File Modes: Reading, Writing, and Appending

The Request Behind an Open

When a program opens a file, it is not immediately holding the file's contents. It is making a request to the operating system. The request identifies a file by name and asks the operating system to locate it and verify that it exists and is readable.

The Open Operation

The operating system has an important role in opening a file. It must locate the file using the name supplied by the program. It must then verify the conditions described by this topic: that the file exists and is readable. If those conditions are satisfied, the operating system returns a file handle. The handle represents the program's connection to the open file; it is not a copy of the file's contents.

Handle and Contents

A file handle and file contents are different things. The file contents are the data stored in the file. The file handle is the reference tool returned after the operating system opens the file. To obtain the actual contents, the program must use operations provided by the file handle. The handle is therefore the key used to access the file, not the data stored inside it.

Access Modes

Reading, writing, and appending are the three access modes named in this topic. A file handle contains metadata about the access to the file, including the file name, the mode, and the encoding. The mode is therefore part of the handle's description of how the program is accessing the open file.

Mistakes to Avoid

• Treating the file handle as if it were the file's contents.

  The source distinguishes the handle, which is a reference tool, from the actual file data.

  Fix: Use operations on the file handle to access the actual contents.

• Skipping the operating system's role.

  Opening asks the operating system to locate the file and verify that it exists and is readable.

  Fix: Trace the request through locating and verification before describing the returned handle.

• Assuming that a mode explains the file's contents.

  Mode is metadata about access, while the contents are the actual stored data.

  Fix: Keep access metadata and file data as separate concepts.

Check Your Understanding

Key Takeaways

1. Opening a file asks the operating system to locate it by name and verify that it exists and is readable.
2. A successful open returns a file handle, which is a reference tool and connection object.
3. The handle is not the actual data stored in the file.
4. The handle contains metadata such as the file name, access mode, and encoding.
5. The actual file contents are accessed through operations provided by the file handle.