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.