sysread is a thin wrapper around the read system call. It would help you to read its documentation (the read(2) man page). From the "Errors" section,
- 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.