Thread: socket and time

Hi,

I'm working on a small school assignment where I should make my program on a Solaris that connects to a server with TCP and gets a 32-bit integer that contains the time which should then be displayed on the command line.

Now, I don't know much unix and although I've done some programming in Java this is my first program in C.

I think I've connected to the server and downloaded the 4 bytes, but now I don't know how to make them display correctly.

Here are my questions:

1) I wish to see the value of the 4 bytes I downloaded. I've made an attempt in the code. But it doesn't look like any of the examples I've got; 2208988800 should correspond to 00:00 1 jan 1970 GMT and -1297728000 should correspond to 00:00 17 nov 1858 GMT. I get -63.

2) I have no idea about the timeformat of these 4 bytes. I think the time format is standardized but I don't know the specification or where to look for it, or how to convert it, so I'd like to know where I should look for this.

3) Why doesn't my casting work on the last line of code? I'd like to print it as an unsigned int, but still get the signed -63.

Seron

Below is my code

Code:

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

#define SERVER_PORT 37

int main() {
    
    char *time;
    int sock, byte_count;
    struct sockaddr_in sin;
    struct hostent *hp;
    char *host;

    host = "roman.ludat.lth.se";
    hp = gethostbyname(host);
    if(!hp) {
	printf("unknown host: %s\n", host);
	exit(-1);
    }

    bzero((char *)&sin, sizeof(sin));
    sin.sin_family = AF_INET;
    bcopy(hp->h_addr, (char *)&sin.sin_addr, hp->h_length);
    sin.sin_port = htons(SERVER_PORT);

    if((sock = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
	perror("socket");
	exit(-1);
    }

    if(connect(sock, (struct sockaddr *)&sin, sizeof(sin)) < 0) {
	perror("connect");
	exit(-1);
    }
    
    byte_count = recv(sock, time, 4, 0);
    printf("*time = %d\n", (unsigned int)*time);
}

Thanks for the quick replies.

I've got the right format now for the time, but it is not showing the correct time, unless that is what the server sends me.

more questions:

1) How could I check that what I have is the right sequence of bits? I don't know what type of machine is at the other end.

2) Am I still using pointers incorrectly?

3) If I put unsigned long int time2 = 0; lower down in the code, for example where the /**/ is the compiler (gcc) complains with a parse error. Why?

Seron

Code:

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

#define SERVER_PORT 37

int main() {
    
    time_t time = 0;
    int sock;
    struct sockaddr_in sin;
    struct hostent *hp;
    char *host = "roman";
    char time_string[27];
    char *tsp = time_string;
    unsigned long int time2 = 0;

    time_string[26] = '\0';

    // for debugging
    printf("time_string = %d\n", time_string);
    printf("time = %d\n", time);
    printf("host = %d\n", host);

    hp = gethostbyname(host);
    if(!hp) {
	printf("unknown host: %s\n", host);
	exit(-1);
    }

    bzero((char *)&sin, sizeof(sin));
    sin.sin_family = AF_INET;
    bcopy(hp->h_addr, (char *)&sin.sin_addr, hp->h_length);
    sin.sin_port = htons(SERVER_PORT);

    if((sock = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
	perror("socket");
	exit(-1);
    }

    if(connect(sock, (struct sockaddr *)&sin, sizeof(sin)) < 0) {
	perror("connect");
	exit(-1);
    }
    
    recv(sock, &time, 4, 0); // put some errorchecking here
    tsp = ctime(&time);

/**/

    // for debugging
    printf("time = %d\n", time);
    memmove( &time2, &time, sizeof(time2) );
    printf("time = %u\n", time2 );

    // prints the time
    printf("%s\n", tsp);
    tsp = ctime(&time2);
    printf("%s\n", tsp);
}

output:

Code:

time_string = -4261488
time = 0
host = 68248
time = -1049019304
time = 3245947992
Sun Oct  4 14:44:56 1936

Sun Oct  4 14:44:56 1936

Thanks for your help. I still get a strange date and time. This part of the code now looks like this:

Code:

recv(sock, &time, 4, 0);
time = ntohl(time);
tsp = ctime(&time);

printf("%s\n", tsp);

I'm going to sleep over it now.

Seron