WinPcap: Capturing the packets without the callback

# Capturing the packets without the callback

The example program in this lesson behaves exactly like the previous program (Opening an adapter and capturing the packets), but it uses pcap_next_ex() instead of pcap_loop().

The callback-based capture mechanism of pcap_loop() is elegant and it could be a good choice in some situations. However, handling a callback is sometimes not practical -- it often makes the program more complex especially in situations with multithreaded applications or C++ classes.

In these cases, pcap_next_ex() retrievs a packet with a direct call -- using pcap_next_ex() packets are received only when the programmer wants them.

The parameters of this function are the same as a capture callback -- it takes an adapter descriptor and a couple of pointers that will be initialized and returned to the user (one to a pcap_pkthdr structure and another to a buffer with the packet data).

In the following program, we recycle the callback code of the previous lesson's example and move it inside main() right after the call to pcap_next_ex().

    #include "pcap.h"

    int main()
    {
    pcap_if_t *alldevs;
    pcap_if_t *d;
    int inum;
    int i=0;
    pcap_t *adhandle;
    int res;
    char errbuf[PCAP_ERRBUF_SIZE];
    struct tm ltime;
    char timestr[16];
    struct pcap_pkthdr *header;
    const u_char *pkt_data;
    time_t local_tv_sec;

        /* Retrieve the device list on the local machine */
        if (pcap_findalldevs_ex(PCAP_SRC_IF_STRING, NULL, &alldevs, errbuf) == -1)
        {
            fprintf(stderr,"Error in pcap_findalldevs: %s\n", errbuf);
            exit(1);
        }

        /* Print the list */
        for(d=alldevs; d; d=d->next)
        {
            printf("%d. %s", ++i, d->name);
            if (d->description)
                printf(" (%s)\n", d->description);
            else
                printf(" (No description available)\n");
        }

        if(i==0)
        {
            printf("\nNo interfaces found! Make sure WinPcap is installed.\n");
            return -1;
        }

        printf("Enter the interface number (1-%d):",i);
        scanf_s("%d", &inum);

        if(inum < 1 || inum > i)
        {
            printf("\nInterface number out of range.\n");
            /* Free the device list */
            pcap_freealldevs(alldevs);
            return -1;
        }

        /* Jump to the selected adapter */
        for(d=alldevs, i=0; i< inum-1 ;d=d->next, i++);

        /* Open the device */
        if ( (adhandle= pcap_open(d->name,          // name of the device
                                  65536,            // portion of the packet to capture.
                                                    // 65536 guarantees that the whole packet will be captured on all the link layers
                                  PCAP_OPENFLAG_PROMISCUOUS,    // promiscuous mode
                                  1000,             // read timeout
                                  NULL,             // authentication on the remote machine
                                  errbuf            // error buffer
                                  ) ) == NULL)
        {
            fprintf(stderr,"\nUnable to open the adapter. %s is not supported by WinPcap\n", d->name);
            /* Free the device list */
            pcap_freealldevs(alldevs);
            return -1;
        }

        printf("\nlistening on %s...\n", d->description);

        /* At this point, we don't need any more the device list. Free it */
        pcap_freealldevs(alldevs);

        /* Retrieve the packets */
        while((res = pcap_next_ex( adhandle, &header, &pkt_data)) >= 0){

            if(res == 0)
                /* Timeout elapsed */
                continue;

            /* convert the timestamp to readable format */
            local_tv_sec = header->ts.tv_sec;
            localtime_s(&ltime, &local_tv_sec);
            strftime( timestr, sizeof timestr, "%H:%M:%S", &ltime);

            printf("%s,%.6d len:%d\n", timestr, header->ts.tv_usec, header->len);
        }

        if(res == -1){
            printf("Error reading the packets: %s\n", pcap_geterr(adhandle));
            return -1;
        }

        return 0;
    }

Why do we use pcap_next_ex() instead of the old pcap_next()? Because pcap_next() has some drawbacks. First of all, it is inefficient because it hides the callback method but still relies on pcap_dispatch(). Second, it is not able to detect EOF, so it's not very useful when gathering packets from a file.

Notice also that pcap_next_ex() returns different values for success, timeout elapsed, error and EOF conditions.

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