What's the difference between O_nonblock and o_ndelay?

The result of O_nonblock and O_ndelay is to make I/O into non-blocking mode (non-blocking), return immediately without reading the data or the write buffer full, without blocking the wait.

The difference is that, in a read operation, if the data is not read, O_ndelay causes the I/O function to return 0 immediately, but this gives rise to a problem, because the read to the end of the file (EOF) is returned 0, so it is impossible to distinguish between the cases. Therefore, O_nonblock is generated, it will return 1 when the data is not read, and set errno to Eagain.

O_ndelay was introduced in earlier versions of System V, and when coding, it is recommended that POSIX-defined o_nonblock,o_nonblock be set at open and Fcntl.

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