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.\"     @(#)open.2	8.2 (Berkeley) 11/16/93
.\"
.Dd May 8, 2003
.Dt OPEN 2
.Os
.Sh NAME
.Nm open
.Nd open or create a file for reading or writing
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In fcntl.h
.Ft int
.Fn open "const char *path" "int flags" "mode_t mode"
.Sh DESCRIPTION
The file name specified by
.Fa path
is opened
for reading and/or writing as specified by the
argument
.Fa flags
and the file descriptor returned to the calling process.
The
.Fa flags
are specified by
.Em or Ns 'ing
the following values.
Applications must specify exactly one of the first three values
(file access modes):
.Bl -tag -offset indent -width O_NONBLOCK
.It Dv O_RDONLY
Open for reading only.
.It Dv O_WRONLY
Open for writing only.
.It Dv O_RDWR
Open for reading and writing.
.El
.Pp
Any combination of the following may be used:
.Bl -tag -offset indent -width O_NONBLOCK
.It Dv O_NONBLOCK
Do not block on open or for data to become available.
.It Dv O_APPEND
Append to the file on each write.
.It Dv O_CREAT
Create the file if it does not exist, in which case the file is
created with mode
.Ar mode
as described in
.Xr chmod 2
and modified by the process' umask value (see
.Xr umask 2 ) .
.It Dv O_TRUNC
Truncate size to 0.
.It Dv O_EXCL
Error if
.Dv O_CREAT
and the file already exists.
.It Dv O_SHLOCK
Atomically obtain a shared lock.
.It Dv O_EXLOCK
Atomically obtain an exclusive lock.
.It Dv O_NOFOLLOW
If last path element is a symlink, don't follow it. This option is
provided for compatibility with other operating systems, but its
security value is questionable.
.It Dv O_DSYNC
If set, write operations will be performed according to synchronized
I/O data integrity completion:
each write will wait for the file data to be committed to stable
storage.
.It Dv O_SYNC
If set, write operations will be performed according to synchronized
I/O file integrity completion:
each write will wait for both the file data and file status to be
committed to stable storage.
.It Dv O_RSYNC
If set, read operations will complete at the same level of
integrity which is in effect for write operations:
if specified together with
.Dv O_SYNC ,
each read will wait for the file status to be committed to stable
storage.
.Pp
Combining
.Dv O_RSYNC
with
.Dv O_DSYNC
only, or specifying it without any other synchronized I/O integrity
completion flag set, has no further effect.
.El
.Pp
Opening a file with
.Dv O_APPEND
set causes each write on the file
to be appended to the end.
If
.Dv O_TRUNC
is specified and the
file exists, the file is truncated to zero length.
.Pp
If
.Dv O_EXCL
is set with
.Dv O_CREAT
and the file already
exists,
.Fn open
returns an error.
This may be used to implement a simple exclusive access locking mechanism.
If
.Dv O_EXCL
is set and the last component of the pathname is
a symbolic link,
.Fn open
will fail even if the symbolic
link points to a non-existent name.
.Pp
If the
.Dv O_NONBLOCK
flag is specified, do not wait for the device or file to be ready or
available.
If the
.Fn open
call would result
in the process being blocked for some reason (e.g., waiting for
carrier on a dialup line),
.Fn open
returns immediately.
This flag also has the effect of making all subsequent I/O on the open file non-blocking.
.Pp
When opening a file, a lock with
.Xr flock 2
semantics can be obtained by setting
.Dv O_SHLOCK
for a shared lock, or
.Dv O_EXLOCK
for an exclusive lock.
If creating a file with
.Dv O_CREAT ,
the request for the lock will never fail
(provided that the underlying filesystem supports locking).
.Pp
If
.Fn open
is successful, the file pointer used to mark the current position within
the file is set to the beginning of the file.
.Pp
When a new file is created it is given the group of the directory
which contains it.
.Pp
The new descriptor is set to remain open across
.Xr execve 2
system calls; see
.Xr close 2
and
.Xr fcntl 2 .
.Pp
The system imposes a limit on the number of file descriptors
open simultaneously by one process.
Calling
.Xr getdtablesize 3
returns the current system limit.
.Sh RETURN VALUES
If successful,
.Fn open
returns a non-negative integer, termed a file descriptor.
Otherwise, a value of \-1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
The named file is opened unless:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded
.Dv {NAME_MAX}
characters, or an entire path name exceeded
.Dv {PATH_MAX}
characters.
.It Bq Er ENOENT
.Dv O_CREAT
is not set and the named file does not exist, or
a component of the path name that must exist does not exist.
.It Bq Er EACCES
Search permission is denied for a component of the path prefix,
the required permissions (for reading and/or writing)
are denied for the given flags, or
.Dv O_CREAT
is specified,
the file does not exist,
and the directory in which it is to be created
does not permit writing.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EISDIR
The named file is a directory, and the arguments specify
it is to be opened for writing.
.It Bq Er EROFS
The named file resides on a read-only file system,
and the file is to be modified.
.It Bq Er EMFILE
The process has already reached its limit for open file descriptors.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er ENXIO
The named file is a character special or block
special file, and the device associated with this special file
does not exist, or
the named file is a
.Tn FIFO ,
.Dv O_NONBLOCK
and
.Dv O_WRONLY
is set and no process has the file open for reading.
.It Bq Er EINTR
The
.Fn open
operation was interrupted by a signal.
.It Bq Er EOPNOTSUPP
.Dv O_SHLOCK
or
.Dv O_EXLOCK
is specified but the underlying filesystem does not support locking.
.It Bq Er ENOSPC
.Dv O_CREAT
is specified,
the file does not exist,
and the directory in which the entry for the new file is being placed
cannot be extended because there is no space left on the file
system containing the directory.
.It Bq Er ENOSPC
.Dv O_CREAT
is specified,
the file does not exist,
and there are no free inodes on the file system on which the
file is being created.
.It Bq Er EDQUOT
.Dv O_CREAT
is specified,
the file does not exist,
and the directory in which the entry for the new file
is being placed cannot be extended because the
user's quota of disk blocks on the file system
containing the directory has been exhausted.
.It Bq Er EDQUOT
.Dv O_CREAT
is specified,
the file does not exist,
and the user's quota of inodes on the file system on
which the file is being created has been exhausted.
.It Bq Er EIO
An I/O error occurred while making the directory entry or
allocating the inode for
.Dv O_CREAT .
.It Bq Er ETXTBSY
The file is a pure procedure (shared text) file that is being
executed and the
.Fn open
call requests write access.
.It Bq Er EFAULT
.Fa path
points outside the process's allocated address space.
.It Bq Er EEXIST
.Dv O_CREAT
and
.Dv O_EXCL
were specified and the file exists.
.It Bq Er EOPNOTSUPP
An attempt was made to open a socket (not currently implemented).
.El
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr close 2 ,
.Xr dup 2 ,
.Xr lseek 2 ,
.Xr read 2 ,
.Xr umask 2 ,
.Xr write 2 ,
.Xr getdtablesize 3
.Sh STANDARDS
The
.Fn open
function conforms to
.St -p1003.1-90 .
The
.Fa flags
values
.Dv O_DSYNC ,
.Dv O_SYNC
and
.Dv O_RSYNC
are extensions defined in
.St -p1003.1b-93 .
.Pp
The
.Dv O_SHLOCK ,
.Dv O_EXLOCK ,
and
.Dv O_NOFOLLOW
flags are non-standard extensions and should not be used if portability
is of concern.
.Sh HISTORY
An
.Fn open
function call appeared in
.At v6 .