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;
}   