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Need some help here... Can't figure out what's wrong :(
here is the error
LNK2019: unresolved external symbol "void __cdecl packet_handler(unsigned char *,struct pcap_pkthdr const *,unsigned char const *)" (?packet_handler@@YAXPAEPBUpcap_pkthdr@@PBE@Z) referenced in...

#include "stdafx.h"
#include <pcap.h>

/* 4 bytes IP address */
typedef struct ip_address{
    u_char byte1;
    u_char byte2;
    u_char byte3;
    u_char byte4;
/* IPv4 header */
typedef struct ip_header{
    u_char  ver_ihl;        // Version (4 bits) + Internet header length (4 bits)
    u_char  tos;            // Type of service 
    u_short tlen;           // Total length 
    u_short identification; // Identification
    u_short flags_fo;       // Flags (3 bits) + Fragment offset (13 bits)
    u_char  ttl;            // Time to live
    u_char  proto;          // Protocol
    u_short crc;            // Header checksum
    ip_address  saddr;      // Source address
    ip_address  daddr;      // Destination address
    u_int   op_pad;         // Option + Padding
/* UDP header*/
typedef struct udp_header{
    u_short sport;          // Source port
    u_short dport;          // Destination port
    u_short len;            // Datagram length
    u_short crc;            // Checksum
/* 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];
u_int netmask;
char packet_filter[] = "ip and udp";
struct bpf_program fcode;
    /* Retrieve the device list */
    if (pcap_findalldevs_ex(PCAP_SRC_IF_STRING, NULL, &alldevs, errbuf) == -1)
        fprintf(stderr,"Error in pcap_findalldevs: %s\n", errbuf);
    /* Print the list */
    for(d=alldevs; d; d=d->next)
        printf("%d. %s", ++i, d->name);
        if (d->description)
            printf(" (%s)\n", d->description);
            printf(" (No description available)\n");
        printf("\nNo interfaces found! Make sure WinPcap is installed.\n");
        return -1;
    printf("Enter the interface number (1-%d):",i);
    scanf_s("%d", &inum);
    if(inum < 1 || inum > i)
        printf("\nInterface number out of range.\n");
        /* Free the device list */
        return -1;
    /* Jump to the selected adapter */
    for(d=alldevs, i=0; i< inum-1 ;d=d->next, i++);
    /* Open the adapter */
    if ( (adhandle= pcap_open(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.
                             PCAP_OPENFLAG_PROMISCUOUS,         // promiscuous mode
                             1000,      // read timeout
                             NULL,      // remote authentication
                             errbuf     // error buffer
                             ) ) == NULL)
        fprintf(stderr,"\nUnable to open the adapter. %s is not supported by WinPcap\n");
        /* Free the device list */
        return -1;
    /* Check the link layer. We support only Ethernet for simplicity. */
    if(pcap_datalink(adhandle) != DLT_EN10MB)
        fprintf(stderr,"\nThis program works only on Ethernet networks.\n");
        /* Free the device list */
        return -1;
    if(d->addresses != NULL)
        /* Retrieve the mask of the first address of the interface */
        netmask=((struct sockaddr_in *)(d->addresses->netmask))->sin_addr.S_un.S_addr;
        /* If the interface is without addresses we suppose to be in a C class network */

    //compile the filter
    if (pcap_compile(adhandle, &fcode, packet_filter, 1, netmask) <0 )
        fprintf(stderr,"\nUnable to compile the packet filter. Check the syntax.\n");
        /* Free the device list */
        return -1;
    //set the filter
    if (pcap_setfilter(adhandle, &fcode)<0)
        fprintf(stderr,"\nError setting the filter.\n");
        /* Free the device list */
        return -1;
    printf("\nlistening on %s...\n", d->description);
    /* At this point, we don't need any more the device list. Free it */
    /* start the capture */
    pcap_loop(adhandle, 0, packet_handler, NULL);
    return 0;
Posted 5-May-13 21:50pm
Updated 5-May-13 22:02pm

1 solution

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Solution 1

Well, the error message clearly says it all: you didn't provide the implementation of the packet_handler function (you provided its declaration and passed it to pcap_loop, but you need also to implement it).

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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