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1 office 1 Unfortunately, the closest thing to a design document is the
2 "README.developer" document in the "doc" directory of the Wireshark
3 source tree; however, although that's useful for people adding new
4 protocol dissectors to Wireshark, it doesn't describe the operations of
5 the "core" of Wireshark.
6  
7 We have no document describing that; however, a quick summary of the
8 part of the code you'd probably be working with is:
9  
10 for every capture file that Wireshark has open, there's a
11 "capture_file" structure - Wireshark currently supports only one
12 open capture file at a time, and that structure is named
13 "cfile" (see the "file.h" header file);
14  
15 that structure has a member "plist", which points to a
16 "frame_data" structure - every link-layer frame that Wireshark
17 has read in has a "frame_data" structure (see the
18 "epan/packet.h" header file), the "plist" member of "cfile"
19 points to the first frame, and each frame has a "next" member
20 that points to the next frame in the capture (or is null for the
21 last frame);
22  
23 each "frame_data" struct has:
24  
25 a pointer to the next frame (null for the last frame);
26  
27 a pointer to the previous frame (null for the first
28 frame);
29  
30 information such as the ordinal number of the frame in
31 the capture, the time stamps for the capture, the size
32 of the packet data in bytes, the size of the frame in
33 bytes (which might not equal the size of the packet data
34 if, for example, the program capturing the packets
35 captured no more than the first N bytes of the capture,
36 for some value of N);
37  
38 the byte offset in the capture file where the frame's
39 data is located.
40  
41 See the "print_packets()" routine in "file.c" for an example of a
42 routine that goes through all the packets in the capture; the loop does
43  
44 for (fdata = cf->plist; fdata != NULL; fdata = fdata->next) {
45  
46 update a progress bar (because it could take a
47 significant period of time to process all packets);
48  
49 read the packet data if the packet is to be printed;
50  
51 print the packet;
52  
53 }
54  
55 The "wtap_seek_read()" call read the packet data into memory; the
56 "epan_dissect_new()" call "dissects" that data, building a tree
57 structure for the fields in the packet.
58