TCP服务器未绑定到特定端口

Question

The problem is that whatever the port I set, the server connects to a different one. For example, in my code I set the port 20000 (but I also tried other numbers), but when I print the port number it's completely different (now it's showing 8270) is this line correct? indirizzo_serv.sin_port=htons(20000);

another issue is that server doesn't accept multiple commands, I mean when I send "time" and get the answer, then I can't send any other command. server code:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<unistd.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<time.h> //per data e ora
#include<locale.h> //per la traduzione di giorno e mese


int main(void)
{
char buf[100]={0},tmp[10]={0};
time_t t; //tempo "grezzo" (rawtime)
struct tm *info; //struttra che contiene le informazioni su data e ora
struct sockaddr_in indirizzo_serv;
int fd1,fd2;
socklen_t len;

setlocale(LC_ALL,"it_IT.utf8"); //per stampare giorno e mese in italiano, ricavato dando da terminale locale -a

indirizzo_serv.sin_family=AF_INET;
indirizzo_serv.sin_port=htons(20000);
indirizzo_serv.sin_addr.s_addr=htonl(INADDR_ANY);
fd1=socket(PF_INET,SOCK_STREAM,0); //socket tcp di rete
bind(fd1,(struct sockaddr*)&indirizzo_serv,sizeof(indirizzo_serv));
listen(fd1,5);

sprintf(tmp,"Numero porta= %d\n",indirizzo_serv.sin_port);
write(STDOUT_FILENO,tmp,strlen(tmp));
bzero(tmp,10);

fd2=accept(fd1,NULL,NULL);

sleep(2);

while(1){
    read(fd2,buf,100);
    if((strcmp(buf,"TIME\n"))==0){
        time(&t);
        info=localtime(&t);
        strftime(buf,100,"Sono le ore %H.%M%n",info);
        write(fd2,buf,strlen(buf));
        bzero(buf,100);
    }
    else if((strcmp(buf,"DATE\n"))==0){
        time(&t);
        info=localtime(&t);
        strftime(buf,100,"Oggi è %A, %e %B %G %n",info);
        write(fd2,buf,strlen(buf));
        bzero(buf,100);
    }
    else if((strcmp(buf,"PORT\n"))==0){
        indirizzo_serv.sin_port=htons(0); //porta casuale
        len=sizeof(indirizzo_serv);
        getsockname(fd1,(struct sockaddr*)&indirizzo_serv,&len);
        sprintf(tmp,"Numero porta= %d\n",indirizzo_serv.sin_port);
        write(STDOUT_FILENO,tmp,strlen(tmp));
        write(fd2,tmp,strlen(tmp)); //invio la porta al client sotto forma di stringa
        bzero(tmp,10);
    }
}

close(fd1);
return 0;
}

client code:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<unistd.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<ctype.h> //per toupper

int main(void)
{
struct sockaddr_in indirizzo;
int fd,i=0,nport;
char buf[100]={0};

indirizzo.sin_family=AF_INET;
inet_aton("127.0.0.1",&indirizzo.sin_addr);
indirizzo.sin_port=htons(10000);

fd=socket(PF_INET,SOCK_STREAM,0);

connect(fd,(struct sockaddr*)&indirizzo,sizeof(indirizzo));

while(1){
    write(STDOUT_FILENO,"Scrivi il comando da inviare al server tra i seguenti: TIME, DATE, PORT.\n",strlen("Scrivi il comando da inviare al server tra i seguenti: TIME, DATE, PORT.\n"));
    read(STDIN_FILENO,buf,sizeof(buf));
    while(buf[i]){
        buf[i]=toupper(buf[i]);
        i++;
    }
    if((strcmp(buf,"PORT\n"))==0){
            bzero(buf,100);
            read(fd,buf,sizeof(buf));
            nport=atoi(buf);
            indirizzo.sin_port=htons(nport);
            connect(fd,(struct sockaddr*)&indirizzo,sizeof(indirizzo));

    }
    write(fd,buf,strlen(buf)); //incio comando al server
    bzero(buf,100);
    read(fd,buf,100); //leggo il messaggio ricevuto dal server
    write(STDOUT_FILENO,buf,strlen(buf)); //stampo a video il messaggio
    bzero(buf,100);
}

close(fd);
return 0;

}

Answer 1

With

sprintf(tmp,"Numero porta= %d\n",indirizzo_serv.sin_port);

you create a string containing the network byte order value of the port. You need to get the host byte order using ntohs :

sprintf(tmp,"Numero porta= %d\n",ntohs(indirizzo_serv.sin_port));

Network byte order is big endian while a modern x86 (or x86_64) based PC is little endian . This is the most likely reason of your problem.

The htons convert a short value from host byte order to network byte order, and the ntohs does the opposite.

Answer 2

This is because you print the port number that is in Network format. Your machine is certainly little-endian which means that bytes to represent numbers are in the opposite of the network ordering.

Since you specified 20000 , in Network ordering this is represented as 0x4E20 (hexadecimal basis) but your machine reads it from the right to the left so reads 0x204E which is 8270 .

Then use ntohs() to convert it back.

Answer 3

可能不严格涉及这种情况，但是在服务器代码中，您声明了tmp [10]，随后又将sprintf声明为10多个字节，还覆盖了* info和indirizzo_serv。