Question

Disclaimer: This is for a homework assignment, but the question is not regarding the assignment, just about general syntax weirdness.

I’m trying to use libpcap in the context of a much larger program, but when I try to get the packet header and data for each packet gcc complains that the third parameter to pcap_next_ex is of an incompatible pointer type. Here’s some sample code to see what I’m talking about:

#include <stdio.h> #include <stdlib.h> #include <pcap.h>  int main() {     pcap_t *pcap;     char pcapErr[PCAP_ERRBUF_SIZE];     struct pcap_pkthdr *pktHeader;     u_char *pktData;      pcap = pcap_open_offline('somefile.pcap', pcapErr);     if (pcap == NULL)     {         fprintf(stderr, 'pcap_open_offline failed: %s\n', pcapErr);         exit(EXIT_FAILURE);     }      while (pcap_next_ex(pcap, &pktHeader, &pktData) == 1)     {         // do things here     }      pcap_close(pcap);      return EXIT_SUCCESS; } 

The man pages indicated that the prototype for pcap_next_ex() is:

int pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header, const u_char **pkt_data) 

How exactly is what I’m passing an incompatible pointer type? Thanks.

Answer 1

Change the declaration of pktData to read:

const u_char *pktData; 

and gcc should stop complaining.