Why does this blocking socket read return zero and no error?

Question

This code running on Linux:

int r, c;
...
assert(0 == (O_NONBLOCK & fcntl(sockfd, F_GETFL, 0)));
errno = 0;
r = read(sockfd, &c, 1);
if (r == 0 && errno == 0) { 
    printf("What gives?\n");
}
...

which is performing a read from a socket, will occasionally return zero (in r) and leave errno set to zero (0) as well. What situation am I encountering? I'd really like to have the read block unless there is an error.

Answer 1

This means that the client finished sending data (eg: did a shutdown for writing), and you already read all available data.

RETURN VALUE

On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number.

Answer 2

Generally the read man page says:

On success, the number of bytes read is returned (zero indicates end of file)

so 0 means that your socket is closed.