Closing a File

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Из курса от партнера Duke University

Interacting with the System and Managing Memory

16 оценок

Duke University

Interacting with the System and Managing Memory

16 оценок

Курс 4 из 4 — Specialization Introduction to Programming in C

The final course in the specialization Introduction to Programming in C will teach you powerful new programming techniques for interacting with the user and the system and dynamically allocating memory. You will learn more sophisticated uses for pointers, such as strings and multidimensional arrays, as well as how to write programs that read and write files and take input from the user. Learning about dynamic memory allocation will allow your programs to perform complex tasks that will be applied in the final part of the specialization project: a Monte Carlo simulation for calculating poker hand probabilities.

Из урока

Module 1: Interacting with the user and system

So far, our programs have had a rather limited interaction with the user or rest of the system, printing some results to standard output (typically to the terminal). Now that we have learned about topics such as strings and arrays, we are ready to learn how to write a program that takes input from the user, takes arguments on the command line, accesses files, and does many other things we typically think of real programs as doing.

Writing to a File2:19

Closing a File2:36

Познакомьтесь с преподавателями

Andrew D. Hilton
Associate Professor of the Practice
Electrical and Computer Engineering
Anne Bracy
Senior Lecturer
Computer Science, Cornell University
Genevieve M. Lipp
Adjunct Professor
Electrical and Computer Engineering/Mechanical Engineering

In this video, we'll look at what happens when fclose is called.

A lot of this is conceptualized because we're not actually going to

delve into the details of what the kernel does or how it interacts with the hardware.

The important thing to understand here

is that our program may have buffered up these writes,

it may be holding them in the file structure,

the C library manages and may not

have actually written them down to the Operating System yet.

Therefore, they're not actually on the disk.

However, when we close the file,

these writes have to actually be propagated down.

So, the first thing that's going to happen if the C library has unwritten data,

is it's going to make a system call to write that into the Operating System.

Here, we see the write system call,

which takes a file descriptor,

the low level communication mechanism to

specify which file between a program and the kernel.

In this case, we're going to assume it's 3.

We wouldn't really know unless we looked in

more detail to figure it out and we don't really care at this point.

The write system call also takes a pointer to the data to write,

as well as how many bytes to write.

In this case, it's 12,

because there are three lines each with four bytes.

Each of these have three numerical digits plus the new line character.

So, this data is going to be written into the operating system's memory.

You'll notice I've written it over here as the actual numerical values of

the bytes rather than the textual representation of the digits.

Just as a reminder that everything is

a number and because this is really just dealing with raw data,

we're not really thinking of it as text anymore.

It's just a bunch of bytes going to a disk.

Next, the C library is going to make the close system call,

passing in the file descriptor because

that's how it communicates with the operating system.

Then the operating system is going to

decide that it should write its buffered data to the disk.

So it's going to write all of these bytes onto the disk somewhere.

Then the disk is going to tell the operating system that it actually succeeded.

It's possible that this fails,

maybe some piece of the disk is gone bad,

in which case it wouldn't be able to write the data,

but let's assume that it succeeds.

So the kernel is going to recognize that this file is closed

and also tell the program that it succeeded in closing the file by returning 0,

which is going to be the return value of fclose.

At this point, the file is closed so we can't do anything else with

it and our call to fclose will finish and will continue executing code.

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