gcc -c -o icmp.o icmp.c -I. -O3 -Wall
icmp.c:23:47: warning: declaration of 'struct icmphdr' will not be visible outside of this
      function [-Wvisibility]
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp);
                                              ^
icmp.c:104:40: error: invalid application of 'sizeof' to an incomplete type
      'struct icmphdr'
  packet_size = sizeof(struct iphdr) + sizeof(struct icmphdr) + packet_details->pa...
                                       ^     ~~~~~~~~~~~~~~~~
icmp.c:93:10: note: forward declaration of 'struct icmphdr'
  struct icmphdr *icmp;
         ^
icmp.c:115:59: error: invalid application of 'sizeof' to an incomplete type
      'struct icmphdr'
  icmp_payload = (char *)(packet + sizeof(struct iphdr) + sizeof(struct icmphdr));
                                                          ^     ~~~~~~~~~~~~~~~~
icmp.c:93:10: note: forward declaration of 'struct icmphdr'
  struct icmphdr *icmp;
         ^
icmp.c:117:23: warning: incompatible pointer types passing 'struct icmphdr *' to parameter
      of type 'struct icmphdr *' [-Wincompatible-pointer-types]
  prepare_headers(ip, icmp);
                      ^~~~
icmp.c:23:56: note: passing argument to parameter 'icmp' here
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp);
                                                       ^
icmp.c:125:7: error: incomplete definition of type 'struct icmphdr'
  icmp->type = packet_details->type;
  ~~~~^
icmp.c:93:10: note: forward declaration of 'struct icmphdr'
  struct icmphdr *icmp;
         ^
icmp.c:126:7: error: incomplete definition of type 'struct icmphdr'
  icmp->checksum = 0;
  ~~~~^
icmp.c:93:10: note: forward declaration of 'struct icmphdr'
  struct icmphdr *icmp;
         ^
icmp.c:127:7: error: incomplete definition of type 'struct icmphdr'
  icmp->checksum = in_cksum((unsigned short *)icmp, sizeof(struct icmphdr) + packe...
  ~~~~^
icmp.c:93:10: note: forward declaration of 'struct icmphdr'
  struct icmphdr *icmp;
         ^
icmp.c:127:53: error: invalid application of 'sizeof' to an incomplete type
      'struct icmphdr'
  icmp->checksum = in_cksum((unsigned short *)icmp, sizeof(struct icmphdr) + packe...
                                                    ^     ~~~~~~~~~~~~~~~~
icmp.c:93:10: note: forward declaration of 'struct icmphdr'
  struct icmphdr *icmp;
         ^
icmp.c:157:42: error: invalid application of 'sizeof' to an incomplete type
      'struct icmphdr'
  enc_MTU = MTU + sizeof(struct iphdr) + sizeof(struct icmphdr);
                                         ^     ~~~~~~~~~~~~~~~~
icmp.c:148:10: note: forward declaration of 'struct icmphdr'
  struct icmphdr *icmp;
         ^
icmp.c:173:59: error: invalid application of 'sizeof' to an incomplete type
      'struct icmphdr'
  icmp_payload = (char *)(packet + sizeof(struct iphdr) + sizeof(struct icmphdr));
                                                          ^     ~~~~~~~~~~~~~~~~
icmp.c:148:10: note: forward declaration of 'struct icmphdr'
  struct icmphdr *icmp;
         ^
icmp.c:178:30: error: incomplete definition of type 'struct icmphdr'
  packet_details->type = icmp->type;
                         ~~~~^
icmp.c:148:10: note: forward declaration of 'struct icmphdr'
  struct icmphdr *icmp;
         ^
icmp.c:179:71: error: invalid application of 'sizeof' to an incomplete type
      'struct icmphdr'
  ...= packet_size - sizeof(struct iphdr) - sizeof(struct icmphdr);
                                            ^     ~~~~~~~~~~~~~~~~
icmp.c:148:10: note: forward declaration of 'struct icmphdr'
  struct icmphdr *icmp;
         ^
icmp.c:233:47: warning: declaration of 'struct icmphdr' will not be visible outside of
      this function [-Wvisibility]
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp)
                                              ^
icmp.c:233:6: error: conflicting types for 'prepare_headers'
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp)
     ^
icmp.c:23:6: note: previous declaration is here
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp);
     ^
icmp.c:243:7: error: incomplete definition of type 'struct icmphdr'
  icmp->code = 0;

icmp.c:233:47: warning: declaration of 'struct icmphdr' will not be visible outside of
      this function [-Wvisibility]
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp)
                                              ^
icmp.c:233:6: error: conflicting types for 'prepare_headers'
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp)
     ^
icmp.c:23:6: note: previous declaration is here
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp);
     ^
icmp.c:243:7: error: incomplete definition of type 'struct icmphdr'
  icmp->code = 0;
  ~~~~^
icmp.c:233:47: note: forward declaration of 'struct icmphdr'
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp)
                                              ^
icmp.c:244:7: error: incomplete definition of type 'struct icmphdr'
  icmp->un.echo.sequence = rand();
  ~~~~^
icmp.c:233:47: note: forward declaration of 'struct icmphdr'
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp)
                                              ^
icmp.c:245:7: error: incomplete definition of type 'struct icmphdr'
  icmp->un.echo.id = rand();
  ~~~~^
icmp.c:233:47: note: forward declaration of 'struct icmphdr'
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp)
                                              ^
icmp.c:246:7: error: incomplete definition of type 'struct icmphdr'
  icmp->checksum = 0;
  ~~~~^
icmp.c:233:47: note: forward declaration of 'struct icmphdr'
void prepare_headers(struct iphdr *ip, struct icmphdr *icmp)
                                              ^
3 warnings and 15 errors generated.
make: *** [Makefile:6: icmp.o] Error 1