C中的套接字；无限循环

Question

因此，我试图编写一个程序，它可以同时接受客户机和服务器上的输入。现在，当我在while(1)循环之外移动这些行时，它是无限循环的，就像我被告知的那样：

sin_size = sizeof their_addr;
        new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);

我被告知其无限循环的原因是因为新套接字正在创建，现在正在侦听，而旧套接字仍然打开并侦听。我试过在客户端和服务器上尝试关闭旧套接字，但是没有什么效果。

清除while循环显然有效，但是套接字在接收到输入后关闭。这样做的目的基本上是制作一个聊天程序。它不需要同时打开服务器和客户端(对于一个A+它是lol的)，但是只要它可以来回发送和接收输入。

我已经包括了客户端和服务器的代码。未来谢谢！

server.c:

    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <errno.h>
    #include <string.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <netdb.h>
    #include <arpa/inet.h>
    #include <sys/wait.h>
    #include <signal.h>

#define PORT "3490"  // the port users will be connecting to

#define BACKLOG 10   // how many pending connections queue will hold

void sigchld_handler(int s)
{
    while(waitpid(-1, NULL, WNOHANG) > 0);
}

// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
    if (sa->sa_family == AF_INET) {
        return &(((struct sockaddr_in*)sa)->sin_addr);
    }

    return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(void)
{
    int sockfd, new_fd;  // listen on sock_fd, new connection on new_fd
    struct addrinfo hints, *servinfo, *p;
    struct sockaddr_storage their_addr; // connector's address information
    socklen_t sin_size;
    struct sigaction sa;
    int yes=1;
    char s[INET6_ADDRSTRLEN];
    int rv;

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE; // use my IP

    if ((rv = getaddrinfo(NULL, PORT, &hints, &servinfo)) != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 1;
    }

    // loop through all the results and bind to the first we can
    for(p = servinfo; p != NULL; p = p->ai_next) {
        if ((sockfd = socket(p->ai_family, p->ai_socktype,
                p->ai_protocol)) == -1) {
            perror("server: socket");
            continue;
        }

        if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes,
                sizeof(int)) == -1) {
            perror("setsockopt");
            exit(1);
        }

        if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
            close(sockfd);
            perror("server: bind");
            continue;
        }

        break;
    }

    if (p == NULL)  {
        fprintf(stderr, "server: failed to bind\n");
        return 2;
    }

    freeaddrinfo(servinfo); // all done with this structure

    if (listen(sockfd, BACKLOG) == -1) {
        perror("listen");
        exit(1);
    }

    sa.sa_handler = sigchld_handler; // reap all dead processes
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    if (sigaction(SIGCHLD, &sa, NULL) == -1) {
        perror("sigaction");
        exit(1);
    }

    printf("server: waiting for connections...\n");

sin_size = sizeof their_addr;
        new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);


    while(1) {  // main accept() loop


        if (new_fd == -1) {
            perror("accept");
            //continue;
        }

        inet_ntop(their_addr.ss_family,
            get_in_addr((struct sockaddr *)&their_addr),
            s, sizeof s);
        printf("server: got connection from %s\n", s);

        if (!fork()) { // this is the child process
            close(sockfd); // child doesn't need the listener
printf("INFIITE LOOP");
//does this because it waiting for another process and it' stays opened and is listnening???
//Maybe look in the client for this
//also said that while loop isn't probably needed and to do 


//probably will need a recv
//input
//send
//recieve
char input[20];
char *pointer;
printf("Type in an input, q to quit: ");
scanf("%s", input);
pointer = input;  //will need to clean this up to be more effcient... later

//if (input[0] == 'q')
//{
//printf("Testing the if");
//close(new_fd);
//need to break out of loop
//}
            if (send(new_fd, pointer, 20, 0) == -1) //need to change the length to the actual length of the input... later.

//int send(int sockfd, const void *msg, int len, int flags);
//sockfd is the socket descriptor you want to send data to (whether it's the one returned by socket() or
//the one you got with accept().) msg is a pointer to the data you want to send, and len is the length of that
//data in bytes. Just set flags to 0. 


//accept needs to be outstide of while loop
//because after accepting, it blocks the socket?
//in the while loop is where all the sending and recieving happens
//after sending, do recieve

                perror("send");
            close(new_fd); 
            exit(0);
        }
        close(new_fd);  // parent doesn't need this
    }


    return 0;
}

Client.c:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>

#include <arpa/inet.h>

#define PORT "3490" // the port client will be connecting to 

#define MAXDATASIZE 100 // max number of bytes we can get at once 

// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
    if (sa->sa_family == AF_INET) {
        return &(((struct sockaddr_in*)sa)->sin_addr);
    }

    return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(int argc, char *argv[])
{
    int sockfd, numbytes;  
    char buf[MAXDATASIZE];
    struct addrinfo hints, *servinfo, *p; 
/*
This is what is in the struct

struct addrinfo {
int ai_flags; // AI_PASSIVE, AI_CANONNAME, etc.
int ai_family; // AF_INET, AF_INET6, AF_UNSPEC
int ai_socktype; // SOCK_STREAM, SOCK_DGRAM
int ai_protocol; // use 0 for "any"
size_t ai_addrlen; // size of ai_addr in bytes
struct sockaddr *ai_addr; // struct sockaddr_in or _in6
char *ai_canonname; // full canonical hostname
struct addrinfo *ai_next; // linked list, next node
};

getaddrinfo() will return a pointer to this
*/
    int rv;
    char s[INET6_ADDRSTRLEN];

    if (argc != 2) {
        fprintf(stderr,"usage: client hostname\n");
        exit(1);
    }

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;

    if ((rv = getaddrinfo(argv[1], PORT, &hints, &servinfo)) != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 1;
    }

    // loop through all the results and connect to the first we can
    for(p = servinfo; p != NULL; p = p->ai_next) {
        if ((sockfd = socket(p->ai_family, p->ai_socktype,
                p->ai_protocol)) == -1) {
            perror("client: socket");
            continue;
        }

        if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
            close(sockfd);
            perror("client: connect");
            continue;
        }

        break;
    }

    if (p == NULL) {
        fprintf(stderr, "client: failed to connect\n");
        return 2;
    }

    inet_ntop(p->ai_family, get_in_addr((struct sockaddr *)p->ai_addr),
            s, sizeof s);
    printf("client: connecting to %s\n", s);

    freeaddrinfo(servinfo); // all done with this structure

    if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {
        perror("recv");
        exit(1);
    }

    buf[numbytes] = '\0';

    printf("client: received '%s'\n",buf);

    close(sockfd); //as soon as client recieves message, it closes the socket. 
//probably needs a while loop in here in order to keep the socket open
//but what are the parameters for the while loop?

    return 0;
}

Answer 1

其中一个问题是服务器。我认为使用getaddrinfo()返回的信息的循环可能会出现问题，但是主处理循环也有问题。

你有：

new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
while(1)
{
    ...

这将只接受第一个连接；除非您的服务器是一次性服务器，否则需要重新接受后续连接。因此，您应该使用：

while ((new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size)) >= 0)
{
    ...process new connection...
    close(new_fd);
}

服务器程序通常不会让用户随时提供响应信息。

这段代码似乎有效。

server.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <sys/wait.h>
#include <signal.h>

#define PORT "3490"

#define BACKLOG 10

void sigchld_handler(int s)
{
    while (waitpid(-1, NULL, WNOHANG) > 0)
        ;
}

void *get_in_addr(struct sockaddr *sa)
{
    if (sa->sa_family == AF_INET)
        return &(((struct sockaddr_in*)sa)->sin_addr);
    return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(void)
{
    int sockfd, new_fd;
    struct addrinfo hints, *servinfo, *p;
    struct sockaddr_storage their_addr;
    socklen_t sin_size;
    struct sigaction sa;
    int yes=1;
    char s[INET6_ADDRSTRLEN];
    int rv;

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;

    if ((rv = getaddrinfo(NULL, PORT, &hints, &servinfo)) != 0)
    {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 1;
    }

    for (p = servinfo; p != NULL; p = p->ai_next)
    {
        if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1)
        {
            perror("server: socket");
            continue;
        }

        if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1)
        {
            perror("setsockopt");
            exit(1);
        }

        if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1)
        {
            close(sockfd);
            perror("server: bind");
            continue;
        }

        break;
    }

    if (p == NULL)
    {
        fprintf(stderr, "server: failed to bind\n");
        return 2;
    }

    freeaddrinfo(servinfo);

    if (listen(sockfd, BACKLOG) == -1)
    {
        perror("listen");
        exit(1);
    }

    sa.sa_handler = sigchld_handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    if (sigaction(SIGCHLD, &sa, NULL) == -1)
    {
        perror("sigaction");
        exit(1);
    }

    printf("server: waiting for connections...\n");

    sin_size = sizeof their_addr;

    while ((new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size)) > 0)
    {
        inet_ntop(their_addr.ss_family,
                get_in_addr((struct sockaddr *)&their_addr),
                s, sizeof s);
        printf("server: got connection from %s\n", s);

        if (fork() == 0)
        {
            close(sockfd);
            char input[] = "This is the response!";
            if (send(new_fd, input, strlen(input), 0) == -1)
                perror("send");
            close(new_fd);
            exit(0);
        }

        close(new_fd);
    }

    return 0;
}

client.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>

#include <arpa/inet.h>

#define PORT "3490"

#define MAXDATASIZE 100

void *get_in_addr(struct sockaddr *sa)
{
    if (sa->sa_family == AF_INET)
    {
        return &(((struct sockaddr_in*)sa)->sin_addr);
    }

    return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(int argc, char *argv[])
{
    int sockfd, numbytes;
    char buf[MAXDATASIZE];
    struct addrinfo hints, *servinfo, *p;

    int rv;
    char s[INET6_ADDRSTRLEN];

    if (argc != 2)
    {
        fprintf(stderr, "usage: client hostname\n");
        exit(1);
    }

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;

    if ((rv = getaddrinfo(argv[1], PORT, &hints, &servinfo)) != 0)
    {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 1;
    }

    for (p = servinfo; p != NULL; p = p->ai_next)
    {
        if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1)
        {
            perror("client: socket");
            continue;
        }

        if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1)
        {
            close(sockfd);
            perror("client: connect (will try again)");
            continue;
        }

        break;
    }

    if (p == NULL)
    {
        fprintf(stderr, "client: failed to connect\n");
        return 2;
    }

    inet_ntop(p->ai_family, get_in_addr((struct sockaddr *)p->ai_addr),
            s, sizeof s);
    printf("client: connecting to %s\n", s);

    freeaddrinfo(servinfo);

    if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1)
    {
        perror("recv");
        exit(1);
    }

    buf[numbytes] = '\0';

    printf("client: received '%s'\n", buf);

    close(sockfd);

    return 0;
}

当服务器在后台运行时，客户端将得到“这是响应！”每次运行时都会发出消息。服务器以迭代方式工作，处理多个请求。有很多清理工作。我需要去研究从accept()中可能出现的哪些错误，可以重新尝试循环。代码采取保守的观点，即如果一个accept()失败，所有后续的也会失败，最好是停止。这是一个设计决策；如果你愿意，你可以改变它。我注意到，一些可能的错误，如EBADF，将是永久的问题，任何数量的重试都无法解决。其他的可能代表短暂的错误。