c - Error message "dereferencing pointer to incomplete type" when I use udphdr -

i write programme udphdr.h. , here header file list:

#include <linux/module.h> #include <linux/kernel.h> #include <linux/netfilter.h> #include <linux/netfilter_ipv4.h> #include <linux/ip.h> #include <linux/tcp.h> #include <linux/udp.h> #include <linux/skbuff.h> #include <linux/init.h> #include <linux/types.h> #include <linux/sched.h> #include <net/sock.h> #include <linux/netlink.h> 

and here code:

void dealudph(struct updhr* udph){         if(udph==null){             printk("udph null!!\n");         }else{             printk("updh %u\n",udph);             printk("udp sport %u\n",udph->source);         } }  static unsigned int hookfunc(unsigned int hooknum, struct sk_buff **skb,     const struct net_device *in, const struct net_device *out,     int (*okfn)(struct sk_buff *)){         struct iphdr *iph=ip_hdr(skb);         struct ethhdr * ethh = eth_hdr(skb);         struct tcphdr *tcph=tcp_hdr(skb);         struct udphdr *udph=udp_hdr(skb);         printk("*********************************************************\n");         dealiph(iph);         dealethh(ethh);         dealtcph(tcph);         dealudph(udph);         printk("*********************************************************\n");         return nf_accept; } 

here makefile:

ifneq ($(kernelrelease),) mymodule-objs:=main.c obj-m += main.o  else pwd := $(shell pwd) kver := $(shell uname -r) kdir := /lib/modules/$(kver)/build  all:     $(make) -c $(kdir) m=$(pwd) modules clean:     rm -rf *.o *.mod.c *.ko *.symvers *order *.markers *- endif 

when compile file, says line: printk("udp sport %u\n",udph->source); error, , error information dereferencing pointer incomplete type.

what matter? how solve it?

you have:

struct updhr* udph 

you mean:

struct udphdr* udph 

(in context of: void dealudph(struct updhr* udph){.)

note long don't need reference members in structure, or allocate copy of structure, can use struct anynameyoulike *ptr parameter without further ado. however, if name appears first time in function prototype (declaration or definition), should warnings like:

$ gcc -g -o3 -std=c11 -wall -wextra -wmissing-prototypes -werror -c xx.c xx.c:2:23: error: ‘struct xyz’ declared inside parameter list [-werror]  void something(struct xyz *ptr)                        ^ xx.c:2:23: error: scope definition or declaration, not want [-werror] xx.c:2:6: error: no previous prototype ‘something’ [-werror=missing-prototypes]  void something(struct xyz *ptr)       ^ cc1: warnings being treated errors rmk: error code 1 $ 

the 'no previous prototype' message accurate , consequence of using -wmissing-prototypes option (in conjunction -werror).

xx.c

#include <stdlib.h> void something(struct xyz *ptr) {   if (ptr == 0)     exit(0); }