On Sep 28, 2012, at 2:01 PM, albert <alo@xxxxxxxxxxxxxx> wrote:
> What/where is/are the function(s) that determine(s) whether the callback for
> next_packet_op is going to be pcap_dump() or some other higher level routine ?
The next_packet_op is a *read* function, not a *write* function, so it would, of course, not be set in pcap_dump(), as that's a *write*-path routine.
It's set in the open-for-reading path, either by pcap_check_header() if it's a pcap file or pcap_ng_check_header() if it's a pcap-ng file.
The open-offline routines (pcap_open_offline() and pcap_fopen_offline()) try calling each of the routines in the check_headers[] array, to check whether the file is a file of the routine's type. If so, the routine sets the next_packet_op to the appropriate routine for that file type and returns 1 to indicate that the correct file type has been found.
> Is this a function that the user must supply ?
No. That's all an internal detail of the current libpcap implementation, so that programs do *not* have to be changed in order to be able to read pcap-ng files. (The current libpcap API limits what types of files can be read - the link-layer header type is per-file, not per-packet or per-interface, in the API, so it doesn't support files with multiple link-layer header types - so *full* support of pcap-ng files will require an additional set of APIs, which programs *would* have to be changed to use. The new APIs will still support pcap, however; requiring programs to know whether they're reading pcap or pcap-ng or... files is an error.)
