String to 2 bytes

[tomchesley]

I am writing a modified udp server, and i almost have it working the way I want, I am only missing the ability to take a string like "8367" and have 2 bytes 32 175 returned
the string is the port # I obtained from the udp server if that helps.
Here is what I have at the moment

#include <stdio.h>
#include <errno.h>
#include <sys/socket.h>
#include <resolv.h>

#define DEFAULT_PORT	1235

int main(int count, char *strings[])
{	int sd;
	int fport;
	int i;
	int port=DEFAULT_PORT;
	struct sockaddr_in addr;
	char buffer[1024];
	char sbuffer[1024];
	char o1[5], o2[5], o3[5], o4[5];  //set string rep of each octet
	int io1, io2, io3, io4;		//set int value for each octet

	if ( count != 2 )
		printf("usage: %s <port>\n...Using default port (%d).\n", strings[0], port);
	else
		port = atoi(strings[1]);
	sd = socket(PF_INET, SOCK_DGRAM, 0);
	bzero(&addr, sizeof(addr));
	addr.sin_family = AF_INET;
	addr.sin_port = htons(port);
	addr.sin_addr.s_addr = INADDR_ANY;
	if ( bind(sd, (struct sockaddr*)&addr, sizeof(addr)) != 0 )
		perror("bind");
	while (1)
	{	int bytes, addr_len=sizeof(addr);

		for (i = 0; i < 12; i++)
		{
			sbuffer[i] = 0;
		}

		bytes = recvfrom(sd, buffer, sizeof(buffer), 0, (struct sockaddr*)&addr, &addr_len);
		printf("msg from %s:%d (%d bytes)\n", inet_ntoa(addr.sin_addr),
						ntohs(addr.sin_port), bytes);

		sscanf(inet_ntoa(addr.sin_addr), "%4[^.].%4[^.].%4[^.].%4[^.]", o1, o2, o3, o4);

		fport = ntohs(addr.sin_port);

		//int of each octet
		io1 = atoi(o1);
		io2 = atoi(o2);
		io3 = atoi(o3);
		io4 = atoi(o4);

		printf("From IP:%s %s %s %s: %d\n", o1, o2, o3, o4, fport);

		//this is what we received in hex
		printf("Received:\n");
		printf("%2x %2x %2x %2x %2x %2x %2x %2x %2x %2x %2x %2x\n",
			buffer[0], buffer[1], buffer[2], buffer[3], buffer[4], buffer[5], 
			buffer[6], buffer[7], buffer[8], buffer[9], buffer[10], buffer[11]);
		//set send buffer to stuff I want
		for (i = 4; i < 12; i++)
		{
			sbuffer[i-4]=buffer[i];
		}
		//this is the ip
		sbuffer[8] = io1;
		sbuffer[9] = io2;
		sbuffer[10] = io3;
		sbuffer[11] = io4;

                //port ???
		sbuffer[12] = fport;
	        sbuffer[13] = fport;

		printf("Sending:\n");
		printf("%2x %2x %2x %2x %2x %2x %2x %2x %2x %2x %2x %2x %2x %2x\n",
			sbuffer[0], sbuffer[1], sbuffer[2], sbuffer[3], sbuffer[4], sbuffer[5], 
			sbuffer[6], sbuffer[7], sbuffer[8], sbuffer[9], sbuffer[10], sbuffer[11],
			sbuffer[12], sbuffer[13]);
		sendto(sd, sbuffer, bytes, 0, (struct sockaddr*)&addr, sizeof(addr));
	}
	close(sd);
}

 

[cpjust]

If you need to convert the string "8367" into the unsigned short 8367, you've almost got it already:

port = atoi(strings[1]);

just change the datatype of port from int to unsigned short, then cast the result to unsigned short:

port = (unsigned short)atoi(strings[1]);

Although you should add some error handling to check if atoi() failed to convert the string and if the number is larger than USHRT_MAX...

 

[tomchesley]

I'm not quite sure how I get 2 bytes back with that.
I have to send back the bytes to the client. Will the above give me "8367" = 32 for the upper byte and 175 for the lower byte?

 

[cpjust]

Yes. A short = 2 bytes (usually) and an int = 4 bytes (usually).

 

[tomchesley]

Ok, thanx, now I got it.

That worked, but what I did instead was take the original int value of fport and did my magic.
how I did it was

int uport; //upper order byte - will be no decimal value
int lport; //lower order byte

uport = fport /256; //set up the upper byte
lport = fport - (uport * 256); //set up the lower byte
//now i have the bytes separated to send back

 

[cpjust]

This should work too:

unsigned short port = 8367;
char portBytes[2] = (char*)port;

or

unsigned short port = 8367;
char LowByte = port | 0xFF;
char HighByte = (port >> 8) | 0xFF;

a union would work too.