socket programming

nickraj · 09-01-2006, 05:54 PM

I have just started network programming in c (unix). After compiling the programm i m getting undefined reference to `socket', undefined reference to `bind' etc. I am unable to figure out the problem , can you help.

nick

tuxdev · 09-01-2006, 06:43 PM

You need to #include <sys/socket.h>

randyding · 09-01-2006, 08:52 PM

If you forgot the #include you should receive

Code:

implicit declaration of function `socket'

So that may not be the problem. It sounds like a link problem.
If you are on Unix (like solaris for example), then add the following to your link options.

Code:

-lnsl -lsocket

primo · 09-01-2006, 09:55 PM

On Solaris you also need to add -lresolv if you use any of the gethostby*() functions.

nickraj · 09-02-2006, 12:08 PM

Quote:

Originally Posted by randyding

If you forgot the #include you should receive

Code:

implicit declaration of function `socket'

So that may not be the problem. It sounds like a link problem.
If you are on Unix (like solaris for example), then add the following to your link options.

Code:

-lnsl -lsocket

Thankx
can u tell me why host to netwok and n/w to host conversion are done ?

paulsm4 · 09-02-2006, 12:45 PM

Byte order.

http://en.wikipedia.org/wiki/Little_endian

nickraj · 09-30-2006, 01:07 PM

I am writting a multithreaded echo server using select() system call. When i run the program either it handles only one client. Or it handles multiple client but no message can be exchanged. Do select() automatically updates the value of readable FD (file descriptor). The following code accepts multiple connection but hangs when clint send msg. I think the problem with the program is that it only checks fd(which is the listen socket of the server) but do not check whether the client FD is readable or not.

Server.c

#include<stdio.h>
#include<sys/socket.h>
#include<sys/types.h>
#include<sys/select.h>
#include<netinet/in.h>

int main(int argc , char* argv[])
{
int fd, fd1;
int x,y;
int size ;
ssize_t n;
int soc[20], sock=0;
int i=0;
int readynum=0;
int maxfd =0,max=-1 ;
int nclient[5];
char buf[21];
struct sockaddr_in server,client;
fd_set readset;
void cfd(int );
/* Validating command line arguments , Error handling for socket()method ,
Assigning the vlaues to the structure server */

if(argc!=3)
{
printf("SYNTAX ERROR !!\n " );
printf("Syntax :<Ip Address> <PORT>\n");
exit(0);
}

if( (fd=socket(AF_INET,SOCK_STREAM,0))==-1 )
{perror("error creating socket");
exit(-1);
}
server.sin_family=AF_INET;
server.sin_port=htons(atoi(argv[2]));
server.sin_addr.s_addr=htonl(INADDR_ANY);
bzero( &(server.sin_zero), 8);

if( (bind(fd,(struct sockaddr*)&server, sizeof(server)) )<0 )
{perror("error in bind ");
exit(-1);
}

/* Listening for the connections , accepting the connections using accept()and s
ending the message to the client after validating the message received from the
client */

if( listen(fd,5)<0 )
{
perror("error in listen");
exit(-1);
}
for( i =0 ;i<5;i++)
nclient[i]=-1;
maxfd=fd;
for(;

{
printf("in loop");
FD_ZERO(&readset);
FD_SET(fd,&readset);
readynum= select(maxfd+1,&readset,NULL,NULL,0);
printf("the FD reay are =%d",readynum);
if(readynum<0) perror("error in select");
else
{

if(( FD_ISSET(fd,&readset))>0)
{
if((fd1=accept(fd,0,0) ) <0 )
perror("Error while accepting connection");
else
{
printf("connection esteblished\n");
if(fd1>maxfd)
maxfd=fd1;
for(i=0;i<=5;i++)
{
if(client[i]==-1)
client[i]=fd1;
break;
}

FD_SET(fd1,&readset);

}
}

}

}

for( i =0;i<=maxfd;i++)
{
printf("in for loop" );
if(FD_ISSET(soc[i],&readset)>0)
{
printf("hello in read ");
n=read(fd1,buf,51);
printf("%d",n );
if(write(fd1,buf,51)<0)
perror("error in writing");

if(n<0)
perror("error in read ");

if(n==0)
{
close(fd1);
FD_CLR(fd1,&readset);
}

if((strcmp(buf,"quit")==0) )
{
int j =0 ;
j=write(fd1,buf,51);
if(j<0) perror("error in send");
nclient[i]=-1;
close(fd1);
FD_CLR(fd1,&readset);

}

}
}

} // Main Class ends

randyding · 09-30-2006, 01:49 PM

I didn't look at your code, but the reason select() is used is to avoid threading, generally.

Its worth while getting a book on socket programming since its so complicated. At the very least take a look at "man select_tut".

pankaj99 · 09-30-2006, 02:26 PM

why you need to add the compile options
'-lnsl -lsocket' under solaris but not under linux?

tuxdev · 09-30-2006, 02:32 PM

Because that's how Solaris is.

Anyway, if you actually want someone to read your code, please put [code] tags on it to preserve indentation.

Wim Sturkenboom · 10-02-2006, 12:23 AM

Two little things that I see:
1)
You never get out of the endless for-loop (the loop that accepts connections)
2)
The loop in bold is only executed once in ANY situation; further client is not an array. I'm amazed that the code generates something that you can execute (my compiuler stops on errors). So if it does, client is probably the cause of the crash.

Code:

    for(;;)
    {
        printf("in loop");
        FD_ZERO(&readset);
        FD_SET(fd,&readset);
        readynum= select(maxfd+1,&readset,NULL,NULL,0);
        printf("the FD reay are =%d",readynum);
        if(readynum<0)
            perror("error in select");
        else
        {
            if(( FD_ISSET(fd,&readset))>0)
            {
                if((fd1=accept(fd,0,0) ) <0 )
                    perror("Error while accepting connection");
                else
                {
                    printf("connection esteblished\n");
                    if(fd1>maxfd)
                        maxfd=fd1;
                    for(i=0;i<=5;i++)
                    {
                        if(client[i]==-1)
                            client[i]=fd1;
                        break;
                    }
                    FD_SET(fd1,&readset);
                }
            }
        }
    } // end_of_for(;;)

// you'll never get here

nickraj · 10-06-2006, 06:36 AM

WEll this is the same program with select() system call
can you tell me the problem . The server must be multithreaded echo server.
server.c

Code:

#include<stdio.h>
#include<sys/socket.h>
#include<sys/types.h>
#include<netinet/in.h>
#include<sys/select.h>

int main(int argc , char *argv[])
 {
        int fd,fd1;
        int maxfd;
        int i;
        int nclient[5];
        char buf[50];
        struct sockaddr_in server , client ;
        fd_set rset;
        fd=socket(AF_INET,SOCK_STREAM,0);
        server.sin_family=AF_INET;
        server.sin_port=htons(atoi(argv[1]));
        server.sin_addr.s_addr=htonl(INADDR_ANY);
        bzero(&(server.sin_zero),8);
        bind(fd,(struct sockaddr*)&server,sizeof(server));
        listen(fd,5);
        for(i=0;i<5;i++)
        {
                nclient[i]=-1;
        }
        for(;;)
        {
                FD_ZERO(&rset);
                FD_SET(fd,&rset);
                for(i=0;i<5;i++)
                {
                        if(nclient[i]>0)
                        {
                                FD_SET(nclient[i],&rset);
                        }
                        printf("\nclient[%d]=%d\n",i,nclient[i]);
                }
        for(;;)
        {
                FD_ZERO(&rset);
                FD_SET(fd,&rset);
                for(i=0;i<5;i++)
                {
                        if(nclient[i]>0)
                        {
                                FD_SET(nclient[i],&rset);
                        }
                        printf("\nclient[%d]=%d\n",i,nclient[i]);
                }
                select(maxfd+1,&rset,NULL,NULL,0);
                if(FD_ISSET(fd,&rset)>0)
                {
                        fd1=accept(fd,0,0);
                        printf("fd id =%d\n",fd1);
                        if(fd1>maxfd)maxfd=fd1;
                        FD_SET(fd1,&rset);
                        for(i=0;i<5;i++)
                        {
                                if(nclient[i]<0)
                                {
                                        nclient[i]=fd1;
                                        break;
                                }
                        }

                }
                if(FD_ISSET(fd1,&rset)>0)
                {
                        read(fd1,buf,sizeof(buf));
                        write(fd1,buf,sizeof(buf));
                        printf("msg from client=%s",buf);
                }
        }
 }


and the client is 
	Code:
	
#include<stdio.h>
#include<sys/socket.h>
#include<sys/types.h>
#include<netinet/in.h>
#include<netdb.h>
#include<unistd.h>

int main (int argc , char*argv[])
 {
        struct sockaddr_in rserver;
        struct hostent *he;
        int fd ;
        char buf[50];
        fd_set rset;

        fd = socket(AF_INET,SOCK_STREAM,0);

        he = gethostbyname(argv[1]);

        rserver.sin_family=AF_INET;
        rserver.sin_port=htons( atoi ( argv[2])  ) ;
        bzero(&(rserver.sin_zero), 8);
        memcpy((unsigned char *) &(rserver.sin_addr) , (unsigned char *)he->h_addr , he
->h_length);
         connect(fd,(struct  sockaddr *)&rserver , sizeof(rserver));
        while(1)
        {
                FD_ZERO(&rset);
                FD_SET(fd,&rset);
                FD_SET(0,&rset);
                select(100,&rset,NULL,NULL,0);
                if(FD_ISSET(0,&rset))
                {
                        read(0,buf,sizeof(buf));
                        write(fd,buf,sizeof(buf));
                }
                if(FD_ISSET(fd,&rset))
                {
                        read(fd,buf,sizeof(buf));
                        printf("from server=%s",buf);
                }
        }
        close(fd);

 }

jlliagre · 10-06-2006, 06:53 AM

Quote:

Originally Posted by pankaj99

why you need to add the compile options
'-lnsl -lsocket' under solaris but not under linux?

Because Linux use Glibc which includes libraries from diverse sources, including SystemV, BSD and others. As an implicit link with glibc is done by the compiler, you do not need to specify it.
Solaris libc on the other hand is strictly SystemV.
Sockets and name service libraries are separated and must be specified.

Wim Sturkenboom · 10-07-2006, 02:17 AM

You're turning some things around:
You are supposed to tell us what in your program does not work (error messages etc) and we will try to point you in the right direction or solve it.

Having said that,
You can check the server part by using telnet; i.e. telnet localhost portnumber. Once that is working, you can concentrate on the client side.

nickraj · 10-07-2006, 11:09 AM

Quote:

Originally Posted by Wim Sturkenboom

You're turning some things around:
You are supposed to tell us what in your program does not work (error messages etc) and we will try to point you in the right direction or solve it.

Having said that,
You can check the server part by using telnet; i.e. telnet localhost portnumber. Once that is working, you can concentrate on the client side.

The problem with the above program is that , when one client connects with the server they both can communicate with each other . What ever client sends , server echo that back to the client. But when the second client connect, the first client can't send any message while second client work fine, So the problem is whenever a new client connects to the server, the previous client do no receive the echo message from the server .