getting port number of a packet in pcap file

[ssh.8768]

Hi all,I am programming in c++ with 'winpcap' . I read a .pcap file and after I want to get the source and destination of packets in that file,but i coulden't get the right answer!!here is a piece of my code:

////I define the sructures here
#pragma pack(1)
    struct sniff_ethernet {
        u_char ether_dhost[ETHER_ADDR_LEN]; /* Destination host address */
        u_char ether_shost[ETHER_ADDR_LEN]; /* Source host address */
        u_short ether_type; /* IP? ARP? RARP? etc */
    };

    /* IP header */
#pragma pack(1)
    struct sniff_ip {
        u_char ip_vhl;      /* version << 4 | header length >> 2 */
        u_char ip_tos;      /* type of service */
        u_short ip_len;     /* total length */
        u_short ip_id;      /* identification */
        u_short ip_off;     /* fragment offset field */
    #define IP_RF 0x8000        /* reserved fragment flag */
    #define IP_DF 0x4000        /* dont fragment flag */
    #define IP_MF 0x2000        /* more fragments flag */
    #define IP_OFFMASK 0x1fff   /* mask for fragmenting bits */
        u_char ip_ttl;      /* time to live */
        u_char ip_p;        /* protocol */
        u_short ip_sum;     /* checksum */
        struct in_addr ip_src;
        struct in_addr ip_dst; /* source and dest address */
    };
#pragma pack(1)
    struct sniff_tcp {
        u_short th_sport;   /* source port */
        u_short th_dport;   /* destination port */
        u_int32_t th_seq;       /* sequence number */
        u_int32_t th_ack;       /* acknowledgement number */

        u_char th_offx2;    /* data offset, rsvd */
    #define TH_OFF(th)  (((th)->th_offx2 & 0xf0) >> 4)
        u_char th_flags;
    #define TH_FIN 0x01
    #define TH_SYN 0x02
    #define TH_RST 0x04
    #define TH_PUSH 0x08
    #define TH_ACK 0x10
    #define TH_URG 0x20
    #define TH_ECE 0x40
    #define TH_CWR 0x80
    #define TH_FLAGS (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG|TH_ECE|TH_CWR)
        u_short th_win;     /* window */
        u_short th_sum;     /* checksum */
        u_short th_urp;     /* urgent pointer */
};

and in the main() I'v write this code for reading the source and destination port:

while (pcap_next_ex(handler, &header, &packet) >= 0)
    {
        ethernet = (struct sniff_ethernet*)(packet);
        ip = (struct sniff_ip*)(packet + SIZE_ETHERNET);
    
        tcp = (struct sniff_tcp*)(packet + SIZE_ETHERNET + size_ip);

        printf("src port: %d dest port: %d \n", tcp->th_sport, tcp->th_dport);
        fprintf(fp,"src port: %d dest port: %d \n", tcp->th_sport, tcp->th_dport);
}

but the result is incorrect? where is the problem?plz help me