into_cstruct t buf ~payload generates an ICMPv4 header from t and writes it into buf at offset 0. payload is used to calculate the ICMPv4 header checksum, but is not included in the generated buffer. into_cstruct may fail if the buffer is of insufficient size.

make_cstruct t ~payload allocates, fills, and returns a Cstruct.t with the header information from t. The payload is used to calculate the ICMPv4 header checksum, but is not included in the generated buffer. make_cstruct allocates 8 bytes for the ICMPv4 header.