FILE Program in C

What is a File?

Abstractly, a file is a collection of bytes stored on a secondary storage device, which is generally a disk of some kind. The collection of bytes may be interpreted, for example, as characters, words, lines, paragraphs and pages from a textual document; fields and records belonging to a database; or pixels from a graphical image. The meaning attached to a particular file is determined entirely by the data structures and operations used by a program to process the file. It is conceivable (and it sometimes happens) that a graphics file will be read and displayed by a program designed to process textual data. The result is that no meaningful output occurs (probably) and this is to be expected. A file is simply a machine decipherable storage media where programs and data are stored for machine usage.

Reading (r):

When an r is used, the file is opened for reading, a w is used to indicate a file to be used for writing, and an a indicates that you desire to append additional data to the data already in an existing file. Most C compilers have other file attributes available; check your Reference Manual for details. Using the r indicates that the file is assumed to be a text file. Opening a file for reading requires that the file already exist. If it does not exist, the file pointer will be set to NULL and can be checked by the program.

/* Program to display the contents of a file on screen */

#include

void main()

{

FILE *fopen(), *fp;

int c;

fp = fopen("prog.c","r");

c = getc(fp) ;

while (c!= EOF)

{

putchar(c);

c = getc(fp);

}

fclose(fp);

}

Writing (w):

When a file is opened for writing, it will be created if it does not already exist and it will be reset if it does, resulting in the deletion of any data already there. Using the w indicates that the file is assumed to be a text file.

Here is the program to create a file and write some data into the file.

#include 

int main()

{

 FILE *fp;

 file = fopen("file.txt","w");

 /*Create a file and add text*/

 fprintf(fp,"%s","This is just an example :)"); /*writes data to the file*/

 fclose(fp); /*done!*/

 return 0;

}

Appending (a):

When a file is opened for appending, it will be created if it does not already exist and it will be initially empty. If it does exist, the data input point will be positioned at the end of the present data so that any new data will be added to any data that already exists in the file. Using the a indicates that the file is assumed to be a text file.

Here is a program that will add text to a file which already exists and there is some text in the file.

#include 

int main()

{

    FILE *fp

    file = fopen("file.txt","a");

    fprintf(fp,"%s","This is just an example :)"); /*append some text*/

    fclose(fp);

    return 0;

}

Reading from a text file:

Now for our first program that reads from a file. This program begins with the familiar include, some data definitions, and the file opening statement which should require no explanation except for the fact that an r is used here because we want to read it.

#include 

   main( )

   {

     FILE *fp;

     char c;

     funny = fopen("TENLINES.TXT", "r");

     if (fp == NULL)

                               printf("File doesn't exist\n");

     else {

      do {

       c = getc(fp); /* get one character from the file

       */

         putchar(c); /* display it on the monitor

       */

       } while (c != EOF); /* repeat until EOF (end of file)

     */

     }

    fclose(fp);

   }

In this program we check to see that the file exists, and if it does, we execute the main body of the program. If it doesn’t, we print a message and quit. If the file does not exist, the system will set the pointer equal to NULL which we can test. The main body of the program is one do while loop in which a single character is read from the file and output to the monitor until an EOF (end of file) is detected from the input file. The file is then closed and the program is terminated. At this point, we have the potential for one of the most common and most perplexing problems of programming in C. The variable returned from the getc function is a character, so we can use a char variable for this purpose. There is a problem that could develop here if we happened to use an unsigned char however, because C usually returns a minus one for an EOF – which an unsigned char type variable is not
capable of containing. An unsigned char type variable can only have the values of zero to 255, so it will return a 255 for a minus one in C. This is a very frustrating problem to try to find. The program can never find the EOF and will therefore never terminate the loop. This is easy to prevent: always have a char or int type variable for use in returning an EOF. There is another problem with this program but we will worry about it when we get to the next program and solve it with the one following that.

After you compile and run this program and are satisfied with the results, it would be a good exercise to change the name of TENLINES.TXT and run the program again to see that the NULL test actually works as stated. Be sure to change the name back because we are still not finished with TENLINES.TXT.