1

实际上，我的问题非常令人愤怒。我先给你看代码。

/*
** listener.c -- a datagram sockets "server" demo
*/
//Original Code: Brian Hall (beej@beej.us)
//Commented and modified by Vishal Kotcherlakota (PID A07124450)

#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 <arpa/inet.h>
#include <netdb.h>

#define MYPORT "4960"   // the port users will be connecting to
#define YOURPORT "4961"
#define MAXBUFLEN 10000 

// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
    //If the socket address struct says it's an IPv4...
    if (sa->sa_family == AF_INET) {
        //...return the IPv4 variable.
        return &(((struct sockaddr_in*)sa)->sin_addr);
    }
    //otherwise, assume it's IPv6, and get the IPv6 variable
    return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(int argc, char *argv[])
{
    int sockfd, sockfdAck; //socket file descriptor (handle)
    struct addrinfo hints, *servinfo, *p, *q;
    int rv;
    int numbytes;
    unsigned int seqNum, stateNum=0, ackNum;
    struct sockaddr_storage their_addr;
    struct timeval recvTime, timeStamp, latency;
    char buf[MAXBUFLEN], junk[MAXBUFLEN];
    size_t addr_len;
    char *ackBack;
    char s[INET6_ADDRSTRLEN];

    if (argc != 2)
    {
        fprintf(stderr, "usage: <executable> <hostname>\n");
        exit(0);
    }

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC; // set to AF_INET to force IPv4
    hints.ai_socktype = SOCK_DGRAM;
    hints.ai_flags = AI_PASSIVE; // use my IP

    if ((rv = getaddrinfo(NULL, MYPORT, &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(q = servinfo; q != NULL; q = q->ai_next) {
        if ((sockfd = socket(q->ai_family, q->ai_socktype,
                q->ai_protocol)) == -1) {
            perror("listener: socket");
            continue;
        }

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

        break;
    }

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

    freeaddrinfo(servinfo);

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC; // set to AF_INET to force IPv4
    hints.ai_socktype = SOCK_DGRAM;

    if ((rv = getaddrinfo(argv[1], BACKPORT, &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 ((sockfdAck = socket(p->ai_family, p->ai_socktype,
                p->ai_protocol)) == -1) {
            perror("listener: socket");
            continue;
        }

        break;
    }

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

    freeaddrinfo(servinfo);
        printf("listener: waiting to recvfrom...\n");
    while (1)
    {

        addr_len = sizeof their_addr;
        if ((numbytes = recvfrom(sockfd, buf, MAXBUFLEN-1 , 0,
            (struct sockaddr *)&their_addr, &addr_len)) == -1) {
            perror("recvfrom");
            exit(1);
        }

        /*printf("listener: got packet from %s\n",
            inet_ntop(their_addr.ss_family,
                get_in_addr((struct sockaddr *)&their_addr),
                s, sizeof s));
        printf("listener: packet is %d bytes long\n", numbytes);
        */
        buf[numbytes] = '\0';
        sscanf(buf,"%u %s",&seqNum, junk);
        if (seqNum == stateNum + 1)
        {
            stateNum = seqNum;
            printf("Ok, state is now %u.\n", stateNum);
        }
        ackBack = malloc(20*sizeof(char));
        sprintf(ackBack, "%u acknowledged\0", stateNum);
        numbytes = sendto(sockfdAck, ackBack, strlen(ackBack), 0, p->ai_addr, p->ai_addrlen);
        if (numbytes == -1);
        {
            perror("sendto");
            exit(1);
        }
        free(ackBack);
    }

    return 0;
}

请原谅草率的代码；我拼命努力按时完成这项任务。目标是使用数据报套接字开发 ARQ 协议。这段代码应该可以工作，但是当我运行它时，我得到一个错误sendto: Success，这意味着我的sendto()调用失败了。我在任何地方都找不到这方面的文档，我变得非常绝望。

4

1 回答 1

6

它与必须无关bind()- 事实上看看这个语法：

if (numbytes == -1) ;        // semicolon !
{
        perror("sendto");
        exit(1);
}

你有一个没有身体的条件，然后是没有条件的身体，它总是执行（如你所见）。

添加 numbytes 的 printf ，你会看到它设置正确，没有错误。

为了避免这种难以发现的错误，我通常将左大括号放在条件之后 - 然后你会立即发现它。但当然，这是公司/项目的编码约定问题。

于 2009-12-12T10:57:55.657 回答