3)Input and Output using file pointers

Having opened a file pointer, you will wish to use it for either input or output. C supplies a set of functions to allow you to do this. All are very similar to input and output functions that you have already met.

Character Input and Output with Files

This is done using equivalents of getchar and putchar which are called getc and putc. Each takes an extra argument, which identifies the file pointer to be used for input or output.

putchar(c) is equivalent to putc(c, stdout)
getchar( ) is equivalent to getc(stdin)

Formatted Input Output with File Pointers

Similarly there are equivalents to the functions printf and scanf which read or write data to files. These are called fprintf and fscanf. You have already seen fprintf being used to write data to stderr.
The functions are used in the same way, except that the fprintf and fscanf take the file pointer as an additional first argument.

Formatted Input Output with Strings

These are the third set of the printf and scanf families. They are called sprintf and sscanf.

sprintf
puts formatted data into a string which must have sufficient space allocated to hold it. This can be done by declaring it as an array of char. The data is formatted according to a control string of the same form as that for p rintf.
sscanf
takes data from a string and stores it in other variables as specified by the control string. This is done in the same way that scanf reads input data into variables. sscanf is very useful for converting strings into numeric v values.

Whole Line Input and Output using File Pointers

Predictably, equivalents to gets and puts exist called fgets and fputs. The programmer should be careful in using them, since they are incompatible with gets and puts. gets requires the programmer to specify the maximum number of characters to be read. fgets and fputs retain the trailing newline character on the line they read or write, wheras gets and puts discard the newline.

When transferring data from files to standard input / output channels, the simplest way to avoid incompatibility with the newline is to use fgets and fputs for files and standard channels too.
For Example, read a line from the keyboard using

fgets(data_string, 80, stdin);
and write a line to the screen using
fputs(data_string, stdout);