module File::Constants

Module File::Constants defines file-related constants.

There are two families of constants here:

Those having to do with file access.
Those having to do with filename globbing.

File constants defined for the local process may be retrieved with method File::Constants.constants:

File::Constants.constants.take(5)
# => [:RDONLY, :WRONLY, :RDWR, :APPEND, :CREAT]

File Access¶ ↑

File-access constants may be used with optional argument mode in calls to the following methods:

Read/Write Access¶ ↑

Read-write access for a stream may be specified by a file-access constant.

The constant may be specified as part of a bitwise OR of other such constants.

Any combination of the constants in this section may be specified.

File::RDONLY¶ ↑

Flag File::RDONLY specifies the stream should be opened for reading only:

filepath = '/tmp/t.tmp'
f = File.new(filepath, File::RDONLY)
f.write('Foo') # Raises IOError (not opened for writing).

File::WRONLY¶ ↑

Flag File::WRONLY specifies that the stream should be opened for writing only:

f = File.new(filepath, File::WRONLY)
f.read # Raises IOError (not opened for reading).

File::RDWR¶ ↑

Flag File::RDWR specifies that the stream should be opened for both reading and writing:

f = File.new(filepath, File::RDWR)
f.write('Foo') # => 3
f.rewind       # => 0
f.read         # => "Foo"

File Positioning¶ ↑

File::APPEND¶ ↑

Flag File::APPEND specifies that the stream should be opened in append mode.

Before each write operation, the position is set to end-of-stream. The modification of the position and the following write operation are performed as a single atomic step.

File::TRUNC¶ ↑

Flag File::TRUNC specifies that the stream should be truncated at its beginning. If the file exists and is successfully opened for writing, it is to be truncated to position zero; its ctime and mtime are updated.

There is no effect on a FIFO special file or a terminal device. The effect on other file types is implementation-defined. The result of using File::TRUNC with File::RDONLY is undefined.

Creating and Preserving¶ ↑

File::CREAT¶ ↑

Flag File::CREAT specifies that the stream should be created if it does not already exist.

If the file exists:

- Raise an exception if File::EXCL is also specified.
- Otherwise, do nothing.

If the file does not exist, then it is created. Upon successful completion, the atime, ctime, and mtime of the file are updated, and the ctime and mtime of the parent directory are updated.

File::EXCL¶ ↑

Flag File::EXCL specifies that the stream should not already exist; If flags File::CREAT and File::EXCL are both specified and the stream already exists, an exception is raised.

The check for the existence and creation of the file is performed as an atomic operation.

If both File::EXCL and File::CREAT are specified and the path names a symbolic link, an exception is raised regardless of the contents of the symbolic link.

If File::EXCL is specified and File::CREAT is not specified, the result is undefined.

POSIX File Constants¶ ↑

Some file-access constants are defined only on POSIX-compliant systems; those are:

File::SYNC.
File::DSYNC.
File::RSYNC.
File::DIRECT.
File::NOATIME.
File::NOCTTY.
File::NOFOLLOW.
File::TMPFILE.

File::SYNC, File::RSYNC, and File::DSYNC¶ ↑

Flag File::SYNC, File::RSYNC, or File::DSYNC specifies synchronization of I/O operations with the underlying file system.

These flags are valid only for POSIX-compliant systems.

File::SYNC specifies that all write operations (both data and metadata) are immediately to be flushed to the underlying storage device. This means that the data is written to the storage device, and the file’s metadata (e.g., file size, timestamps, permissions) are also synchronized. This guarantees that data is safely stored on the storage medium before returning control to the calling program. This flag can have a significant impact on performance since it requires synchronous writes, which can be slower compared to asynchronous writes.
File::RSYNC specifies that any read operations on the file will not return until all outstanding write operations (those that have been issued but not completed) are also synchronized. This is useful when you want to read the most up-to-date data, which may still be in the process of being written.
File::DSYNC specifies that all data write operations are immediately to be flushed to the underlying storage device; this differs from File::SYNC, which requires that metadata also be synchronized.

Note that the behavior of these flags may vary slightly depending on the operating system and filesystem being used. Additionally, using these flags can have an impact on performance due to the synchronous nature of the I/O operations, so they should be used judiciously, especially in performance-critical applications.

File::NOCTTY¶ ↑

Flag File::NOCTTY specifies that if the stream is a terminal device, that device does not become the controlling terminal for the process.