int
open(
const char *path
, int flags
, mode_t mode
)
path
is opened
for reading and/or writing as specified by the
argument
flags
and the file descriptor returned to the calling process.
The
flags
are specified by
or'ing
the values listed below.
Applications must specify exactly one of the first three values
(file access methods):
O_RDONLY
O_WRONLY
O_RDWR
Any combination of the following may be used:
O_NONBLOCK
O_APPEND
O_CREAT
O_TRUNC
O_EXCL
O_CREAT
and the file already exists.
O_SHLOCK
O_EXLOCK
O_NOFOLLOW
O_DSYNC
O_SYNC
O_RSYNC
O_SYNC
,
each read will wait for the file status to be committed to stable
storage.
Combining
O_RSYNC
with
O_DSYNC
only, or specifying it without any other synchronized I/O integrity
completion flag set, has no further effect.
O_ALT_IO
O_NOCTTY
O_DIRECT
To meet the alignment requirements for direct I/O, the file offset,
the length of the I/O and the address of the buffer in memory must all
be multiples of
DEV_BSIZE
(512 bytes).
If the I/O request is made
using an interface that supports scatter/gather via struct iovec, each
element of the request must meet the above alignment constraints.
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, do not wait for the device or file to be ready or
available.
If the
open()
call would result
in the process being blocked for some reason (e.g., waiting for
carrier on a dialup line),
open(
)
returns immediately.
This flag also has the effect of making all subsequent I/O on the open file non-blocking.
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 filesystem supports locking).
If
open()
is successful, 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. Calling getdtablesize(3) returns the current system limit.
)
returns a non-negative integer, termed a file descriptor.
Otherwise, a value of -1 is returned and
errno
is set to indicate the error.
EPERM
]
ENOTDIR
]
ENAMETOOLONG
]
NAME_MAX
characters, or an entire path name exceeded
PATH_MAX
characters.
ENOENT
]
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.
EACCES
]
O_CREAT
is specified,
the file does not exist,
and the directory in which it is to be created
does not permit writing.
ELOOP
]
EISDIR
]
EROFS
]
EMFILE
]
ENFILE
]
ENXIO
]
O_NONBLOCK
and
O_WRONLY
is set and no process has the file open for reading.
EINTR
]
)
operation was interrupted by a signal.
EOPNOTSUPP
]
O_SHLOCK
or
O_EXLOCK
is specified but the underlying filesystem does not support locking.
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.
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.
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.
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.
EIO
]
O_CREAT
.
ETXTBSY
]
)
call requests write access.
EFAULT
]
path
points outside the process's allocated address space.
EEXIST
]
O_CREAT
and
O_EXCL
were specified and the file exists.
EOPNOTSUPP
]
)
function conforms to
ISO/IEC 9945-1:1990 (``POSIX.1'') .
The
flags
values
O_DSYNC
,
O_SYNC
and
O_RSYNC
are extensions defined in
IEEE Std 1003.1b-1993 (``POSIX.1'') .
The
O_SHLOCK
,
O_EXLOCK
,
and
O_NOFOLLOW
flags are non-standard extensions and should not be used if portability
is of concern.
)
function call appeared in
Version 6 AT&T UNIX
.