CODEWITHSUNDEEP
cpointerslibpcap
MicrosoctCprog
MicrosoctCprog

Reputation: 490

pointer to packet len in libpcap

In libpcap I have this code for sniffing and printing packets length

int main(int argc, char *argv[])
 {
    pcap_t *handle;         /* Session handle */
    char *dev;          /* The device to sniff on */
    char errbuf[PCAP_ERRBUF_SIZE];  /* Error string */
    struct bpf_program fp;      /* The compiled filter */
    char filter_exp[] = "port 23";  /* The filter expression */
    bpf_u_int32 mask;       /* Our netmask */
    bpf_u_int32 net;        /* Our IP */
    struct pcap_pkthdr header;  /* The header that pcap gives us */
    const u_char *packet;       /* The actual packet */

    /* Define the device */
    dev = pcap_lookupdev(errbuf);
    if (dev == NULL) {
        fprintf(stderr, "Couldn't find default device: %s\n", errbuf);
        return(2);
    }
    /* Find the properties for the device */
    if (pcap_lookupnet(dev, &net, &mask, errbuf) == -1) {
        fprintf(stderr, "Couldn't get netmask for device %s: %s\n", dev, errbuf);
        net = 0;
        mask = 0;
    }
    
    /* Open the session in promiscuous mode */
    handle = pcap_open_live(dev, BUFSIZ, 1, 1000, errbuf);
    if (handle == NULL) {
        fprintf(stderr, "Couldn't open device %s: %s\n", dev, errbuf);
        return(2);
    }
    /* Compile and apply the filter */
    if (pcap_compile(handle, &fp, filter_exp, 0, net) == -1) {
        fprintf(stderr, "Couldn't parse filter %s: %s\n", filter_exp, pcap_geterr(handle));
        return(2);
    }
    if (pcap_setfilter(handle, &fp) == -1) {
        fprintf(stderr, "Couldn't install filter %s: %s\n", filter_exp, pcap_geterr(handle));
        return(2);
    }
    while(1)
    {
        packet = pcap_next(handle, &header);
        printf("packet len =  [%d]\n", header.len);
    }
    pcap_close(handle);
    return(0);
 }

I want to set pointer to header.len before loop and print it each iteration:

bpf_u_int32 * len= &header.len
while(1)
{
    packet = pcap_next(handle, &header);
    printf("packet len =  [%d]\n", *len);
}

Does that work or can the address of header.len change in each iteration?

Upvotes: 0

Views: 164

Answers (1)

user13951124
user13951124

Reputation: 176

The address of header won't change within that loop.

However, there's no reason to do

bpf_u_int32 * len= &header.len
while(1)
{
    packet = pcap_next(handle, &header);
    printf("packet len =  [%d]\n", *len);
}

rather than just

while(1)
{
    packet = pcap_next(handle, &header);
    printf("packet len =  [%d]\n", header.len);
}

You'll get the same answer and, in fact, a compiler may even generate the same code for that. (This isn't your grandfather's C; C compilers do a lot more optimizing and other code transformation than they did in the old days.)

Putting a pointer to header.len into a variable, and dereferencing the pointer, isn't inherently more efficient. The compiler might well generate code to do that if, for example, "load/push what a register points to" is more efficient than "load/push from an offset from what a register points to".

Upvotes: 1

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