Reading File Contents with File Methods
Opening a file asks the operating system to locate the file by name and verify it exists and is readable.
The Request to Open
When a program opens a file, it is not immediately receiving all of the file's contents. The program is asking the operating system to locate a file by name and verify that the file exists and is readable. If that check succeeds, the operating system returns a file handle that the program can use to access the file.
Opening a file establishes access to the file. It does not, by itself, mean that the actual data has been placed in the file handle.
The Operating System Check
The operating system performs the locating and verification work. It uses the file name supplied by the program to locate the file, then checks whether the file exists and is readable. A successful result is not the file's data itself; it is a file handle, which is a reference tool for working with the open file.
Conditions for Access
Two conditions are central to a successful open operation: the named file must exist, and it must be readable. If either condition is not met, the operating system cannot provide the successful file access represented by the handle.
Handle and Contents
A file handle is a connection object that provides a way to work with an open file. It contains metadata about the access, including the file name, mode, and encoding. It is a reference tool, not the actual data stored in the file.
Think of the handle as the key to the file rather than the contents of the file. To obtain the actual file contents, you must use methods on the file handle. The handle gives the program a connection and information about that connection; the methods provide the path to the stored data.
A Complete Opening Trace
Opening a Readable File
A program asks to open a file named notes.txt. The file exists and is readable. What does the program receive, and what does it still need to do to obtain the contents?
Request: The program asks the operating system to open notes.txt.
Locate: The operating system uses the supplied name to locate the file.
Verify: The operating system verifies that the file exists and is readable.
Return: Because both conditions are satisfied, the operating system returns a file handle.
Read: The program must use methods on that file handle to read the actual contents. The handle itself is not the contents.
The successful open produces a file handle containing or providing access information such as the file name, mode, and encoding. The actual file data is obtained through methods on the handle.
The sequence has two different outcomes to keep separate: opening creates or returns the access connection, while reading through the handle obtains the file data.
Mistakes About File Access
Treating the file handle as if it were the file contents.
The handle is a reference tool and connection object, not the data itself.
Fix:
Use methods on the file handle to read the actual contents.Assuming that naming a file is enough to open it successfully.
The operating system must locate the file and verify both its existence and readability.
Fix:
Treat existence and readability as required conditions for successful access.Ignoring the information associated with the handle.
A file handle contains metadata about access, including the file name, mode, and encoding.
Fix:
Recognize the handle as a connection object that describes and enables access to the open file.
Check Your Understanding
A program requests a file by name. Explain the operating system's role, state the two conditions needed for successful opening, and identify what the program receives before it reads the actual contents.
Hints
- Start with what the operating system must locate and verify.
- Separate the returned reference from the data stored in the file.
- Name the three kinds of access information associated with the handle.
- A strong answer says that opening asks the operating system to locate the named file and verify that it exists and is readable. When that succeeds, the operating system returns a file handle. The handle is a connection object and reference tool, not the file data; methods on the handle are needed to read the contents. The handle also provides access information such as the file name, mode, and encoding.
Key Takeaways
- Opening a file asks the operating system to locate it by name and verify that it exists and is readable.
- A successful open returns a file handle, which is a reference tool and connection object.
- The file handle is not the actual data stored in the file.
- Methods on the file handle are required to read the file contents.
- A file handle provides access information such as the file name, mode, and encoding.