— open or create a file for
reading or writing
char *path, int
fd, const char
*path, int flags,
The file name specified by 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 argument may indicate the file is
to be created if it does not exist (by specifying the
O_CREAT flag), in which case the file is created
with a mode specified by an additional argument of type
mode_t as described in
chmod(2) and modified by the process' umask value (see
The flags specified are a bitwise OR of the following values. Exactly one of the first three values (file access modes) must be specified:
Any combination of the following flags may additionally be used:
- Do not block on open or for data to become available.
- Append on each write.
- Create file if it does not exist. An additional argument of type mode_t must be supplied to the call.
- Truncate size to 0.
- Error if
O_CREATis set and file exists.
- Perform synchronous I/O operations.
- Atomically obtain a shared lock.
- Atomically obtain an exclusive lock.
- If last path element is a symlink, don't follow it.
FD_CLOEXEC(the close-on-exec flag) on the new file descriptor.
- Error if path does not name a directory.
Opening a file with
causes each write on the file to be appended to the end. If
O_TRUNC and a writing mode are specified and the
file exists, the file is truncated to zero length. If
O_EXCL is set with
and the file already exists,
an error. This may be used to implement a simple exclusive access locking
mechanism. If either of
O_NOFOLLOW are 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. If the
O_SYNC flag is set, all I/O operations on the file
will be done synchronously.
A FIFO should either be opened with
O_RDONLY or with
The behavior for opening a FIFO with
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
O_CREAT, the request for the lock will never
fail (provided that the underlying filesystem supports locking).
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. getdtablesize(3) returns the current system limit.
function is equivalent to
open() except that where
path specifies a relative path, the file to be opened
is determined relative to the directory associated with file descriptor
fd instead of the current working directory.
is passed the special value
AT_FDCWD (defined in
<fcntl.h>) in the
fd parameter, the current working directory is used
and the behavior is identical to a call to
open() returns a
non-negative integer, termed a file descriptor. Otherwise, a value of -1 is
returned and errno is set to indicate the error.
openat() functions will fail if:
- A component of the path prefix is not a directory.
O_DIRECTORYis specified and path does not name a directory.
- A component of a pathname exceeded
NAME_MAXcharacters, or an entire pathname (including the terminating NUL) exceeded
O_CREATis not set and the named file does not exist.
- A component of the pathname that must exist does not exist.
- 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.
O_CREATis specified, the file does not exist, and the directory in which it is to be created does not permit writing.
- Too many symbolic links were encountered in translating the pathname, or
O_NOFOLLOWflag was specified and the target is a symbolic link.
- The named file is a directory, and the arguments specify it is to be opened for writing.
- The flags specified for opening the file are not valid.
- The named file resides on a read-only file system, and the file is to be modified.
- The process has already reached its limit for open file descriptors.
- The system file table is full.
- The named file is a character special or block special file, and the device associated with this special file does not exist.
- The named file is a FIFO, the
O_WRONLYflags are set, and no process has the file open for reading.
open() operation was interrupted by a signal.
O_EXLOCKis specified but the underlying filesystem does not support locking.
O_NONBLOCKand one of
O_EXLOCKis specified and the file is already locked.
O_CREATis 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.
O_CREATis specified, the file does not exist, and there are no free inodes on the file system on which the file is being created.
O_CREATis 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.
O_CREATis 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.
- An I/O error occurred while making the directory entry or allocating the
- The file is a pure procedure (shared text) file that is being executed and
open() call requests write access.
- path points outside the process's allocated address space.
O_EXCLwere specified and the file exists.
- The file named by path is flagged append-only but
O_APPENDwas not specified in flags.
- An attempt was made to open a socket (not currently implemented).
- An attempt was made to open a terminal device that requires exclusive access and the specified device has already be opened.
openat() function will
- The path argument specifies a relative path and the
fd argument is neither
AT_FDCWDnor a valid file descriptor.
- The path argument specifies a relative path and the fd argument is a valid file descriptor but it does not reference a directory.
- The path argument specifies a relative path but search permission is denied for the directory which the fd file descriptor references.
chflags(2), chmod(2), close(2), dup(2), flock(2), lseek(2), read(2), umask(2), write(2), getdtablesize(3)
openat() functions conform to IEEE
Std 1003.1-2008 (“POSIX.1”).
POSIX specifies three different flavors
for synchronous I/O:
OpenBSD, these are all equivalent.
O_EXLOCK flags are non-standard extensions and
should not be used if portability is of concern.
open() system call first appeared in
Version 1 AT&T UNIX. The
flags argument has been supported since
4.2BSD. Before that, a dedicated
creat() system call had to be used to create new
files; it appeared in Version 1 AT&T
UNIX, was deprecated in 4.3BSD-Reno, and
removed in OpenBSD 5.0.
openat() system call has been
available since OpenBSD 5.0.
O_TRUNC flag requires that one of
O_WRONLY also be
EINVAL is returned.