Unix Programming - About the function inet

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About the function inet_ntoa()

learning_C++

2004-09-29, 8:07 pm

I have two programs: one for client and one for server.
In the client.c,
-------------------------------------------------
cin2.sin_family = AF_INET;
inet_aton("130.233.248.203",&(cin2.sin_addr));
cin2.sin_port = htons(PORT);
a1=inet_ntoa(cin2.sin_addr);
printf("The IP value is: %s\n", a1);
//printf("Trying to connect to %s = %s =
%s\n",HOST,inet_ntoa(cin.sin_addr),inet_ntoa(cin2.sin\
_addr));
printf("Trying to connect to %s = %s =
%s\n",HOST,inet_ntoa(cin.sin_addr),a1);
----
a1=130.233.248.203. But after the function printf()is called it
changed to a1=130.233.248.198.
-------------------------------------------------------------
My command line argument is : client 2+3
The output is "Server responded with 2 + 3 = 5"
But when I input client 2*3
The output is: "client: No match."
What is wrong?
--------------------------------------------------------------
Please help me and thanks,

/*for client.c*/
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>

#define PORT 0x1234 /* define a port number for the service
*/
#define HOST "idefix.hut.fi"
#define bufsize 20

void main (argc,argv)

int argc;
char *argv[];

{ struct sockaddr_in cin,cin2;
struct hostent *hp;
char buffer[bufsize];
char *a1;
int sd;

/* Either by hostname ........ lookup in DNS */

if ((hp = gethostbyname(HOST)) == NULL)
{
perror("gethostbyname: ");
exit(1);
}

memset(&cin,0,sizeof(cin));

cin.sin_family = AF_INET;
cin.sin_addr.s_addr = ((struct in_addr *)(hp->h_addr))->s_addr;
cin.sin_port = htons(PORT);
/* Or by IP numerical address ........ */

memset(&cin2,0,sizeof(cin2));

cin2.sin_family = AF_INET;
inet_aton("130.233.248.203",&(cin2.sin_addr));
cin2.sin_port = htons(PORT);
a1=inet_ntoa(cin2.sin_addr);
printf("The IP value is: %s\n", a1);
//printf("Trying to connect to %s = %s =
%s\n",HOST,inet_ntoa(cin.sin_addr),inet_ntoa(cin2.sin\
_addr));
printf("Trying to connect to %s = %s =
%s\n",HOST,inet_ntoa(cin.sin_addr),a1);
if ((sd = socket(AF_INET,SOCK_STREAM,0)) == -1)
{
perror("socket");
exit(1);
}

if (connect(sd,(struct sockaddr *)&cin,sizeof(cin)) == -1)
{
perror("connect");
exit(1);
}
memset(buffer, 0, sizeof(buffer));
sprintf(buffer,"%s",argv[1]);//now we use only two arguments
//sprintf(buffer,"%s + %s",argv[1],argv[3]);
printf("the command line argv[3] is %s:\n", argv[3]);
printf("The input value is : %s\n",buffer);
if (send(sd,buffer,strlen(buffer),0) == -1)
{
perror ("send");
exit(1);
}

if (recv(sd,buffer,bufsize,0) == -1)
{
perror("recv");
exit (1);
}

printf ("Server responded with %s\n",buffer);

close (sd);
}

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

#define PORT 0x1234
#define bufsize 20
#define queuesize 5
#define true 1
#define false 0

int DoService(char *);

void main ()

{ struct sockaddr_in cin;
struct sockaddr_in sin;
struct hostent *hp;
char buffer[bufsize];
int sd, sd_client, addrlen;

memset(&sin,0,sizeof(sin)); /* Another way to zero memory */
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY; /* Broadcast address */
sin.sin_port = htons(PORT);

if ((sd = socket(AF_INET,SOCK_STREAM,0)) == -1)
{
perror("socket");
exit(1);
}
if (bind(sd,(struct sockaddr *)&sin,sizeof(sin)) == -1) /* Must have
this on server */
{
perror("bind");
exit(1);
}

if (listen(sd,queuesize) == -1)
{
perror("listen");
exit(1);
}

while (true)
{
addrlen=sizeof(struct sockaddr_in);
if ((sd_client = accept(sd,(struct sockaddr *)&cin,&addrlen)) ==
-1)
//if ((sd_client = accept(sd,(struct sockaddr *)&cin,&addrlen))
== -1)
{
printf("the value of sd_client= %d\n",sd_client);
perror("accept");
exit(1);
}

printf("server: got connection from %s\n",
inet_ntoa(cin.sin_addr));
memset(buffer,0,sizeof(buffer));
printf("The former value is: %s\n", buffer);
if (recv(sd_client,buffer,sizeof(buffer),0)
== -1)
//printf("The received value is : %s\n", buffer);
{
perror("recv");
exit(1);
}
printf("The received value is : %s\n", buffer);

if (!DoService(buffer)){

break;
}

if (send(sd_client,buffer,strlen(buffer),0)
== -1)
{
perror("send");
exit(1);
}

close (sd_client);
}

close (sd);
printf("Server closing down...\n");
}

int DoService(buffer)

char *buffer;

/* This is the protocol section. Here we must */
/* check that the incoming data are sensible */

{ int a=0,b=0;
char c;

printf("Received: %s\n",buffer);
// memset(buffer, 0,sizeof(buffer));
sscanf(buffer,"%d%c%d\n",&a,&c,&b);

if (a > 0 && b> 0)
{
switch(c){
case '+':
sprintf(buffer,"%d + %d = %d",a,b,a+b);
return true;
case '-':
sprintf(buffer,"%d - %d = %d",a,b,a-b);
return true;
case '*':
sprintf(buffer,"%d * %d = %d",a,b,a*b);
return true;
case '/':
sprintf(buffer,"%d / %d = %d",a,b,a/b);
return true;
default:
return true;
}
}
else
{
if (strncmp("halt",buffer,4) == 0)
{
sprintf(buffer,"Server closing down!");
return false;
}
else
{
sprintf(buffer,"Invalid protocol");
return true;
}
}
}

Måns Rullgård

2004-09-29, 8:07 pm

learning_c@hotmail.com (learning_C++) writes:

> I have two programs: one for client and one for server.
> In the client.c,
> -------------------------------------------------
> cin2.sin_family = AF_INET;
> inet_aton("130.233.248.203",&(cin2.sin_addr));
> cin2.sin_port = htons(PORT);
> a1=inet_ntoa(cin2.sin_addr);
> printf("The IP value is: %s\n", a1);
> //printf("Trying to connect to %s = %s =
> %s\n",HOST,inet_ntoa(cin.sin_addr),inet_ntoa(cin2.sin\
> _addr));
> printf("Trying to connect to %s = %s =
> %s\n",HOST,inet_ntoa(cin.sin_addr),a1);
> ----
> a1=130.233.248.203. But after the function printf()is called it
> changed to a1=130.233.248.198.

$ man inet_ntoa
[...]
The inet_ntoa() function converts the Internet host address in given in
network byte order to a string in standard numbers-and-dots notation.
The string is returned in a statically allocated buffer, which subse-
quent calls will overwrite.

> -------------------------------------------------------------
> My command line argument is : client 2+3
> The output is "Server responded with 2 + 3 = 5"
> But when I input client 2*3
> The output is: "client: No match."
> What is wrong?

You are using tcsh, which although by many considered wrong, is your
choice. Remember the special meaning of * in the shell? Apparently
you have no files matching the pattern 2*3, in which case tcsh prints
this message. The solution is to quote the argument, "2*3". I'm
pretty sure this is covered in the tcsh man page, but I don't have it
installed so I can't check.

You should really get used to reading man pages. It's often much
quicker than posting a question here and waiting for an answer.

--
Måns Rullgård
mru@mru.ath.cx

Nils O. Selåsdal

2004-09-30, 8:59 pm

On Wed, 29 Sep 2004 12:57:21 -0700, learning_C++ wrote:

> I have two programs: one for client and one for server.
> In the client.c,
> -------------------------------------------------
> cin2.sin_family = AF_INET;
> inet_aton("130.233.248.203",&(cin2.sin_addr));
> cin2.sin_port = htons(PORT);
> a1=inet_ntoa(cin2.sin_addr);
> printf("The IP value is: %s\n", a1);
> //printf("Trying to connect to %s = %s =
> %s\n",HOST,inet_ntoa(cin.sin_addr),inet_ntoa(cin2.sin\
> _addr));
> printf("Trying to connect to %s = %s =
> %s\n",HOST,inet_ntoa(cin.sin_addr),a1);
> ----
> a1=130.233.248.203. But after the function printf()is called it
> changed to a1=130.233.248.198.

inet_ntoa is not reentrant.
consider;
a1=inet_ntoa(cin2.sin_addr);
Now a1 points to some internal buffer the inet_ntoa knows about.
later ...
a2=inet_ntoa(cin.sin_addr);
Now the internal buffer is overwritten by this new value
That means a1 still points to the same buffer, which is now changed.

Check if you have the inet_ntoa_r function which allows you to provide
a buffer.
Or copy the string returned from inet_ntoa so subsequent calls doesn't
over write it.

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