The syntax is:
-c hexc:nr[,hexc:nr,...]
As example, filling with 16 byte counter with following:
ffffffff3333337a7a7a7a88882a0b0c
is possible with following pattern:
-c ff:4,33:3,7a:4,88:2,2a:1,0b:1,0c:1
To fill with zeroes (or any other static byte) is simple:
-c 0:16
Note that any bytes that tend to overflow counter space are dropped, and any
unused bytes not filled by pattern are set to zeroes.