100% cpu usage with a libpcap simple example - c

Question

while running the code below, one of the CPU cores reaches 100% usage. With or without traffic. What is wrong?
Example code:
#include <stdio.h>
#include <stdlib.h>
#include <pcap.h>
#include <errno.h>
void my_callback(u_char *args, const struct pcap_pkthdr* pkthdr, const u_char*
packet)
{
//nothing, nothing at all...
//printf("+");
}
int main(int argc,char **argv)
{
int i;
char *dev;
char errbuf[PCAP_ERRBUF_SIZE];
pcap_t* descr;
const u_char *packet;
struct bpf_program fp; /* hold compiled program */
bpf_u_int32 maskp; /* subnet mask */
bpf_u_int32 netp; /* ip */
if(argc != 2){
fprintf(stdout, "Usage: %s \"expression\"\n"
,argv[0]);
return 0;
}
/* Now get a device */
dev = pcap_lookupdev(errbuf);
if(dev == NULL) {
fprintf(stderr, "%s\n", errbuf);
exit(1);
}
/* Get the network address and mask */
pcap_lookupnet(dev, &netp, &maskp, errbuf);
/* open device for reading in promiscuous mode */
descr = pcap_open_live(dev, BUFSIZ, 1,-1, errbuf);
if(descr == NULL) {
printf("pcap_open_live(): %s\n", errbuf);
exit(1);
}
/* Now we'll compile the filter expression*/
if(pcap_compile(descr, &fp, argv[1], 0, netp) == -1) {
fprintf(stderr, "Error calling pcap_compile\n");
exit(1);
}
/* set the filter */
if(pcap_setfilter(descr, &fp) == -1) {
fprintf(stderr, "Error setting filter\n");
exit(1);
}
/* loop for callback function */
pcap_loop(descr, -1, my_callback, NULL);
return 0;
}
compile with: gcc example.c -o example -lpcap
run with: ./example "tcp" or the filter you like.
As you can see it is the typical example, the main and the callback function for the loop: pcap_loop(descr, -1, my_callback, NULL);
The callback is empty (useless) but it is just to show that the problem is not in the callback.

Answer 1

You specified timeout -1 here:
descr = pcap_open_live(dev, BUFSIZ, 1,-1, errbuf);
It turns pcap_loop into a busy loop, as poll continuously times out instantly.
Use something like 1000 (milliseconds) if you have no reason for other value.