Manual Pages  — OPEN

The openat() function is equivalent to the open() function except in the case where the path specifies a relative path. For openat() and relative path, the file to be opened is determined relative to the directory associated with the file descriptor fd instead of the current working directory. The flag parameter and the optional fourth parameter correspond exactly to the parameters of open(). If openat() is passed the special value AT_FDCWD in the fd parameter, the current working directory is used and the behavior is identical to a call to open().

When openat() is called with an absolute path, it ignores the fd argument.

In capsicum(4) capability mode, open() is not permitted. The path argument to openat() must be strictly relative to a file descriptor fd. path must not be an absolute path and must not contain ".." components which cause the path resolution to escape the directory hierarchy starting at fd. Additionally, no symbolic link in path may target absolute path or contain escaping ".." components. fd must not be AT_FDCWD.

If the vfs.lookup_cap_dotdot sysctl(3) MIB is set to zero, ".." components in the paths, used in capability mode, are completely disabled. If the vfs.lookup_cap_dotdot_nonlocal MIB is set to zero, ".." is not allowed if found on non-local filesystem.

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_EXEC          open for execute only
O_SEARCH        open for search only, an alias for O_EXEC
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        ignored
O_TTY_INIT      ignored
O_DIRECTORY     error if file is not a directory
O_CLOEXEC       set FD_CLOEXEC upon open
O_VERIFY        verify the contents of the file
O_RESOLVE_BENEATH       path resolution must not cross the fd directory

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 and synchronously be written to disk.

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 FreeBSD , but is present for POSIX compatibility. The open() system call will not assign controlling terminals on FreeBSD .

O_TTY_INIT may be used to ensure the OS restores the terminal attributes when initially opening a TTY. This is the default on FreeBSD , but is present for POSIX compatibility. The initial call to open() on a TTY will always restore default terminal attributes on FreeBSD .

O_DIRECTORY may be used to ensure the resulting file descriptor refers to a directory. This flag can be used to prevent applications with elevated privileges from opening files which are even unsafe to open with O_RDONLY, such as device nodes.

O_CLOEXEC may be used to set FD_CLOEXEC flag for the newly returned file descriptor.

O_VERIFY may be used to indicate to the kernel that the contents of the file should be verified before allowing the open to proceed. The details of what "verified" means is implementation specific. The run-time linker (rtld) uses this flag to ensure shared objects have been verified before operating on them.

O_RESOLVE_BENEATH returns ENOTCAPABLE if any intermediate component of the specified relative path does not reside in the directory hierarchy beneath the starting directory. Absolute paths or even the temporal escape from beneath of the starting directory is not allowed.

When fd is opened with O_SEARCH, execute permissions are checked at open time. The fd may not be used for any read operations like getdirentries(2). The primary use for this descriptor will be as the lookup descriptor for the *at() family of functions.

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.

If a sleeping open of a device node from devfs(5) is interrupted by a signal, the call always fails with EINTR, even if the SA_RESTART flag is set for the signal. A sleeping open of a fifo (see mkfifo(2)) is restarted as normal.

When a new file is created it is given the group of the directory which contains it.

Unless O_CLOEXEC flag was specified, the new descriptor is set to remain open across execve(2) system calls; see close(2), fcntl(2) and O_CLOEXEC description.

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.