can't capture ppp packets using winpcap sometimes - winpcap

Question

I am using winpcap on my Windows XP, wishing to capture ppp packets with my WCDMA card.
I have installed winpcap 4.1.3 and Microsoft Network Monitor 3.4.
I compile the example code basic_dump from the winpcap developer's pack. It can always list the ppp card:
1. \Device\NPF_GenericDialupAdapter (Adapter for generic dialup and VPN capture)
2. \Device\NPF_{04F9E6B9-214E-4FE5-892B-0419694392E1} (WAN (PPP/SLIP) Interface)
Sometimes it captures packets and prints timestamp and length as expected, sometimes it just prints nothing even when I am surfing the Internet.
Below is the code.
#ifdef _MSC_VER
/*
* we do not want the warnings about the old deprecated and unsecure CRT functions
* since these examples can be compiled under *nix as well
*/
#define _CRT_SECURE_NO_WARNINGS
#endif
#include "pcap.h"
/* prototype of the packet handler */
void packet_handler(u_char *param, const struct pcap_pkthdr *header, const u_char *pkt_data);
int main()
{
pcap_if_t *alldevs;
pcap_if_t *d;
int inum;
int i=0;
pcap_t *adhandle;
char errbuf[PCAP_ERRBUF_SIZE];
/* Retrieve the device list */
if(pcap_findalldevs(&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("%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 */
/* Open the adapter */
if ((adhandle= pcap_open_live(d->name, // name of the device
65536, // portion of the packet to capture.
// 65536 grants that the whole packet will be captured on all the MACs.
0, // promiscuous mode (nonzero means promiscuous)
1000, // read timeout
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);
/* start the capture */
pcap_loop(adhandle, 0, packet_handler, NULL);
pcap_close(adhandle);
return 0;
}
/* Callback function invoked by libpcap for every incoming packet */
void packet_handler(u_char *param, const struct pcap_pkthdr *header, const u_char *pkt_data)
{
struct tm *ltime;
char timestr[16];
time_t local_tv_sec;
/*
* unused parameters
*/
(VOID)(param);
(VOID)(pkt_data);
/* convert the timestamp to readable format */
local_tv_sec = header->ts.tv_sec;
ltime=localtime(&local_tv_sec);
strftime( timestr, sizeof timestr, "%H:%M:%S", ltime);
printf("%s,%.6d len:%d\n", timestr, header->ts.tv_usec, header->len);
}