Custom packet dissector data structure

Question

Suppose I have a custom protocol called cust_prot. One of the field is of 4 bytes length and has a flag bit as follows:

      The 5th bit of the 2nd byte of this field is a flag variable.

So, I wanted to know what will be the data structure. I have made the following structure but am not sure whether we need to do masking or not.

     .
     .
     { &hf_cust_prot_skip5,
         { "CUST PROT SKIP5", "cust_prot.skip5",
         FT_UINT32, BASE_DEC,
         NULL, 0x0,
         NULL, HFILL }
     },
     { &hf_cust_prot_cap1,
         { "CUST PROT CAP1", "cust_prot.skip5.cap1",
         FT_BOOLEAN, BASE_DEC,
         NULL, 0x0,               /*Confused about this mask field*/
         NULL, HFILL }
     },
     .
     .

Also if someone can provide the way to display that flag bit, it will be very useful.

Answer 1

 { &hf_cust_prot_cap1,
     { "CUST PROT CAP1", "cust_prot.skip5.cap1",
     FT_BOOLEAN, 32,
     NULL, 0x080000,               /*"5th bit" of 2nd byte counting from the left ? */
     NULL, HFILL }
 },

Use the usual proto_tree_add_item(...) to display the field.

Note that the exact format of the display of the field can be varied by changing the field width (32) and bit mask and using the correct offset in the proto_tree_add_item()` call.

E.G.,: usimg 8 as the field width, a bit mask of 0x08 and an offset of 1 from will display the field as 1 bit in 8 bits.

Try experimenting with different values.

This is all explained in doc/README.dissector.