how to dissect a packet with offset 20 in the dissect function

Question

I am new to this dissector coding.please help me in solving this.Here is my sample code.I wanted to dissect a 4 byte. By taking first 20 bits for i bit and next 12 bits for j bit.But I didnt get the correct value when printed in decimal.

I tried using tvb_get_htonl function instead of tvb_get_guint8 function in the below code.But I didnt get correct result.

Please let me know how to dissect this 4 byte data.

dissect() {

proto_tree_add_uint(tree, hf_i_bit,tvb,offset,4,(tvb_get_guint8( tvb,offset & 0xFFFFF000) >> 12));

proto_tree_add_uint(tree,hf_ j_bit,tvb,offset,2,tvb_get_guint8(tvb,offset &0x00000FFF)); }

proto_register{

{ &hf_ i_bit,

{ "i bit", "x.i", FT_UINT32, BASE_DEC,NULL, 0, NULL, HFILL } },

{ &hf_ j_bit,

{ "j bit", "x.j", FT_UINT16, BASE_DEC,NULL, 0, NULL, HFILL } }, }

Accepted Answer

Your code cannot as tvb_get_guint8 only retrieves a single byte (as indicated in the documentation). You should try something like:

dissect() {
proto_tree_add_item(tree, hf_i_bit,tvb,offset,4,ENC_BIG_ENDIAN);
proto_tree_add_uint(tree,hf_ j_bit,tvb,offset,2, ENC_BIG_ENDIAN); }
proto_register{
{ &hf_ i_bit,
{ "i bit", "x.i", FT_UINT32, BASE_DEC,NULL, 0xFFFFF000, NULL, HFILL } },
{ &hf_ j_bit,
{ "j bit", "x.j", FT_UINT16, BASE_DEC,NULL, 0x0FFF, NULL, HFILL } }, }