ZC - indicate EOF with -1.

zc: When input functions reach end-of-file on this stream, they will return a value of -1. If you do not specify "zc", input functions will return zero instead. Using -1 as EOF is standard in C; thus the C library function "fopen" always invokes OPEN using "zc" as a default. C programs that invoke OPEN directly rather than through "fopen" should specify "zc" explicitly.
"zc" lets you process files that contain '*0' characters without confusing them with EOF marks.