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Practical C Programming, 3rd Edition by Steve Oualline

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Chapter 14. File Input/Output

I the heir of all the ages, in the foremost files of time.

Alfred, Lord Tennyson

A file is a collection of related data. C treats a file as a series of bytes. Many files reside on disk; however, devices like terminals, printers, and magnetic tapes are also considered files.

The C library contains a large number of routines for manipulating files. The declarations for the structures and functions used by the file functions are stored in the standard include file <stdio.h>. Before doing anything with files, you must put the line:

#include <stdio.h>

at the beginning of your program.

The declaration for a file variable is:

FILE *file-variable;      /* comment */

For example:

#include <stdio.h> 
FILE *in_file;  /* file containing the input data */

Before a file can be used, it must be opened using the function fopen. fopen returns a pointer to the file structure for the file. The format for fopen is:

file-variable = fopen(name, mode);

where:

file-variable

is a file variable. A value of NULL is returned on error.

name

is the actual name of the file (data.txt, temp.dat, etc.).

mode

indicates if the file is to be read or written. mode is "w" for writing and "r" for reading. The flag “b” can be added to indicate a binary file. Omitting the binary flag indicates an ASCII (text) file. (See Section 14.2 for a description of ASCII and binary files.)

Flags can be combined. So “wb” is used to write a binary file.

The function returns a file handle that will be used in subsequent I/O operations. ...

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