Resource management in Rust

For simplicity, we omit error handling in this example. Consider the following pseudo-code.

WRITE_TO_FILE()
1   file_descriptor ← OPEN_FILE("MyDocument.txt")
2   text_to_write ← "Hello, World!"
3   WRITE_TEXT(file_descriptor, text_to_write)
4   CLOSE_FILE(file_descriptor)

On line 1, we call the function OPEN_FILE and we pass as parameter the name of the file we want to open. This function OPEN_FILE, which we do not give pseudo-code for, communicates with the operating system and asks to open the file with the given file name.

In general, an application can open more than one file. So if the application asks the operating system to write to a file, the operating system has to know: "Which file are you referring to?". For this reason, the function OPEN_FILE returns a number that will be used later on in the communication with the operating system to specify which file the application is talking about. In the case of a file, the number is called a file descriptor. In most libraries, the file descriptor is wrapped in another data type that represents the file. In our example, we just assume that we get the bare file descriptor.

Line 2 is nothing special: we just store the text we want to write in a variable.

On line 3, we call the function WRITE_TEXT to write this text to the file. The function WRITE_TEXT asks the operating system to write the given text to a file. The first parameter, for which we pass the variable file_descriptor specifies to which file the text should be written.

On line 4, we call the function CLOSE_FILE to close the file. Again, we pass the variable file_descriptor to specify which file must be closed.

Exercise. In the previous example, the programmer can make a bug for instance by forgetting to close the file. There are many mistakes like that. Write down a number of them. We will come back to this later on.