error: dereferencing pointer to incomplete type
 

hi everyone!
im working on my client/server chat program and i had "error: dereferencing pointer to incomplete type" during the compilation of my clientchat.c

Here is the code:

#include <stdio.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <string.h>
#include <stdlib.h>
#include <netdb.h>
#include <unistd.h>
#include <signal.h>
#include <time.h>

int main(int argc,char *argv[])
{
struct sockaddr_in clientaddr;
pid_t pid;
int clientfd,sendbytes;
struct hostent *host;
char *buf,*buf_r;

if(argc < 4)
{
printf("usage:\n");
printf("%s host port name\n",argv[0]);
exit(1);
}

host = gethostbyname(argv[1]);
if((clientfd = socket (AF_INET,SOCK_STREAM,0)) == -1)
{
perror("socket\n");
exit(1);
}
clientaddr.sin_family = AF_INET;
clientaddr.sin_port = htons((uint16_t)atoi(argv[2]));
clientaddr.sin_addr = *((struct in_addrr *) host->h_addr);
bzero(&(clientaddr.sin_zero),0);
if(connect(clientfd,(struct sockaddr *)&clientaddr,sizeof(struct sockaddr)) == -1)
{
perror("connect\n");
exit(1);
}

buf=(char *)malloc(120);
memset(buf,0,120);

buf_r=(char *)malloc(100);

if( recv(clientfd,buf,100,0) == -1)
{
perror("recv: ");
exit(1);
}
printf("\n%s\n",buf);

pid = fork();

while(1)
{
if(pid > 0)
{
//get_cur_time(time_str);

strcpy(buf,argv[3]);
strcat(buf,":");
memset(buf_r,0,100);
//gets(buf_r);
fgets(buf_r,100,stdin);
strncat(buf,buf_r,strlen(buf_r)-1);
//strcat(buf,time_str);
//printf("---%s\n",buf);
if((sendbytes = send(clientfd,buf,strlen(buf),0)) == -1)
{
perror("send\n");
exit(1);
}
}
else if(pid == 0)
{
memset(buf,0,1000);
if(recv(clientfd,buf,100,0) <=0)
{
perror("recv:");
close(clientfd);
raise(SIGSTOP);
exit(1);
}
printf("%s\n",buf);
}
else
perror("fork");
}
close(clientfd);
return 0;
}

Please help me with this.
Thank u much.

Is the extra r on that line a typo?

Without that r your code compiles.

One of the header prototypes is probably not coded exactly the way the compiler wants it. I think there is a "no deference" switch for gcc and g++ that you could try. I'm not able to locate the exact code, because I need more context from the compiler, and preferably some debug traces.

I have a hunch uint16 type is a culprit. Try using that header from the kernel headers instead of the libc6 one.

thank u so much. no more errors on both my clientchat.c and serverchat.c but why is it that there is still no exe file. it is supposed to have a chat program between the client and the server.

when i tried

./clientchat acer-d56ca9a39d 350

the result is -bash: ./clientchat: No such file or directory

same result i got for serverchat

how did you compile your code? What was the command line?

using cygwin terminal i compiled my serverchat as:
gcc -Wall -o server serverchat.c

and my clientchat as
gcc -Wall -o client clientchat.c

there were no errors displayed alr

so your server program is server (instead or serverchat), your client program is client (instead of clientchat).
The name of the created program is the string after the -o option. see the man page of gcc...

where can i check on that?

man g++

so what shall i do to make my program an exe file. so that i can have the expected chat between the server and the client.

How can i make my codes in hello world program into a single chat program where the client can send messages to the server and the server can send replies to the client?

here are my codes for the hello world program:

server.c


#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/utsname.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <unistd.h>

int _GetHostName(char *buffer, int length);

const char MESSAGE[]="Hello, World!\n";
const int BACK_LOG=5;

int main(int argc, char *argv[]){
int serverSocket=0,on=0,port=0,status=0,childPid=0;
struct hostent *hostPtr=NULL;
char hostname[80]="";
struct sockaddr_in serverName={0};

if(2!=argc){
fprintf(stderr,"Usage: %s <port>\n",argv[0]);
exit(1);
}
port=atoi(argv[1]);
serverSocket=socket(PF_INET,SOCK_STREAM,IPPROTO_TCP);
if(-1==serverSocket){
perror("socket()");
exit(1);
}

on=1;
status=setsockopt(serverSocket,SOL_SOCKET,SO_REUSEADDR,
(const char*)&on,sizeof(on));
if(-1==status){
perror("setsockopt(...,SO_REUSEADDR,...)");
}
{
struct linger linger={0};
linger.l_onoff=1;
linger.l_linger=30;
status=setsockopt(serverSocket,SOL_SOCKET,SO_LINGER,
(const char*)&linger,sizeof(linger));
if(-1==status){
perror("setsockopt(...,SO_LINGER,...)");
}
}

status=_GetHostName(hostname,sizeof(hostname));
if(-1==status){
perror("_GetHostName()");
exit(1);
}

hostPtr=gethostbyname(hostname);
if(NULL==hostPtr){
perror("gethostbyname()");
exit(1);
}

(void)memset(&serverName,0,sizeof(serverName));
(void)memcpy(&serverName.sin_addr,hostPtr->h_addr,hostPtr->h_length);

serverName.sin_family=AF_INET;
serverName.sin_port=htons(port);

status=bind(serverSocket,(struct sockaddr*)&serverName,sizeof(serverName));
if(-1==status){
perror("bind()");
exit(1);
}

status=listen(serverSocket,BACK_LOG);
if(-1==status){
perror("listen()");
exit(1);
}

for(;{
struct sockaddr_in clientName={0};
int slaveSocket, clientLength=sizeof(clientName);

(void)memset(&clientName,0,sizeof(clientName));

slaveSocket=accept(serverSocket,
(struct sockaddr*)&clientName,&clientLength);
if(-1==slaveSocket){
perror("accept()");
exit(1);
}

childPid=fork();

switch(childPid){

case -1:/*ERROR */
perror("fork()");
exit(1);
case 0 :/*child process */
close(serverSocket);
if(-1==getpeername(slaveSocket,
(struct sockaddr*)&clientName,&clientLength)){
perror("getpeername()");
}else{
printf("Connection request from %s\n",
inet_ntoa(clientName.sin_addr));
}

write(slaveSocket,MESSAGE,strlen(MESSAGE));
close(slaveSocket);
exit(0);
default:/*parent process */
close(slaveSocket);
}
}
return 0;
}

int _GetHostName(char *buffer,int length){
struct utsname sysname={0};
int status=0;

status=uname(&sysname);
if(-1!=status){
strncpy(buffer,sysname.nodename,length);
}
return(status);
}

------
client.c


#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

int main(int argc,char *argv[]){
int clientSocket,remotePort,status=0;
struct hostent *hostPtr=NULL;
struct sockaddr_in serverName={0};
char buffer[256]="";
char *remoteHost=NULL;

if(3!=argc){
fprintf(stderr,"Usage: %s <serverHost> <serverPort>\n",argv[0]);
exit(1);
}
remoteHost=argv[1];
remotePort=atoi(argv[2]);
clientSocket=socket(PF_INET,SOCK_STREAM,IPPROTO_TCP);
if(-1==clientSocket){
perror("socket()");
exit(1);
}

hostPtr=gethostbyname(remoteHost);
if(NULL==hostPtr){
hostPtr=gethostbyaddr(remoteHost,strlen(remoteHost),AF_INET);
if(NULL==hostPtr){
perror("Error resolving server address ");
exit(1);
}
}
serverName.sin_family=AF_INET;
serverName.sin_port=htons(remotePort);
(void)memcpy(&serverName.sin_addr,hostPtr->h_addr,hostPtr->h_length);
status=connect(clientSocket,(struct sockaddr*)&serverName,
sizeof(serverName));
if(-1==status){
perror("connect()");
exit(1);
}

while(0<(status=read(clientSocket,buffer,sizeof(buffer)-1)))
printf("%d: %s",status,buffer);
if(-1==status)perror("read()");
close(clientSocket);
return 0;
}

thank u!

Hi Joyce,
Please be advised that your posted code sample contains a likely heap-based buffer overflow which an attacker might be able to leverage to exploit the system. Use of strcpy() should be limited to instances where the source buffer is not user-controlled or has already been bounds checked. strncpy(buf, argv[3], 120) would prevent this type of attack in the event that argv[3] does not null terminate within 120 bytes.

For more information on how this could compromise your system, please check out my blog regarding stack-based buffer overflows which are even more dangerous. (Heap based overflows can certainly still lead to code execution and crashes.)

VERT Vuln School: Stack Buffer Overflows 101

Part 1: Introducing the Bug
Part 2: Explaining the Stack
Part 3: Exploiting the Bug

Regards,
Craig