Echo input into output, at the right place - c

Question

I want to insert the input given to a program into its output, in the moment it is read.
For instance, given the program:
#include <stdio.h>
int main() {
double x, y;
printf("Soma de dois números:\n");
printf("Digite o primeiro número: ");
scanf("%lf", &x);
printf("Digite o segundo número: ");
scanf("%lf", &y);
double soma = x + y;
printf("%lf + %lf = %lf\n", x, y, soma);
return 0;
}
when it is run with input redirection, I want the input inserted into the output, like the following:
$ echo -e "10\n25\n" | ./a.out
sum of two numbers
first number: 12
second number: 25
12.000000 + 25.000000 = 37.000000
Is there any library or tool to help me with that?
Edited Context: I intend to use this for correcting program exercises in a programming course. I want to see/show the program input and output as if it were run in an interactive console with the input being typed interactively, although the input is redirect from a text file.

Answer 1

As I noted in a comment:
There isn't an easy way to do that. The problem is that when you type at the terminal, the terminal driver echoes the characters typed. When you have I/O redirection (from a pipe or a file), the terminal driver is not involved, and neither the disk driver nor the pipe driver echoes to standard output.
One option might be to use a pseudo-terminal for the input to your program — but I've not tried it so that might not work. It's also fairly fiddly to set up (though there might well be a utility available on Linux, in particular, to do the job).
Or you could test to see whether the input is a terminal, doing nothing special if it is a terminal, and echoing the result of the input if it is. POSIX function isatty() determines whether the given file descriptor is a terminal or not.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(void)
{
printf("Enter a number: ");
double d;
if (scanf("%lf", &d) != 1)
{
fprintf(stderr, "Failed to read a number!\n");
exit(EXIT_FAILURE);
}
if (!isatty(STDIN_FILENO))
printf("%lf\n", d);
printf("You entered: %lf\n", d);
return 0;
}
With that saved as source file tty47.c, I can run:
$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror -Wmissing-prototypes -Wstrict-prototypes -fno-common tty47.c -o tty47
$ tty47
Enter a number: 2.718282
You entered: 2.718282
$ echo "3.141593" | tty47
Enter a number: 3.141593
You entered: 3.141593
$ tty47 <<<'1.618034'
Enter a number: 1.618034
You entered: 1.618034
$
The first run accepts the input I type at the terminal, and that is echoed by the terminal driver. The other two runs know that the input comes from a pipe and a file respectively, and they echo the input. Note that the echo is not guaranteed to be exactly what the user entered — I cheated by typing numbers with 6 decimal places. Use fewer places and you get trailing zeros; use more and the result is rounded. You'll have to decide whether that matters.
An alternative strategy would read the input using getchar() or equivalent and echo each character with putchar() or equivalent if the input is not a terminal, and then use sscanf() to parse the data. That's definitely fiddlier.