why read() behaviour changed after redirection?

Question

I have encountered a problem while programming in Linux using C, and I have narrowed it down to the following question.

#define BUF_SIZE 10

int main()
{
        int cnt;
        char buf[BUF_SIZE];
        cnt = read(STDIN_FILENO, buf, BUF_SIZE);
        printf("cnt = %d\n", cnt);
        return 0;
}

Given the code above, if I use the terminal as input, and put in 3 characters and then hit enter, then I got "cnt = 4".

./a.out 
abc
cnt = 4

But if I use redirection, and use a file, which contains two lines, as the input, read() does not stop at the newline character, but proceed and read the whole file. Like this

//inside input_file. This line is not in the file.
123
456

Then I got

./a.out < input_file
cnt = 8

Now here is my question. Why read() does not stop after reading the first line in the file ? What is the difference between a terminal input and a file input ?

Some references to explain this would be appreciated.