Packet Sniffer using winpcap missing some packets

Question

I'm new to WinPcap and C/C++. I'm trying to make a packet sniffer. I've followed the example code from https://www.binarytides.com/code-packet-sniffer-c-winpcap.

The code looks like this:

/*
Simple Sniffer with winpcap , prints ethernet , ip , tcp , udp and icmp headers along with data dump in hex
*/

#include "stdio.h"
#include "winsock2.h"   //need winsock for inet_ntoa and ntohs methods

#define HAVE_REMOTE
#include "pcap.h"   //Winpcap :)

#pragma comment(lib , "ws2_32.lib") //For winsock
#pragma comment(lib , "wpcap.lib") //For winpcap

//some packet processing functions
void ProcessPacket (u_char* , int); //This will decide how to digest

void print_ethernet_header (u_char*);
void PrintIpHeader (u_char* , int);
void PrintIcmpPacket (u_char* , int);
void print_udp_packet (u_char* , int);
void PrintTcpPacket (u_char* , int);
void PrintData (u_char* , int);

// Set the packing to a 1 byte boundary
//#include "pshpack1.h"
//Ethernet Header
typedef struct ethernet_header
{
    UCHAR dest[6];
    UCHAR source[6];
    USHORT type;
}   ETHER_HDR , *PETHER_HDR , FAR * LPETHER_HDR , ETHERHeader;

//Ip header (v4)
typedef struct ip_hdr
{
    unsigned char ip_header_len:4; // 4-bit header length (in 32-bit words) normally=5 (Means 20 Bytes may be 24 also)
    unsigned char ip_version :4; // 4-bit IPv4 version
    unsigned char ip_tos; // IP type of service
    unsigned short ip_total_length; // Total length
    unsigned short ip_id; // Unique identifier

    unsigned char ip_frag_offset :5; // Fragment offset field

    unsigned char ip_more_fragment :1;
    unsigned char ip_dont_fragment :1;
    unsigned char ip_reserved_zero :1;

    unsigned char ip_frag_offset1; //fragment offset

    unsigned char ip_ttl; // Time to live
    unsigned char ip_protocol; // Protocol(TCP,UDP etc)
    unsigned short ip_checksum; // IP checksum
    unsigned int ip_srcaddr; // Source address
    unsigned int ip_destaddr; // Source address
} IPV4_HDR;

//UDP header
typedef struct udp_hdr
{
    unsigned short source_port; // Source port no.
    unsigned short dest_port; // Dest. port no.
    unsigned short udp_length; // Udp packet length
    unsigned short udp_checksum; // Udp checksum (optional)
} UDP_HDR;

// TCP header
typedef struct tcp_header
{
    unsigned short source_port; // source port
    unsigned short dest_port; // destination port
    unsigned int sequence; // sequence number - 32 bits
    unsigned int acknowledge; // acknowledgement number - 32 bits

    unsigned char ns :1; //Nonce Sum Flag Added in RFC 3540.
    unsigned char reserved_part1:3; //according to rfc
    unsigned char data_offset:4; /*The number of 32-bit words in the TCP header.
    This indicates where the data begins.
    The length of the TCP header is always a multiple
    of 32 bits.*/

    unsigned char fin :1; //Finish Flag
    unsigned char syn :1; //Synchronise Flag
    unsigned char rst :1; //Reset Flag
    unsigned char psh :1; //Push Flag
    unsigned char ack :1; //Acknowledgement Flag
    unsigned char urg :1; //Urgent Flag

    unsigned char ecn :1; //ECN-Echo Flag
    unsigned char cwr :1; //Congestion Window Reduced Flag

    ////////////////////////////////

    unsigned short window; // window
    unsigned short checksum; // checksum
    unsigned short urgent_pointer; // urgent pointer
} TCP_HDR;

typedef struct icmp_hdr
{
    BYTE type; // ICMP Error type
    BYTE code; // Type sub code
    USHORT checksum;
    USHORT id;
    USHORT seq;
} ICMP_HDR;
// Restore the byte boundary back to the previous value
//#include <poppack.h>

FILE *logfile;
int tcp=0,udp=0,icmp=0,others=0,igmp=0,total=0,i,j;
struct sockaddr_in source,dest;
char hex[2];

//Its free!
ETHER_HDR *ethhdr;
IPV4_HDR *iphdr;
TCP_HDR *tcpheader;
UDP_HDR *udpheader;
ICMP_HDR *icmpheader;
u_char *data;

int main()
{
    u_int i, res , inum ;
    u_char errbuf[PCAP_ERRBUF_SIZE] , buffer[100];
    u_char *pkt_data;
    time_t seconds;
    struct tm tbreak;
    pcap_if_t *alldevs, *d;
    pcap_t *fp;
    struct pcap_pkthdr *header;

    fopen_s(&logfile , "log.txt" , "w");
    
    if(logfile == NULL) 
    {
        printf("Unable to create file.");
    }

    /* The user didn't provide a packet source: Retrieve the local device list */
    if (pcap_findalldevs_ex(PCAP_SRC_IF_STRING, NULL, &alldevs, errbuf) == -1)
    {
        fprintf(stderr,"Error in pcap_findalldevs_ex: %s\n", errbuf);
        return -1;
    }
    
    i = 0;
    /* Print the list */
    for(d=alldevs; d; d=d->next)
    {
        printf("%d. %s\n    ", ++i, d->name);

        if (d->description)
        {
            printf(" (%s)\n", d->description);
        }
        else
        {
            printf(" (No description available)\n");
        }
    }
        
    if (i==0)
    {
        fprintf(stderr,"No interfaces found! Exiting.\n");
        return -1;
    }

    printf("Enter the interface number you would like to sniff : ");
    scanf_s("%d" , &inum);

    
    /* Jump to the selected adapter */
    for (d=alldevs, i=0; i< inum-1 ;d=d->next, i++);
        
    /* Open the device */
    if ( (fp= pcap_open(d->name,
                        100 /*snaplen*/,
                        PCAP_OPENFLAG_PROMISCUOUS /*flags*/,
                        20 /*read timeout*/,
                        NULL /* remote authentication */,
                        errbuf)
                        ) == NULL)
    {
        fprintf(stderr,"\nError opening adapter\n");
        return -1;
    }

    //read packets in a loop :)
    while((res = pcap_next_ex( fp, &header, &pkt_data)) >= 0)
    {
        if(res == 0)
        {
            // Timeout elapsed
            continue;
        }
        seconds = header->ts.tv_sec;
        localtime_s( &tbreak , &seconds);
        strftime (buffer , 80 , "%d-%b-%Y %I:%M:%S %p" , &tbreak );
        //print pkt timestamp and pkt len
        //fprintf(logfile , "\nNext Packet : %ld:%ld (Packet Length : %ld bytes) " , header->ts.tv_sec, header->ts.tv_usec, header->len);
        fprintf(logfile , "\nNext Packet : %s.%ld (Packet Length : %ld bytes) " , buffer , header->ts.tv_usec, header->len);
        ProcessPacket(pkt_data , header->caplen);
    }
    
    if(res == -1)
    {
        fprintf(stderr, "Error reading the packets: %s\n" , pcap_geterr(fp) );
        return -1;
    }
    
    return 0;
}

void ProcessPacket(u_char* Buffer, int Size)
{
    //Ethernet header
    ethhdr = (ETHER_HDR *)Buffer;
    iphdr = (IPV4_HDR *)(Buffer + sizeof(ETHER_HDR));
    int z = ntohs(iphdr->ip_total_length);
    total+=z;
    
    //Ip packets
    if(ntohs(ethhdr->type) == 0x0800)
    {
        //ip header
        iphdr = (IPV4_HDR *)(Buffer + sizeof(ETHER_HDR));
     
        
        switch (iphdr->ip_protocol) //Check the Protocol and do accordingly...
        {
            case 1: //ICMP Protocol
            icmp++;
            PrintIcmpPacket(Buffer,Size);
            break;

            case 2: //IGMP Protocol
            igmp++;
            break;

            case 6: //TCP Protocol
            tcp++;
            PrintTcpPacket(Buffer,Size);
            break;

            case 17: //UDP Protocol
            udp++;
            print_udp_packet(Buffer,Size);
            break;

            default: //Some Other Protocol like ARP etc.
            others++;
            break;
        }
    }
    
    printf("TCP : %d UDP : %d ICMP : %d IGMP : %d Others : %d Total Size : %d\r" , tcp , udp , icmp , igmp , others , total);
}

/*
    Print the Ethernet header
*/
void print_ethernet_header (u_char* buffer )
{
    ETHER_HDR *eth = (ETHER_HDR *)buffer;
    
    fprintf(logfile,"\n");
    fprintf(logfile,"Ethernet Header\n");
    fprintf(logfile , " |-Destination Address : %.2X-%.2X-%.2X-%.2X-%.2X-%.2X \n", eth->dest[0] , eth->dest[1] , eth->dest[2] , eth->dest[3] , eth->dest[4] , eth->dest[5] );
    fprintf(logfile , " |-Source Address      : %.2X-%.2X-%.2X-%.2X-%.2X-%.2X \n", eth->source[0] , eth->source[1] , eth->source[2] , eth->source[3] , eth->source[4] , eth->source[5] );
    fprintf(logfile , " |-Protocol            : 0x%.4x \n" , ntohs(eth->type) );
}

/*
    Print the IP header for IP packets
*/
void PrintIpHeader (unsigned char* Buffer, int Size)
{
    int iphdrlen = 0;

    iphdr = (IPV4_HDR *)(Buffer + sizeof(ETHER_HDR));
    iphdrlen = iphdr->ip_header_len*4;

    memset(&source, 0, sizeof(source));
    source.sin_addr.s_addr = iphdr->ip_srcaddr;

    memset(&dest, 0, sizeof(dest));
    dest.sin_addr.s_addr = iphdr->ip_destaddr;

    print_ethernet_header(Buffer);

    fprintf(logfile,"\n");
    fprintf(logfile,"IP Header\n");
    fprintf(logfile," |-IP Version : %d\n",(unsigned int)iphdr->ip_version);
    fprintf(logfile," |-IP Header Length : %d DWORDS or %d Bytes\n",(unsigned int)iphdr->ip_header_len,((unsigned int)(iphdr->ip_header_len))*4);
    fprintf(logfile," |-Type Of Service : %d\n",(unsigned int)iphdr->ip_tos);
    fprintf(logfile," |-IP Total Length : %d Bytes(Size of Packet)\n",ntohs(iphdr->ip_total_length));
    fprintf(logfile," |-Identification : %d\n",ntohs(iphdr->ip_id));
    fprintf(logfile," |-Reserved ZERO Field : %d\n",(unsigned int)iphdr->ip_reserved_zero);
    fprintf(logfile," |-Dont Fragment Field : %d\n",(unsigned int)iphdr->ip_dont_fragment);
    fprintf(logfile," |-More Fragment Field : %d\n",(unsigned int)iphdr->ip_more_fragment);
    fprintf(logfile," |-TTL : %d\n",(unsigned int)iphdr->ip_ttl);
    fprintf(logfile," |-Protocol : %d\n",(unsigned int)iphdr->ip_protocol);
    fprintf(logfile," |-Checksum : %d\n",ntohs(iphdr->ip_checksum));
    fprintf(logfile," |-Source IP : %s\n",inet_ntoa(source.sin_addr));
    fprintf(logfile," |-Destination IP : %s\n",inet_ntoa(dest.sin_addr));
}

/*
    Print the TCP header for TCP packets
*/
void PrintTcpPacket(u_char* Buffer, int Size)
{
    unsigned short iphdrlen;
    int header_size = 0 , tcphdrlen , data_size;

    iphdr = (IPV4_HDR *)(Buffer + sizeof(ETHER_HDR));
    iphdrlen = iphdr->ip_header_len*4;

    tcpheader = (TCP_HDR*)( Buffer + iphdrlen + sizeof(ETHER_HDR) );
    tcphdrlen = tcpheader->data_offset*4;
    
    data = ( Buffer + sizeof(ETHER_HDR) + iphdrlen + tcphdrlen );
    data_size = (Size - sizeof(ETHER_HDR) - iphdrlen - tcphdrlen );

    fprintf(logfile,"\n\n***********************TCP Packet*************************\n");

    PrintIpHeader(Buffer,Size);

    fprintf(logfile,"\n");
    fprintf(logfile,"TCP Header\n");
    fprintf(logfile," |-Source Port : %u\n",ntohs(tcpheader->source_port));
    fprintf(logfile," |-Destination Port : %u\n",ntohs(tcpheader->dest_port));
    fprintf(logfile," |-Sequence Number : %u\n",ntohl(tcpheader->sequence));
    fprintf(logfile," |-Acknowledge Number : %u\n",ntohl(tcpheader->acknowledge));
    fprintf(logfile," |-Header Length : %d DWORDS or %d BYTES\n" , (unsigned int)tcpheader->data_offset,(unsigned int)tcpheader->data_offset*4);
    fprintf(logfile," |-CWR Flag : %d\n",(unsigned int)tcpheader->cwr);
    fprintf(logfile," |-ECN Flag : %d\n",(unsigned int)tcpheader->ecn);
    fprintf(logfile," |-Urgent Flag : %d\n",(unsigned int)tcpheader->urg);
    fprintf(logfile," |-Acknowledgement Flag : %d\n",(unsigned int)tcpheader->ack);
    fprintf(logfile," |-Push Flag : %d\n",(unsigned int)tcpheader->psh);
    fprintf(logfile," |-Reset Flag : %d\n",(unsigned int)tcpheader->rst);
    fprintf(logfile," |-Synchronise Flag : %d\n",(unsigned int)tcpheader->syn);
    fprintf(logfile," |-Finish Flag : %d\n",(unsigned int)tcpheader->fin);
    fprintf(logfile," |-Window : %d\n",ntohs(tcpheader->window));
    fprintf(logfile," |-Checksum : %d\n",ntohs(tcpheader->checksum));
    fprintf(logfile," |-Urgent Pointer : %d\n",tcpheader->urgent_pointer);
    fprintf(logfile,"\n");
    fprintf(logfile," DATA Dump ");
    fprintf(logfile,"\n");

    fprintf(logfile,"IP Header\n");
    PrintData( (u_char*)iphdr , iphdrlen);

    fprintf(logfile,"TCP Header\n");
    PrintData( (u_char*)tcpheader , tcphdrlen );

    fprintf(logfile,"Data Payload\n");
    PrintData( data , data_size );

    fprintf(logfile,"\n###########################################################\n");
}

/*
    Print the UDP header for UDP packets
*/
void print_udp_packet(u_char *Buffer,int Size)
{
    int iphdrlen = 0 , data_size = 0;

    iphdr = (IPV4_HDR *)(Buffer + sizeof(ETHER_HDR));
    iphdrlen = iphdr->ip_header_len*4;

    udpheader = (UDP_HDR*)( Buffer + iphdrlen + sizeof(ETHER_HDR) );
        
    data = ( Buffer + sizeof(ETHER_HDR) + iphdrlen + sizeof(UDP_HDR) );
    data_size = (Size - sizeof(ETHER_HDR) - iphdrlen - sizeof(UDP_HDR) );

    fprintf(logfile,"\n\n***********************UDP Packet*************************\n");

    PrintIpHeader(Buffer,Size);

    fprintf(logfile,"\nUDP Header\n");
    fprintf(logfile," |-Source Port : %d\n",ntohs(udpheader->source_port));
    fprintf(logfile," |-Destination Port : %d\n",ntohs(udpheader->dest_port));
    fprintf(logfile," |-UDP Length : %d\n",ntohs(udpheader->udp_length));
    fprintf(logfile," |-UDP Checksum : %d\n",ntohs(udpheader->udp_checksum));

    fprintf(logfile,"\n");
    
    fprintf(logfile,"IP Header\n");
    PrintData( (u_char*)iphdr , iphdrlen);

    fprintf(logfile,"UDP Header\n");
    PrintData((u_char*)udpheader , sizeof(UDP_HDR));

    fprintf(logfile,"Data Payload\n");
    PrintData(data ,data_size);

    fprintf(logfile,"\n###########################################################\n");
}

void PrintIcmpPacket(u_char* Buffer , int Size)
{
    int iphdrlen = 0 , icmphdrlen = 0 , data_size=0;

    iphdr = (IPV4_HDR *)(Buffer + sizeof(ETHER_HDR));
    iphdrlen = iphdr->ip_header_len*4;

    icmpheader = (ICMP_HDR*)( Buffer + iphdrlen + sizeof(ETHER_HDR) );
    
    data = ( Buffer + sizeof(ETHER_HDR) + iphdrlen + sizeof(ICMP_HDR) );
    data_size = (Size - sizeof(ETHER_HDR) - iphdrlen - sizeof(ICMP_HDR) );

    fprintf(logfile,"\n\n***********************ICMP Packet*************************\n");
    PrintIpHeader(Buffer,Size);

    fprintf(logfile,"\n");

    fprintf(logfile,"ICMP Header\n");
    fprintf(logfile," |-Type : %d",(unsigned int)(icmpheader->type));

    if((unsigned int)(icmpheader->type)==11) 
    {
        fprintf(logfile," (TTL Expired)\n");
    }
    else if((unsigned int)(icmpheader->type)==0) 
    {
        fprintf(logfile," (ICMP Echo Reply)\n");
    }

    fprintf(logfile," |-Code : %d\n",(unsigned int)(icmpheader->code));
    fprintf(logfile," |-Checksum : %d\n",ntohs(icmpheader->checksum));
    fprintf(logfile," |-ID : %d\n",ntohs(icmpheader->id));
    fprintf(logfile," |-Sequence : %d\n",ntohs(icmpheader->seq));
    fprintf(logfile,"\n");

    fprintf(logfile , "IP Header\n");
    PrintData( (u_char*)iphdr , iphdrlen);

    fprintf(logfile , "ICMP Header\n");
    PrintData( (u_char*)icmpheader , sizeof(ICMP_HDR) );

    fprintf(logfile , "Data Payload\n");
    PrintData(data , data_size);

    fprintf(logfile,"\n###########################################################\n");
}

/*
    Print the hex values of the data
*/
void PrintData (u_char* data , int Size)
{
    unsigned char a , line[17] , c;
    int j;
    
    //loop over each character and print
    for(i=0 ; i < Size ; i++)
    {
        c = data[i];
        
        //Print the hex value for every character , with a space
        fprintf(logfile," %.2x", (unsigned int) c);
        
        //Add the character to data line
        a = ( c >=32 && c <=128) ? (unsigned char) c : '.';
        
        line[i%16] = a;
        
        //if last character of a line , then print the line - 16 characters in 1 line
        if( (i!=0 && (i+1)%16==0) || i == Size - 1)
        {
            line[i%16 + 1] = '\0';
            
            //print a big gap of 10 characters between hex and characters
            fprintf(logfile ,"          ");
            
            //Print additional spaces for last lines which might be less than 16 characters in length
            for( j = strlen(line) ; j < 16; j++)
            {
                fprintf(logfile , "   ");
            }
            
            fprintf(logfile , "%s \n" , line);
        }
    }
    
    fprintf(logfile , "\n");
}

The program works fine when I run it. I tried to download some file and the total size displayed on the program is correct.

Then I tried to remove the logfile and change the fprintf() to printf().

When I run my program again, it displays full log information on the console instead of the log file, but when I tried to download some file, and check the total size, it's less than the file size, so I think there are some packets missing.

I've tried to insert every packet into a database but some packets are also missing.

Is there any way to fix this, so that the program can display real-time packet sniffing without missing some packets?