Linux网络编程：recv的返回值

recv是网络编程中用于TCP传输接收数据的接口，由于工作中一直没有接触到网络编程，虽然知道这个函数，但是没有使用经验，所有对接口细节不是很了解。

直到前阵子在项目中调试第三方的源码时，跟踪到这个函数，才发现它的返回值和我想象的不太一样。

recv函数的原型：

ssize_t recv(int sockfd, void* buf, size_t len, int flags);

之前知道的是如果读取成功，会返回实际读取到的字节数，但是如果读取不到数据呢？一直想当然的认为会返回0，其实并不是，当读取不到数据的时候，返回的是-1，错误码是11，对应的含义是：Resource temporarily unavailable，这里的资源自然是指要读取的数据，因为没有数据可以读取，所以资源不可用。

那么返回值为0表示什么意思？答案是当对端关闭了socket的时候（是真正地关闭，会引发四次握手，而不是减少引用），我们再去调用recv读取数据，会返回0。

下面是测试代码，连接建立后，服务端会网socket发送数据，每次发送后sleep5秒，当发送四次数据后，关闭socket。为了测试在没有数据可读的情况下调用recv的返回值，我们在客户端把socket设置为非阻塞模式，当服务端在sleep时，socket没有数据可读，调用recv后会返回-1，打印出来的错误码是11，当服务端关闭了socket后，再次调用recv则返回0。

服务端代码：

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

using namespace std;

#define MAX_OF_LISTEN_QUEUE 10
#define MAXBUF 1024

int main(){
    int sockFd;
    int destSockFd;
    struct sockaddr_in source;
    struct sockaddr_in dest;
    char buff[MAXBUF+1];

    memset(&source, 0, sizeof(source));
    source.sin_family = AF_INET;
    source.sin_port = htons(8235);
    source.sin_addr.s_addr = inet_addr("192.168.1.109");

    sockFd = socket(AF_INET, SOCK_STREAM, 0);
    if(sockFd < 0){
        cout << "created server socket error!" << endl; 
    }

    if(bind(sockFd, (struct sockaddr*)&source, sizeof(source)) != 0){
        cout << "server bind error!" << endl;   
    }

    if(listen(sockFd, MAX_OF_LISTEN_QUEUE) != 0){
        cout << "server listen error!" << endl;
    }

    socklen_t len = sizeof(dest);   
    while(1){
        destSockFd = accept(sockFd, (struct sockaddr*)&dest, &len);
        if(destSockFd == -1){
            cout << "server accept error!" << endl;
        }
        else{
            cout << "server accept a connection!" << endl;
            break;  
        }
    }

    int hasSend;
    strcpy(buff, "This message from server!");
    int count = 0;
    while(1){
        count++;
        if(count > 4)
            break;
        sleep(5);
        cout << "server sending message!" << endl;
        hasSend = send(destSockFd, buff, strlen(buff), 0); 
        if(hasSend == -1){
            cout << "server send message error!" << endl;       
        }
        if(hasSend == strlen(buff)){
            cout << "server send OK!" << endl;      
        }
        else{
            cout << "server only send " << hasSend << "bytes!" << endl;     
        }
    }   

    close(destSockFd);  
    cout << "server will exit!" << endl;    
}

客户端代码：

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

using namespace std;

#define MAXSLEEP 128
#define MAXBUF 1024

int main(){
    int sockFd;
    struct sockaddr_in dest;
    struct sockaddr_in source;
    char buff[MAXBUF+1];

    sockFd = socket(AF_INET, SOCK_STREAM, 0);
    if(sockFd < 0){
        cout << "created client socket error!" << endl; 
    }

    memset(&dest, 0, sizeof(dest));
    dest.sin_family = AF_INET;
    dest.sin_port = htons(8235);
    dest.sin_addr.s_addr = inet_addr("192.168.1.109");

    memset(&source, 0, sizeof(source));
    source.sin_family = AF_INET;
    source.sin_port = htons(8234);
    source.sin_addr.s_addr = inet_addr("192.168.1.109");

    if(bind(sockFd, (struct sockaddr*)&source, sizeof(source)) != 0){
        cout << "client bind error!" << endl;   
    }

    //use exponential backoff
    int nsec;
    for(nsec = 1; nsec <= MAXSLEEP; nsec <<= 1){
        if(connect(sockFd, (struct sockaddr*)&dest, sizeof(dest)) == 0){
            cout << "connect to server successfully, will break" << endl;
            break;  
        }
        if(nsec <= MAXSLEEP/2){
            cout << "connect to server error, will sleep " << nsec << "seconds!" << endl;
            sleep(nsec);        
        }
    }

    int flag = fcntl(sockFd, F_GETFL, 0);
    flag |= O_NONBLOCK;
    fcntl(sockFd, F_SETFL, flag);

    int recvLen;

    //setsockopt(sockFd, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout));
    while(1){
        cout << "client recving message!" << endl;
        recvLen = recv(sockFd, buff, MAXBUF, 0);
        if(recvLen == -1){
            cout << "client recv message error!" << endl;
            if(errno == 11)     
                cout << "no data to read!" << endl;
        }
        else if(recvLen == 0){
            cout << "server has close!" << endl;
            break;  
        }
        else{
            buff[25] = '\0';
            cout << "client recv " << recvLen << " bytes, message is:" << buff << endl;     
        }
        sleep(2);
    }

    close(sockFd);
    cout << "client will exit!" << endl;
}