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Unfortunately, the closest thing to a design document is the
"README.developer" document in the "doc" directory of the Wireshark
source tree; however, although that's useful for people adding new
protocol dissectors to Wireshark, it doesn't describe the operations of
the "core" of Wireshark.
We have no document describing that; however, a quick summary of the
part of the code you'd probably be working with is:
for every capture file that Wireshark has open, there's a
"capture_file" structure - Wireshark currently supports only one
open capture file at a time, and that structure is named
"cfile" (see the "file.h" header file);
that structure has a member "plist", which points to a
"frame_data" structure - every link-layer frame that Wireshark
has read in has a "frame_data" structure (see the
"epan/packet.h" header file), the "plist" member of "cfile"
points to the first frame, and each frame has a "next" member
that points to the next frame in the capture (or is null for the
last frame);
each "frame_data" struct has:
a pointer to the next frame (null for the last frame);
a pointer to the previous frame (null for the first
information such as the ordinal number of the frame in
the capture, the time stamps for the capture, the size
of the packet data in bytes, the size of the frame in
bytes (which might not equal the size of the packet data
if, for example, the program capturing the packets
captured no more than the first N bytes of the capture,
for some value of N);
the byte offset in the capture file where the frame's
data is located.
See the "print_packets()" routine in "file.c" for an example of a
routine that goes through all the packets in the capture; the loop does
for (fdata = cf->plist; fdata != NULL; fdata = fdata->next) {
update a progress bar (because it could take a
significant period of time to process all packets);
read the packet data if the packet is to be printed;
print the packet;
The "wtap_seek_read()" call reads the packet data into memory; the
"epan_dissect_new()" call "dissects" that data, building a tree
structure for the fields in the packet.