指向libpcap中数据包len的指针
在指向libpcap中数据包len的指针,c,pointers,libpcap,C,Pointers,Libpcap,在libpcap中,我有这个用于嗅探和打印数据包长度的代码 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;
libpcap
中,我有这个用于嗅探和打印数据包长度的代码
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);
}
我想在循环之前设置指向header.len
的指针,并在每次迭代中打印它:
bpf_u_int32 * len= &header.len
while(1)
{
packet = pcap_next(handle, &header);
printf("packet len = [%d]\n", *len);
}
在每次迭代中,
header.len
的地址会改变吗?在该循环中,header
的地址不会改变
然而,没有理由这么做
bpf_u_int32 * len= &header.len
while(1)
{
packet = pcap_next(handle, &header);
printf("packet len = [%d]\n", *len);
}
而不仅仅是
while(1)
{
packet = pcap_next(handle, &header);
printf("packet len = [%d]\n", header.len);
}
您将得到相同的答案,事实上,编译器甚至可能为此生成相同的代码。(这不是你祖父的C;C编译器比以前做了更多的优化和其他代码转换。)
将指向
header.len
的指针放入变量中,并取消对指针的引用,从本质上讲并不更有效。例如,如果“加载/推送寄存器指向的对象”比“从寄存器指向的偏移量加载/推送对象”更有效,编译器可能会生成代码来实现这一点。为什么需要另一个指向header.len的指针?直接引用它有什么不对?