libc/sys/open.2

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.\"     @(#)open.2	8.2 (Berkeley) 11/16/93
.\" $FreeBSD$
.\"
.Dd November 16, 1993
.Dt OPEN 2
.Os
.Sh NAME
.Nm open
.Nd open or create a file for reading or writing
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include <fcntl.h>
.Ft int
.Fn open "const char *path" "int flags" "..."
.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
argument may indicate the file is to be
created if it does not exist (by specifying the
.Dv O_CREAT
flag).
In this case
.Nm
requires a third argument
.Fa "mode_t mode" ,
and the file is created with mode
.Fa mode
as described in
.Xr chmod 2
and modified by the process' umask value (see
.Xr umask 2 ) .
.Pp
The flags specified are formed by
.Em or Ns 'ing
the following values
.Pp
.Bd -literal -offset indent -compact
O_RDONLY	open for reading only
O_WRONLY	open for writing only
O_RDWR		open for reading and writing
O_NONBLOCK	do not block on open
O_APPEND	append on each write
O_CREAT		create file if it does not exist
O_TRUNC		truncate size to 0
O_EXCL		error if create and file exists
O_SHLOCK	atomically obtain a shared lock
O_EXLOCK	atomically obtain an exclusive lock
O_DIRECT	eliminate or reduce cache effects
O_FSYNC		synchronous writes
O_NOFOLLOW	do not follow symlinks
.Ed
.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.
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.
If the
.Dv O_NONBLOCK
flag is specified and 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.
The first time the process attempts to perform I/O on the open
file it will block (not currently implemented).
.Pp
If
.Dv O_FSYNC
is used in the mask, all writes will
immediately be written to disk,
the kernel will not cache written data
and all writes on the descriptor will not return until
the data to be written completes.
.Pp
If
.Dv O_NOFOLLOW
is used in the mask and the target file passed to
.Fn open
is a symbolic link then the
.Fn open
will fail.
.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
.Dv O_DIRECT
may be used to minimize or eliminate the cache effects of reading and writing.
The system will attempt to avoid caching the data you read or write.
If it cannot avoid caching the data,
it will minimize the impact the data has on the cache.
Use of this flag can drastically reduce performance if not used with care.
.Pp
If successful,
.Fn open
returns a non-negative integer, termed a file descriptor.
It returns -1 on failure.
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.
.Xr Getdtablesize 2
returns the current system limit.
.Sh IMPLEMENTATION NOTES
In the non-threaded library
.Fn open
is implemented as the
.Va open
syscall.
.Pp
In the threaded library, the
.Va open
syscall is assembled to
.Fn _thread_sys_open
and
.Fn open
is implemented as a function which disables thread rescheduling
and calls
.Fn _thread_sys_open .
Before returning,
.Fn open
enables thread rescheduling.
.Sh RETURN VALUES
If successful,
.Fn open
returns a non-negative integer, termed a file descriptor.
It returns -1 on failure, and sets
.Va errno
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 255 characters,
or an entire path name exceeded 1023 characters.
.It Bq Er ENOENT
.Dv O_CREAT
is not set and the named file does not exist.
.It Bq Er ENOENT
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.
.It Bq Er EACCES
The required permissions (for reading and/or writing)
are denied for the given flags.
.It Bq Er EACCES
.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 EMLINK
.Dv O_NOFOLLOW
was specified and the target is a symbolic link.
.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.
.It Bq Er ENXIO
The named file is a fifo, no process has
it open for reading, and the arguments specify it is
to be opened for writing.
.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 EWOULDBLOCK
.Dv O_NONBLOCK
and one of
.Dv O_SHLOCK
or
.Dv O_EXLOCK
is specified and the file is locked.
.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).
.It Bq Er EINVAL
An attempt was made to open a descriptor with an illegal combination
of
.Dv O_RDONLY ,
.Dv O_WRONLY ,
and
.Dv O_RDWR .
.El
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr close 2 ,
.Xr dup 2 ,
.Xr getdtablesize 2 ,
.Xr lseek 2 ,
.Xr read 2 ,
.Xr umask 2 ,
.Xr write 2
.Sh HISTORY
An
.Fn open
function call appeared in
.At v6 .