENAMETOOLONG
                     Pathname resolution of a symbolic link  pro-
                     duced  an  intermediate  result whose length
                     exceeds {PATH_MAX}.


     ENOMEM
                     The path argument names a STREAMS  file  and
                     the system is unable to allocate resources.

     ETXTBSY
                     The file is a pure procedure  (shared  text)
                     file  that  is  being  executed and oflag is
                     O_WRONLY or O_RDWR.

Examples
     Example 1 Open a file for writing by the owner.


     The following example opens the file  /tmp/file,  either  by
     creating  it  if it does not already exist, or by truncating
     its length to 0 if it does exist. If the call creates a  new
     file,  the  access  permission  bits in the file mode of the
     file are set to permit reading and writing by the owner, and
     to permit reading only by group members and others.



     If the call to open() is successful, the file is opened  for
     writing.


       #include <fcntl.h>
       ...
       int fd;
       mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
       char *filename = "/tmp/file";
       ...
       fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, mode);
       ...


     Example 2 Open a file using an existence check.


     The following example uses the open()  function  to  try  to
     create  the LOCKFILE file and open it for writing. Since the
     open() function specifies the O_EXCL flag, the call fails if
     the  file  already  exists.  In  that  case, the application
     assumes that someone else is updating the password file  and
     exits.


       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>
       #define LOCKFILE "/etc/ptmp"
       ...
       int pfd; /* Integer for file descriptor returned by open() call. */
       ...
       if ((pfd = open(LOCKFILE, O_WRONLY | O_CREAT | O_EXCL,
               S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)

       {
               fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
               exit(1);
       }
       ...


     Example 3 Open a file for writing.


     The following example opens a file for writing, creating the
     file  if  it does not already exist. If the file does exist,
     the system truncates the file to zero bytes.


       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>
       #define LOCKFILE "/etc/ptmp"
       ...
       int pfd;
       char filename[PATH_MAX+1];
       ...
       if ((pfd = open(filename, O_WRONLY | O_CREAT | O_TRUNC,
               S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
       {
               perror("Cannot open output file\n"); exit(1);
       }
       ...

Usage
     The open() function has a transitional interface for  64-bit
     file  offsets.   See  lf64(5).  Note  that using open64() is
     equivalent to using open() with O_LARGEFILE set in oflag.

Attributes
     See attributes(5) for descriptions of the  following  attri-
     butes:



     tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i)  ATTRI-
     BUTE  TYPEATTRIBUTE  VALUE  _ Interface StabilityCommitted _
     MT-LevelAsync-Signal-Safe _ StandardFor  open(),  see  stan-
     dards(5).

See Also
     Intro(2), chmod(2),  close(2),  creat(2),  dup(2),  exec(2),
     fcntl(2),   getmsg(2),  getrlimit(2),  lseek(2),  putmsg(2),
     read(2),   setpflags(2),   stat(2),   umask(2),    write(2),
     attropen(3C),  fcntl.h(3HEAD),  stat.h(3HEAD), unlockpt(3C),
     attributes(5), lf64(5), privileges(5), standards(5), tpd(5),
     connld(7M), streamio(7I)

Notes
     Hierarchical Storage Management (HSM) file systems can some-
     times cause long delays when opening a file, since HSM files
     must be recalled from secondary storage.
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