Nonblocking read sometimes drops data

Question

I'm attempting to write interprocess communication for the following scenario:

• A "reader" (the program I am writing) listens continuously on a fifo.
• Writers appear and disappear in serial, like running echo "[string]" > fifo_in repeatedly.

---

Here is the reader logic I am using (as far as I know). The executable should be run from a directory containing mkfifo the_fifo:

#include <iostream>
#include <string>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
                                    
int main(int argc, char* argv[]) {
    int fd = open("the_fifo", O_RDONLY | O_NONBLOCK);
    char buf[32];
    std::string line  = "";
    while(1) {
        ssize_t n_read = read(fd, buf, sizeof buf);
        if(n_read > 0) line.append(buf, n_read);
        std::cout << line;
        std::cout.flush();
        line = "";
    }
    return 0;
}

This executable should print characters as they are written into the_fifo.

Occasionally when one runs echo "[string]" > fifo_in somewhere else on the system while this executable is running, the executable does not print the string. What is going wrong?