c++系统()挂起，使用netcat连接到不同线程中的套接字

Question

我在一个单独的线程上启动一个简单的套接字服务器，并尝试在主线程上使用netcat连接到它。这是代码：

#include <cstdlib>
#include <sys/socket.h>
#include <unistd.h>
#include <netinet/in.h>
#include <iostream>
#include <string>
#include <cstring>
using namespace std;
void* callback(void*);
bool isServerReady = false;
int main()
{
    pthread_t thread;
    int result = pthread_create(&thread, 0, callback, 0);
    if (result != 0) {
        cout << "[ERROR] Unable to create thread" << endl;
    }
    while (!isServerReady) {
    }
    int returnValue = system("echo TEST | netcat localhost 1234");
    cout << "Return value: " << returnValue << endl;
    return 0;
}
void* callback(void* threadId)
{
    int serverSocketId = socket(AF_INET, SOCK_STREAM, 0);
    if (serverSocketId == -1) {
        cout << "[SERVER ERROR] Unable to create server socket." << endl;
        return (void*)-1;
    }
    int serverPort = 1234;
    struct sockaddr_in serverAddress;
    serverAddress.sin_family = AF_INET;
    serverAddress.sin_port = htons(serverPort);
    serverAddress.sin_addr.s_addr = INADDR_ANY;
    int bindReturn = bind(serverSocketId, (struct sockaddr*)&serverAddress, sizeof(serverAddress));
    if (bindReturn == -1) {
        cout << "[SERVER ERROR] Unable to bind server socket." << endl;
        return (void*) -1;
    }
    isServerReady = true;
    int waitQueueSize = 5;
    listen(serverSocketId, waitQueueSize);
    struct sockaddr_in clientAddress;
    socklen_t clientAddressSize = sizeof(clientAddress);
    int clientSocketId = accept(serverSocketId, (struct sockaddr*)&clientAddress, &clientAddressSize);
    if (clientSocketId == -1) {
        cout << "[SERVER ERROR] Unable to create client socket." << endl;
        return (void*)-1;
    }
    char clientBuffer[256];
    bzero(clientBuffer, 256);
    int charsRead = read(clientSocketId, clientBuffer, 255);
    if (charsRead == -1) {
        cout << "[SERVER ERROR] Unable to read client socket." << endl;
        return (void*)-1;
    }
    string serverMessage = "You wrote: ";
    serverMessage.append(clientBuffer);
    int charsWritten = write(clientSocketId, (char*)serverMessage.c_str(), (int)serverMessage.length());
    if (charsWritten == -1) {
        cout << "[SERVER ERRROR] Error writing to client socket." << endl;
        return (void*)-1;
    }
    int closeReturn = close(serverSocketId);
    if (closeReturn == -1) {
        cout << "[SERVER ERROR] Error closing server socket." << endl;
        return (void*)-1;
    }
    return (void*)0;
}

然而，system函数永远不会返回，使程序挂起：服务器的响应"You wrote: TEST"正确地打印在控制台上，但之后程序就挂起了，没有到达打印返回值的行。如果我将相同的服务器代码提取到另一个程序中，启动它，并从命令行使用相同的netcat调用，我会得到相同的响应消息，然后netcat退出，而不需要我做任何事情。

这是编译命令：

g++ -std=c++11 test.cpp -o test -lpthread && ./test

和编译器版本：

g++ (Ubuntu 4.8.4-2ubuntu1~14.04.1) 4.8.4

为什么您认为netcat在从system()内部调用时没有返回？

Answer 1

多亏了@kfsone，问题只是我的服务器从未关闭客户端套接字，从而阻止客户端netcat意识到服务器不再侦听，并使其无限期地等待。要解决此问题：

// ...
string serverMessage = "You wrote: ";
serverMessage.append(clientBuffer);
int charsWritten = write(clientSocketId, (char*)serverMessage.c_str(), (int)serverMessage.length());
if (charsWritten == -1) {
    cout << "[SERVER ERRROR] Error writing to client socket." << endl;
    return (void*)-1;
}
// ADDED THIS:
int closeClientReturn = close(clientSocketId);
if (closeClientReturn == -1) {
    cout << "[SERVER ERROR] Error closing client socket." << endl;
    return (void*)-1;
}
int closeReturn = close(serverSocketId);
if (closeReturn == -1) {
    cout << "[SERVER ERROR] Error closing server socket." << endl;
    return (void*)-1;
}
return (void*)0;