Opening a file returns a pointer to a FILE structure  (defined in Opening a file returns a pointer to a FILE structure  (defined in stdio.h).
standard input  =  pointer stdin
standard output = pointer stdout
standard error = pointer stderr

The standard library provides many functions for reading data from files and for writing data to files.
Functions that work with files:

fgetc(input stream) - reads one character from the input stream.
fputc(input stream) - writes one character to the input stream.
fgets and fputs - are used to read and write strings to and from the input stream.
fscanf and fprintf - are the corresponding file commands to scanf and printf.

Creating a Sequential Access File:
C imposes no structure on a file.
The programmer must provide the file structure to meet the requirements of his/her application.
To do this the programmer must understand the data that must be processed.
Use a predetermined format or design the storage structure from scratch.

Identifying the file stream:
Before a file can be used within a C program, a pointer variable must be created to refer to the file stream.
The FILE type is used to declare a file pointer variable.

              FILE *Myfileptr;

This declaration identifies a pointer which refers to a file stream.
Each file stream that will be used must have a unique pointer defined.
Opening a file stream
Before a file can be used with in a C program it must be opened for the type of operation that will be performed.
This is accomplished by using the fopen command.  

Opening a file
        fopen(“filename”,”mode”);

Filename can be represented by either a string constant or a variable.
               fopen(“a:\\myfile.dat”,”w”);
               filename = “a:\\myfile.data”;
               fopen(filename,”w”);

mode can be represented by a “r” or by a “w”.
               fopen(“a:\\myfile.dat”,”w”);
               filename = “a:\\myfile.data”;
               fopen(filename,”r”);

The “w” indicates write mode and the “r” indicates read mode.

Read & Write
If  you try to open a file in write mode and the file doesn’t exist a new file will be created.
If the file does exist it will be opened and any contents of the file will be discarded without warning.
If you try to open a file in read mode and it doesn’t exist you will receive an error message.

Error checking
When opening files it is wise to check for errors.
The following code snippet will provide error checking.
         If ((myptr = fopen(“myfile.dat”,”r”) == NULL)
              printf(“Error opening file!!”);
         else
             /* Continue doing whatever you wanted */

Operations on files
Reading from a file (opened with the “r”).
          fscanf(Myfileptr,”%s”, first_string);
          fgets(first_string,10, Myfileptr);
          fscanf(Myfileptr,”%d”, &A_Number);
Writing to a file (opened with the “w”).
   fprintf(Myfileptr,”%s\n”,first_string);
          fputs(first_string, Myfileptr);
          fprintf(Myfileptr,”%d\n”, A_Number);

File Operations
A file can be opened for only one operation at a time.
You cannot write and read from a file at the same time.
You must close the file and reopen it in the mode you wish to operate.
                        fclose(filename);
                        fclose(Myfileptr);

End of File Marker
All files in C end with and End-of-File marker or what is called an All files in C end with and End-of-File marker or what is called an <EOF>.
This marker is automatically placed after the last element of data within the file by the operating system.
You can use this marker to determine when you have reached the end of a file.