C套接字編程：recv總是失敗

Question

我想了解用於大學考試的C套接字編程。 我做這個簡單的例子：

socket_server.c

#include <sys/socket.h> 
#include <stdio.h> 
#include <string.h> 
#include <unistd.h> 
#include <sys/types.h> 
#include <arpa/inet.h> 

#define SA struct sockaddr 
#define MAX_MESS_LEN 16 

int main(){ 
    struct sockaddr_in my_addr, cl_addr; 
    int ret, len, sk, cn_sk; 
    char cl_ip[INET_ADDRSTRLEN + 1], msg[MAX_MESS_LEN + 1]; 

    sk = socket(AF_INET, SOCK_STREAM, 0); //creo socket TCP/IPv4 listener 
    printf("socket created! :)\n"); 
    memset(&my_addr, 0, sizeof(my_addr)); //azzero struttura my_addr 
    my_addr.sin_family = AF_INET; //famiglia protocolli IPv4 
    my_addr.sin_addr.s_addr = htonl(INADDR_ANY); //in ascolto su qualsiasi interfaccia 
    my_addr.sin_port = htons(1234); //porta del socket 

    ret = bind(sk, (SA *) &my_addr, sizeof(my_addr)); //lego il socket alla struttura indirizzo 
    printf("socket binded! :)\n"); 
    ret = listen(sk, 10); //mi pongo in ascolto sul socket con massimo 10 richieste pendenti 
    printf("socket listening (on any interface with port 1234)! :)\n"); 
    len = sizeof(cl_addr); 
    printf("socket is going to wait for a request... zzzzZZZ\n"); 
    cn_sk = accept(sk, (SA *) &cl_addr, &len); //creo il socket connected per la richiesta in cima alla lista 
    printf("socket accepted request! :D\n"); 
    inet_ntop(AF_INET, &cl_addr.sin_addr, cl_ip, INET_ADDRSTRLEN); //converto indirizzo IPv4 cl_addr.sin_addr in stringa (cl_ip) 
    printf("IP client: %s\n", cl_ip); 

    ret = recv(sk, (void *) msg, MAX_MESS_LEN, MSG_WAITALL); //attendo la ricezione di tutti i caratteri della stringa inviata sul socket dal client 
    msg[MAX_MESS_LEN + 1] = '\0'; 
    printf("received data (%d bytes): %s\n", ret, msg); 
    if(ret == -1 || ret < MAX_MESS_LEN) 
     printf("recv ERROR! :(\n"); 

    close(sk); //chiudo il server 
} 

socket_client.c

#include <sys/socket.h> 
#include <stdio.h> 
#include <string.h> 
#include <arpa/inet.h> 
#define SA struct sockaddr 
#define MSG_LEN 17 

int main(){ 
    struct sockaddr_in srv_addr; 
    int ret, sk; 
    char msg[MSG_LEN]; 

    sk = socket(AF_INET, SOCK_STREAM, 0); 
    printf("Socket created! :)\n"); 
    memset(&srv_addr, 0, sizeof(srv_addr)); 
    srv_addr.sin_family = AF_INET; 
    srv_addr.sin_port = htons(1234); 
    ret = inet_pton(AF_INET, "192.168.1.132", &srv_addr.sin_addr); 

    printf("Trying to establish a connection with 192.168.1.132 on port 1234..."); 
    ret = connect(sk, (SA *) &srv_addr, sizeof(srv_addr)); //faccio una richiesta sul socket 
    if(ret != -1) printf("Connection established! :D\n"); 
    else printf(" connection error :(\n"); 

    strcpy(msg, "something to send"); //scrivo messaggio in una stringa 
    printf("sending message: %s\n", msg); 
    ret = send(sk, (void *) msg, strlen(msg), 0); //invio il messaggio sul socket 
    if(ret == -1 || ret < strlen(msg)) 
     printf("send ERROR! :(\n"); 

    close(sk); 
} 

並編譯它：

gcc socket_client.c -o socket_client 
gcc socket_server.c -o socket_server 

也許這是一個愚蠢的問題，但服務器中的recv函數總是失敗，這裏有一個輸出示例：

服務器：

socket created! :) 
socket binded! :) 
socket listening (on any interface with port 1234)! :) 
socket is going to wait for a request... zzzzZZZ 
socket accepted request! :D 
IP client: 192.168.1.132 
received data (-1 bytes): ���� 
recv ERROR! :(

客戶：

Socket created! :) 
Trying to establish a connection with 192.168.1.132 on port 1234...Connection established! :D 
sending message: something to send 

什麼問題？我不明白！ :(我跟着一步一步走，我的老師的指導。

我在arch linux 64位運行;） 謝謝你的迴應！

Answer 1

cn_sk = accept(sk, (SA *) &cl_addr, &len); //creo il socket connected per la richiesta in cima alla lista 
... 
ret = recv(sk, (void *) msg, MAX_MESS_LEN, MSG_WAITALL); //at 

你所連接的插座cn_sk在客戶端上不recv，而是在監聽套接字sk。如果你想檢查errno，你可能會看到ENOTCONN。