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System Calls                                              open(2)



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
     argument 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 fd argument. If the  fd  argu-
     ment  has  the special value AT_FDCWD, a relative path argu-
     ment will be resolved relative to the current working direc-
     tory.  If  the path argument is absolute, the fd 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 file  descriptor
     flag associated with the new file descriptor is 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  three
     values (file access modes) below in the value of oflag:

     O_RDONLY
           Open for reading only.




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System Calls                                              open(2)



     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.

     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_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 follows (see creat(2)):  a  bitwise-
           AND  is  performed  on  the  file-mode  bits  and  the
           corresponding bits in the complement 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 permission 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  syn-
           chronized   I/O   file   integrity   completion   (see
           fcntl(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_DSYNC
           Write I/O operations on the file  descriptor  complete
           as  defined by synchronized I/O data integrity comple-
           tion.

     O_EXCL
           If O_CREAT and O_EXCL are set,  open()  fails  if  the
           file  exists.  The check for the existence of the file



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System Calls                                              open(2)



           and the creation of the file if it does not  exist  is
           atomic  with  respect  to  other  processes  executing
           open() naming the same filename in the same  directory
           with  O_EXCL  and  O_CREAT set. If O_CREAT is not set,
           the effect is undefined.

     O_LARGEFILE
           If set, the offset maximum in the open  file  descrip-
           tion  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_NONBLOCK or O_NDELAY
           These flags may 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:

           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.

           If O_NONBLOCK and O_NDELAY are clear:

           An open() for reading  only  blocks  until  a  process
           opens  the  file  for  writing.  An open() for writing
           only blocks until a process opens the file  for  read-
           ing.

           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
           written  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:

           If O_NONBLOCK or O_NDELAY is set:

           The open() function returns without blocking  for  the
           device  to  be ready or available. Subsequent behavior
           of the device is device-specific.



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System Calls                                              open(2)



           If  O_NONBLOCK and O_NDELAY are clear:

           The open() function blocks until the device  is  ready
           or available before returning.

           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 complete as  defined  by  synchronized  I/O
           data integrity completion.  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 comple-
           tion.

     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_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;  providing  an absolute path results in a normal
           file reference.

     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.



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System Calls                                              open(2)



     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 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,  or the file exists and the permissions speci-
           fied by oflag are denied, or the file does  not  exist
           and  write  permission is denied for the parent direc-
           tory of the file to be created, or O_TRUNC  is  speci-
           fied and write permission is denied.

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

     EDQUOT



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System Calls                                              open(2)



           The file does not exist,  O_CREAT  is  specified,  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.

     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 supplied and the under-
           lying 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.

     EMFILE
           OPEN_MAX file descriptors are currently  open  in  the
           calling process.

     EMULTIHOP
           Components of path require hopping to multiple  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



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System Calls                                              open(2)



           to an empty string.

     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  contain  the
           new  file cannot be expanded, the file does not exist,
           and O_CREAT is specified.

     ENOSYS
           The device specified by path does not support the open
           operation.

     ENOTDIR
           A component of the path prefix is not a directory or a
           relative  path  was  supplied to openat(), the O_XATTR
           flag was not supplied, and the  file  descriptor  does
           not not refer to a directory.

     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 associ-
           ated with this special file does not exist.

     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:

     EBADF The fildes argument is not a valid open file  descrip-
           tor or is not AT_FTCWD.

     The open() function may fail if:




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System Calls                                              open(2)



     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  produced  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.

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.

     When a UFS file system is mounted with logging enabled, file
     system  transactions  that  free blocks from files might not
     actually add those freed blocks to the  file  system's  free
     list  until  some  unspecified  time  in  the  future.  This
     behavior improves file system performance but does not  con-
     form  to the POSIX, Single UNIX Specification, SPARC Confor-
     mance Definition, System  V  Application  Binary  Interface,
     System  V Interface Definition, and X/Open Portability Guide
     Standards, which  require  that  freed  space  be  available
     immediately.  To enable standards conformance regarding file
     deletions or to address the problem of  not  being  able  to
     grow  files  on a relatively full UFS file system even after
     files  have  been  deleted,   disable   UFS   logging   (see
     mount_ufs(1M).

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

     ____________________________________________________________
    |       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
    |_____________________________|_____________________________|
    | Interface Stability         | See below.                  |
    |_____________________________|_____________________________|
    | MT-Level                    | Async-Signal-Safe           |
    |_____________________________|_____________________________|





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System Calls                                              open(2)



     The open() function is Standard;. The openat()  function  is
     Evolving.

SEE ALSO
     mount_ufs(1M),  intro(2),  chmod(2),   close(2),   creat(2),
     dup(2),    exec(2),   fcntl(2),   getmsg(2),   getrlimit(2),
     lseek(2), putmsg(2), read(2), stat(2),  umask(2),  write(2),
     attropen(3C),   unlockpt(3C),  attributes(5),  fcntl(3HEAD),
     lf64(5), stat(3HEAD), 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.









































SunOS 5.9           Last change: 10 Dec 2001                    9