为什么pcap_next_ex的第三个参数是不兼容的指针类型？

#include <pcap.h>
#include<stdio.h>

int main() {
    pcap_t *handle;
    char errbuf[PCAP_ERRBUF_SIZE];
    const u_char *packet;
    struct pcap_pkthdr header;
    struct pcap_pkthdr *header_ptr;

    handle = pcap_open_offline("example.pcap", errbuf);
    if (handle == NULL) {
        fprintf(stderr, "Couldn't open file: %s\n", errbuf);
        return 1;
    }

    while ((packet = pcap_next_ex(handle, &header_ptr, NULL)) != NULL) {
        // Process the packet
    }

    pcap_close(handle);
    return 0;
}