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Inside Macintosh: Files /

Chapter 1 - Introduction to File Management / About Files

File Access Characteristics

A file can be open or closed. Your application can perform certain operations, such as reading and writing data, only on open files. It can perform other operations, such as deleting, only on closed files.

When you open a file, the File Manager reads information about the file from its volume and stores that information in a file control block (FCB). The File Manager also creates an access path to the file, a description of the route to be followed when accessing the file. The access path specifies the volume on which the file is located and the location of the file on the volume. Each access path is assigned a unique file reference number (some number greater than 0) by which your application refers to the path. Multiple access paths can be opened to the same file.

For each open access path to a file, the File Manager maintains a current position marker, called the file mark, to keep track of where it is in the file during a read or write operation. The mark is the number of the next byte that will be read or written; each time a byte is read or written, the mark is moved. When, during a write operation, the mark reaches the number of the last byte currently allocated to the file, the File Manager adds another clump to the file.

You can read bytes from and write bytes to a file either singly or in sequences of virtually unlimited length. You can specify where each read or write operation should begin by setting the mark or specifying an offset; if you don't, the operation begins at the current file mark.

Each time you want to read or write a file's data, you need to pass the address of a data buffer, a part of RAM (usually in your application's heap). The File Manager uses the buffer when it transfers data to or from your application. You can use a single buffer for each read or write operation, or change the address and size of the buffer as necessary.

When your application writes data to a file, the File Manager transfers the data from your application's data buffer and writes it to the disk cache, a part of RAM (usually in the System heap). The File Manager uses the disk cache as an intermediate buffer when reading data from or writing it to the file system. When your application requests
that data be read from a file, the File Manager looks for the data in the disk cache
and transfers it to your application's data buffer if the data is found in the cache; otherwise, the File Manager reads the requested bytes from the disk and puts them in your data buffer.

Note

You can also read a continuous stream of characters or a line of characters from a file. In the first case, you ask the File Manager to read a specific number of bytes: When that many have been read, or when the mark reaches the logical end-of-file, the read operation terminates. In the second case, called newline mode, the read operation terminates when either of the above conditions is met or when a specified character, the newline character, is read. The newline character is usually Return (ASCII code $0D), but it can be any character. Information about newline mode is associated with each access path to a file and can differ from one access path to another. See the chapter "File Manager" in this book for more information about newline mode.