Scieneer Common Lisp 1.3.9 online documentation

ext:read-chars stream string start requested &optional waitp[Function]

Attempt to read the requested number of characters into the string starting at start, returning the number of characters read or -1 if at the end-of-file. On pipes or similar devices, this function returns as soon as any data is available, even if the amount read is less than requested and the end-of-file has not been reached. When waitp is true, wait for at least one character before returning, otherwise return zero if no input is available.