Files and I/O streams in C

This tutorial will teach you all about C file system (file handling in C) from very basic to advance. Let's starts with streams and files.

Streams and Files

Before going to discuss about C file system, it is necessary to know about the term streams and files or the difference between streams and files. C I/O system supplies a consistent interface to the programmer independent of the actual device being accessed i.e., the C I/O system provides a level of abstraction between the programmer and the device and this abstraction is called as stream, and the actual device is called as file. Let's discuss about streams.

Streams

The C file system is designed to work with a large variety of devices, including terminals, disk drives, and tape drives. However each devices is very different, the buffered file system transforms each into a logical device called a stream. There are the following two types of streams:

• Text Streams - A text stream is a sequence of characters.
• Binary Streams - A binary stream is a sequence of bytes that has a one-to-one correspondence to the bytes in the external device, i.e., no character translation occurs.

Files

In C language, a file may be anything from a disk file to a terminal or printer. You can associate a stream with a specific file by performing an open operation. And once a file is opened, information can be exchanged between it and your program.

C File System Basics

The C file system is composed of several interrelated functions. Here the following table lists the most common of these functions. These functions require the header file <stdio.h>

Function Name	Use
fopen()	Opens a file
fclose()	Closes a file
putc()	Writes a character to a file
fputc()	Same as putc()
getc()	Reads a character from a file
fgetc()	Same as getc()
fgets()	Reads a string from a file
fputs()	Writes a string to a file
fseek()	Seeks to a specified byte in a file
ftell()	Returns the current file position
fprintf()	Is to a file what printf() is to the console
fscanf()	Is to a file what scanf() is to the console
feof()	Returns true if end-of-file is reached
ferror()	Returns true if an error has occurred
rewind()	Resets the file position indicator to the beginning of the file
remove()	Erases a file
fflush()	Flushes a file

The File Pointer

The file pointer is simply the common thread that unites the C I/O system. A file pointer is a pointer to a structure of type FILE. it points to the information that defines various things about the file, including its name, status, and the current position of the file. Here is the statement to use the file pointers in your C program:

FILE *fp;

Opening a File in CTo open a file in C, use fopen() function. The fopen() function opens a stream for use and links a file with that stream. And then it returns the file pointer associated with that file. Here is the prototype of the function fopen()

FILE *fopen(const char *filename, const char *mode)

Here, the filename is the name of the file and mode is the file access mode. Here is the list of all access mode that you can use in opening your file:

Mode	Use
r	Open an existing text file for reading
w	Create a text file for writing
a	Append to a text file
rb	Open a binary file for reading
wb	Create a binary file for writing
ab	Append to a binary file
r+	Open a text file for reading/writing
w+	Create a text file for reading/writing
a+	Append or create a text file for reading/writing
r+b	Open a binary file for reading/writing
w+b	Create a binary file for reading/writing
a+b	Append or create a binary file for reading/writing

Here the following code fragment uses fopen() function to open a file named myfile.txt for reading

    FILE *fp;

    fp = fopen("myfile.txt", "r");

Closing a File in C

To close a file in C, use fclose() function. This function closes a stream that was opened by a call to fopen(). Here is the prototype of the fclose() function

        int fclose(FILE *fp);

Here, fp is the file pointer returned by the call to the function fopen( ).

Writing a Character to a File in C

The C I/O system defines the following two functions that is used in outputting a character to a file. Both functions are equivalent.

        putc( )

        fputc( )

Here is the prototype of the function putc( )

        int putc(int ch, FILE *fp);

Here fp is the file pointer returned by the function fopen(), and ch is the character to be output. If a putc( ) operation is successful, it returns the character written. Otherwise, it returns EOF (end-of-file).

Reading a Character from a File in C

The C I/O system defines the following two functions that is used in inputting a character from a file. Both the functions are equivalent.

        getc( )

        fgetc( )

Here is the prototype of the function getc( )

        int getc(FILE *fp);

Here fp is the file pointer of type FILE returned by fopen( ) function. The function getc( ) returns an EOF when the end of the file has been reached.

C File I/O Examples

Now let's take some examples of C File I/O.

C Reading a File Example

This program reads a file entered by the user and displays the content of the file on the output screen. Before running this program, first create any file say "myfile.txt" and writes some content in the file. Here we have written the following content inside this file:

Single_core_developers

Now concentrate on the program given below.

/* C File I/O - Reading a File in C*/

#include<stdio.h>

#include<conio.h>

#include<stdlib.h>

void main( )

{

FILE *fp;

char ch, fname[20];

clrscr( );

printf("Enter filename with extension: ");

gets(fname);

fp = fopen(fname, "r");

if(!fp)

{

printf("Error in opening the file..!!");

exit(1);

}

ch = getc(fp); //read one character

while(ch != EOF)

{

putchar(ch); //print on the screen

ch = getc(fp); //read one character

}

fclose(fp); //close file after use

getch( );

}

IF you are using Turbo C/C++ Compiler then always use clrscr( ); and getch( ); .In other compiler you can ignore it.