udp server in C

udpserv.c

/* 
 * udpserver.c - A simple UDP echo server 
 * usage: udpserver <port>
 */

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

#define BUFSIZE 1024

/*
 * error - wrapper for perror
 */
void error(char *msg)
{
	perror(msg);
	exit(1);
}

int main(int argc, char **argv)
{
	int sockfd;		/* socket */
	int portno;		/* port to listen on */
	int clientlen;		/* byte size of client's address */
	struct sockaddr_in serveraddr;	/* server's addr */
	struct sockaddr_in clientaddr;	/* client addr */
	struct hostent *hostp;	/* client host info */
	char *buf;		/* message buf */
	char *hostaddrp;	/* dotted decimal host addr string */
	int optval;		/* flag value for setsockopt */
	int n;			/* message byte size */

	/* 
	 * check command line arguments 
	 */
	if (argc != 2) {
		fprintf(stderr, "usage: %s <port>\n", argv[0]);
		exit(1);
	}
	portno = atoi(argv[1]);

	/* 
	 * socket: create the parent socket 
	 */
	sockfd = socket(AF_INET, SOCK_DGRAM, 0);
	if (sockfd < 0)
		error("ERROR opening socket");

	/* setsockopt: Handy debugging trick that lets 
	 * us rerun the server immediately after we kill it; 
	 * otherwise we have to wait about 20 secs. 
	 * Eliminates "ERROR on binding: Address already in use" error. 
	 */
	optval = 1;
	setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR,
		   (const void *)&optval, sizeof(int));

	/*
	 * build the server's Internet address
	 */
	bzero((char *)&serveraddr, sizeof(serveraddr));
	serveraddr.sin_family = AF_INET;
	serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
	serveraddr.sin_port = htons((unsigned short)portno);

	/* 
	 * bind: associate the parent socket with a port 
	 */
	if (bind(sockfd, (struct sockaddr *)&serveraddr,
		 sizeof(serveraddr)) < 0)
		error("ERROR on binding");

	/* 
	 * main loop: wait for a datagram, then echo it
	 */
	clientlen = sizeof(clientaddr);
	while (1) {

		/*
		 * recvfrom: receive a UDP datagram from a client
		 */
		buf = malloc(BUFSIZE);
		n = recvfrom(sockfd, buf, BUFSIZE, 0,
			     (struct sockaddr *)&clientaddr, &clientlen);
		if (n < 0)
			error("ERROR in recvfrom");

		/* 
		 * gethostbyaddr: determine who sent the datagram
		 */
		hostp = gethostbyaddr((const char *)&clientaddr.sin_addr.s_addr,
				      sizeof(clientaddr.sin_addr.s_addr),
				      AF_INET);
		if (hostp == NULL)
			error("ERROR on gethostbyaddr");
		hostaddrp = inet_ntoa(clientaddr.sin_addr);
		if (hostaddrp == NULL)
			error("ERROR on inet_ntoa\n");
		/*printf("server received %d bytes\n", n); */

		/* 
		 * sendto: echo the input back to the client 
		 */
		n = sendto(sockfd, buf, n, 0,
			   (struct sockaddr *)&clientaddr, clientlen);
		if (n < 0)
			error("ERROR in sendto");
	}
}

Do we really need to malloc() the buffer at every loop cycle?

Probably it is a mistake. I am pretty sure you can malloc() before entering the loop. And then, inside the loop, you should do a memset() to reset the buffer every cycle.

sure it is a mistake, and a serious one as memory allocated is never freed (and could not be as previous buf pointer value is lost/overwritten each time).
also there is no need to memset() as data length is supplied on input.

I'm having trouble with recvfrom, error 15, which is non blocking errror

More over... There is no need to declare int to be casted later into uint.... And call atoul() to get unsigned value instead of atoi()...
...
int portno;
...
portno = atoi(argv[1]);
...
serveraddr.sin_port = htons((unsigned short)portno);
...

Thanks a lot for sharing. HOWEVER, it does not show 802.1q tags similar to another program that I got from here.
I can capture the complete ethernet frame including the VLAN tags using tcpdump. For some reason, these programs drop/exclude 802.1q tags.
Does anybody know the reason?