udp回射客户端

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

const int maxline=4096;

char *sock_ntop(const struct sockaddr *sa, socklen_t salen) {
	char portstr[8];
	static char str[128];
	
	switch(sa->sa_family) {
	case AF_INET: {
		struct sockaddr_in * sin = (struct sockaddr_in *)sa;
		if(inet_ntop(AF_INET, &sin->sin_addr, str, sizeof(str)) == NULL) {
			return NULL;
		}
		if(ntohs(sin->sin_port) != 0) {
			snprintf(portstr, sizeof(portstr), ":%d", ntohs(sin->sin_port));
			strcat(str, portstr);
		}
		return str;
	}
	}
}

void dg_cli(FILE *fp, int sockfd, const struct sockaddr * pservaddr, socklen_t servlen) {
	int n;
	char sendline[maxline], recvline[maxline+1];
	char errbuff[maxline];
	socklen_t len;
	struct sockaddr *preply_addr;

	if((preply_addr=malloc(servlen)) == NULL) {
		strerror_r(errno, errbuff, maxline);
		fprintf(stderr, "malloc error: %s\n", errbuff);
		exit(-1);
	}

	while(fgets(sendline, maxline, fp) != NULL) {
		if(sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen)<0) {
			strerror_r(errno, errbuff, maxline);
			fprintf(stderr, "sendto error: %s\n", errbuff);
			exit(-1);
		}

		len=servlen;
		if((n=recvfrom(sockfd, recvline, maxline, 0, preply_addr, &len))<0) {
			strerror_r(errno, errbuff, maxline);
			fprintf(stderr, "recvfrom error: %s\n", errbuff);
			exit(-1);
		}
		printf("reply from %s\n", sock_ntop(preply_addr, len));

		if(len != servlen || memcmp(pservaddr, preply_addr, len) != 0) {
			printf("reply from %s (ignored)\n", sock_ntop(preply_addr, len));
			continue;
		}
		recvline[n]=0;
		fputs(recvline, stdout);
	}
}

int main(int argc, char **argv) {
	int sockfd;
	struct sockaddr_in servaddr;
	char errbuff[maxline];

	if(argc != 2) {
		fprintf(stderr, "usage: udpcli <IPaddress>\n");
		exit(-1);
	}

	if((sockfd=socket(AF_INET, SOCK_DGRAM, 0))<0) {
		strerror_r(errno, errbuff, maxline);
		fprintf(stderr, "socket error: %s\n", errbuff);
		exit(-1);
	}
	
	memset(&servaddr, 0, sizeof(servaddr));
	servaddr.sin_family = AF_INET;
	servaddr.sin_port = htons(9999);
	inet_pton(AF_INET, argv[1], &servaddr.sin_addr);

	dg_cli(stdin, sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr));
	
	return 0;
}