Can read() from socket return EWOULDBLOCK?

Question

My code uses epoll for multiplexing. After EPOLLIN is encountered it reads one byte from ready socket.

read(fd, &onebyte, 1);

Can the above line return EWOULDBLOCK?

Answer 1

yes,

Errors:

...

EAGAIN or EWOULDBLOCK

The file descriptor fd refers to a socket and has been marked nonblocking (O_NONBLOCK), and the read would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities

reference: the read() man description