Correct Code - Non-blocking pipe with popen

Question

There are tons of questions about non blocking pipes, but there are NO examples of code that can be copy&paste (with little correction) and used.

I got the idea

Answer 1

Yes. If the read call returns with the error value (-1) and errno is set to EAGAIN, that means that no data is available, so you continue the loop to try again. If you got rid of the errno, errors would be effectively ignored, and your program would probably crash. Imagine if you did remove it: When read returned -1, but, say, the error was that the pipe was broken (the other end closed it), you would just keep trying to loop and enter an infinite loop. Bad idea.

Answer 2

In my Linux multithreaded application (Ubuntu 16.04, x86_64, Native POSIX Threads, libc.so.6 version Ubuntu GLIBC 2.23-0ubuntu9) this code

int fd = fileno(pipe);
fcntl(fd, F_SETFL, O_NONBLOCK);
while (true)
        {
            ssize_t r = read(fd, buff, sizeof(buff));
            if ((r == -1) && (errno == EAGAIN)) // really need errno?
                continue;
            else
            {
                if (r > 0)
                    result += buff;
                else
                    break;
            }
        }

even with fctnl(...) causes fail as not only output of command but many another arbitrary values from RAM are often read. I think it is due to thread unsafety of the code.

(Example: execute echo 3 values, 10, 30, and 40, in parallel by 20 threads, read the values and display only the last value. Output: 40 QIh MIh 40 40 30...)

Another code from this answer works fine due to fgets implementation is thread-safe:

while (!feof(pipe))
{
    if (fgets(buff, sizeof(buff), pipe) != NULL)
    result += buff;
}