open
- open or create a file for reading or writing
LIBRARY
Lb libc
SYNOPSIS
#include <fcntl.h> int
open (const char *path int flags ...);
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
O_CREAT
flag).
In this case
open ();
requires a third argument
Fa mode_t mode ,
and the file is created with mode
Fa mode
as described in
chmod(2)
and modified by the process' umask value (see
umask(2)).
The flags specified are formed by
or 'ing
the following values
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_SYNC synchronous writes
O_NOFOLLOW do not follow symlinks
O_NOCTTY don't assign controlling terminal
Opening a file with
O_APPEND
set causes each write on the file
to be appended to the end.
If
O_TRUNC
is specified and the
file exists, the file is truncated to zero length.
If
O_EXCL
is set with
O_CREAT
and the file already
exists,
open ();
returns an error.
This may be used to
implement a simple exclusive access locking mechanism.
If
O_EXCL
is set and the last component of the pathname is
a symbolic link,
open ();
will fail even if the symbolic
link points to a non-existent name.
If the
O_NONBLOCK
flag is specified and the
open ();
system call would result
in the process being blocked for some reason (e.g., waiting for
carrier on a dialup line),
open ();
returns immediately.
The descriptor remains in non-blocking mode for subsequent operations.
If
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.
O_SYNC
is a synonym for
O_FSYNC
required by
POSIX
If
O_NOFOLLOW
is used in the mask and the target file passed to
open ();
is a symbolic link then the
open ();
will fail.
When opening a file, a lock with
flock(2)
semantics can be obtained by setting
O_SHLOCK
for a shared lock, or
O_EXLOCK
for an exclusive lock.
If creating a file with
O_CREAT
the request for the lock will never fail
(provided that the underlying file system supports locking).
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.
O_NOCTTY
may be used to ensure the OS does not assign this file as the
controlling terminal when it opens a tty device.
This is the default on
Fx ,
but is present for
POSIX
compatibility.
The
open ();
system call will not assign controlling terminals on
Fx .
If successful,
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.
When a new file is created it is given the group of the directory
which contains it.
The new descriptor is set to remain open across
execve(2)
system calls; see
close(2)
and
fcntl(2).
The system imposes a limit on the number of file descriptors
open simultaneously by one process.
The
getdtablesize(2)
system call returns the current system limit.
RETURN VALUES
If successful,
open ();
returns a non-negative integer, termed a file descriptor.
It returns -1 on failure, and sets
errno
to indicate the error.
ERRORS
The named file is opened unless:
Bq Er ENOTDIR
A component of the path prefix is not a directory.
Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or an entire path name exceeded 1023 characters.
Bq Er ENOENT
O_CREAT
is not set and the named file does not exist.
Bq Er ENOENT
A component of the path name that must exist does not exist.
Bq Er EACCES
Search permission is denied for a component of the path prefix.
Bq Er EACCES
The required permissions (for reading and/or writing)
are denied for the given flags.
Bq Er EACCES
O_TRUNC
is specified and write permission is denied.
Bq Er EACCES
O_CREAT
is specified,
the file does not exist,
and the directory in which it is to be created
does not permit writing.
Bq Er EPERM
O_CREAT
is specified, the file does not exist, and the directory in which it is to be
created has its immutable flag set, see the
chflags(2)
manual page for more information.
Bq Er EPERM
The named file has its immutable flag set and the file is to be modified.
Bq Er EPERM
The named file has its append-only flag set, the file is to be modified, andO_TRUNC
is specified or
O_APPEND
is not specified.
Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
Bq Er EISDIR
The named file is a directory, and the arguments specify
it is to be modified.
Bq Er EROFS
The named file resides on a read-only file system,
and the file is to be modified.
Bq Er EROFS
O_CREAT
is specified and the named file would reside on a read-only file system.
Bq Er EMFILE
The process has already reached its limit for open file descriptors.
Bq Er ENFILE
The system file table is full.
Bq Er EMLINK
O_NOFOLLOW
was specified and the target is a symbolic link.
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.
Bq Er ENXIO
O_NONBLOCK
is set, the named file is a fifo,
O_WRONLY
is set, and no process has the file open for reading.
Bq Er EINTR
The
open ();
operation was interrupted by a signal.
Bq Er EOPNOTSUPP
O_SHLOCK
or
O_EXLOCK
is specified but the underlying file system does not support locking.
Bq Er EOPNOTSUPP
The named file is a special file mounted through a file system that
does not support access to it (e.g. NFS).
Bq Er EWOULDBLOCK
O_NONBLOCK
and one of
O_SHLOCK
or
O_EXLOCK
is specified and the file is locked.
Bq Er ENOSPC
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.
Bq Er ENOSPC
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.
Bq Er EDQUOT
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.
Bq Er EDQUOT
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.
Bq Er EIO
An I/O error occurred while making the directory entry or
allocating the inode for
O_CREAT
Bq Er ETXTBSY
The file is a pure procedure (shared text) file that is being
executed and the
open ();
system call requests write access.
Bq Er EFAULT
The
Fa path
argument
points outside the process's allocated address space.
Bq Er EEXIST
O_CREAT
and
O_EXCL
were specified and the file exists.
Bq Er EOPNOTSUPP
An attempt was made to open a socket (not currently implemented).
Bq Er EINVAL
An attempt was made to open a descriptor with an illegal combination
of
O_RDONLYO_WRONLY
and
O_RDWR
