How to read a line from stdin, blocking until the newline is found?

I'm trying to read one line at a time, of arbitrary length, from stdin at the command line. I'm not sure if I'll be able to include GNU readline and would prefer to use a library function.

The documentation I've read suggests that getline ought to work, but in my experiments it doesn't block. My sample program:

#include <stdio.h>
int main()
{
    char *line = NULL;
    if (getline(&line, NULL, stdin) == -1) {
        printf("No line\n");
    } else {
        printf("%s\n", line);
    }
    return 0;
}

produces No line, which makes it unsuitable for accepting user input.

How do I do this? I know it should be trivial, but I haven't been able to figure it out.