# usually behave like "head -n 1"
a
ret: 0
# never consume more than one line
1
2
ret: 0
# add a newline
abc
ret: 1
# print one newline on empty input

ret: 1
# add a newline, return 1 when EOF
xyz
ret: 1
# print one newline on empty input, return 1 when EOF

ret: 1
# large line of zero bytes
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00100000  0a                                                |.|
00100001
ret: 0