The GNU readline package

// A program to demonstrate the use of the GNU readline package for
// improving the ease of interaction with a program.

// Michael Ashley / UNSW / 20 April 2004

#include <stdio.h>
#include <math.h>

// The following two lines are necessary for GNU readline:

#include <readline/readline.h>
#include <readline/history.h>

int main(void) {
    char *cp;
    float x;
    int ret;

    printf("This program calculates square roots\n");

    // Loop forever (we will break out of the loop in certain conditions).

    while (1) {

        // Grab an input line from the user. readline automatically
        // allocates space for the line, and returns a pointer to it. We
        // store the pointer in our variable "cp".

        cp = readline("please input a number (ENTER to exit) > ");

	// If nothing was entered, readline gives us a NULL pointer. We
        // test for this and exit the program.

	if (cp == NULL) {
	    printf("\n");
	    break;
	}

	// Else we add the new line to the history buffer, so that the user
	// can use the up/down arrow keys to navigate through previous
	// responses.

	add_history(cp);
	
	// Now, try to read a floating point number from the input line.
        // "ret" will be 1 if this worked; EOF at end-of-file; and something
        // else if an error occurred.

	ret = sscanf(cp, "%f", &x);

	// We have now finished with the input line, so we free the space 
        // that readline() allocated for it ("man readline" explains why you
        // should do this).

	free(cp);

	// Now check the results from sscanf.

	if (ret == 1) {
  	    printf ("The square root of %f is %f\n", x, sqrt(x));
	} else if (ret == EOF) {
	    break;
	} else {
	    printf ("that wasn't a valid number...\n");
	}
    }

    return 0;
}

To compile the above program, do, e.g., "gcc example.c -lm -lreadline -lcurses".