套接字 sendto() 返回 EINVAL

Question

我正在尝试在 C 中发送一个 UDP 数据包。我有以下sendto()：

char* msg = "Hello";

//ret is the return value of getaddrinfo, the address is AF_INET (IPv4)
//and the sock_type is SOCK_DGRAM (UDP)
struct sockaddr_in *ip = (struct sockaddr_in *)ret->ai_addr;

if ((sendto(sock, msg, strlen(msg), 0, (struct sockaddr *)ip, 
                sizeof(struct sockaddr *))) != -1) {
    printf("msg sent successfully");
} else {
    printf("Error sending msg: %s\n", strerror(errno));
}

但是，它返回一个错误，说明存在无效参数。查看联机帮助页，我真的无法判断哪一个是无效参数。有什么想法吗？

编辑：这是我所有的代码

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

int main(int argc, const char *argv[])
{
    /*
     * Help the technically challenged among us who have no idea
     * what on God's green Earth they are doing with this thing.
    */
    if (argc != 2) {
        printf("usage: routetracer <ip address or hostname>\n");
        return -1;
    }

    /*
     * hints- parameters for return value of getaddrinfo
     * ret- return value of getaddrinfo
     */
    struct addrinfo hints, *ret;
    int status;
    char ipv4[INET_ADDRSTRLEN];
    int ttl = 0;
    char* msg = "Hello";
    int last_hop = 0;

    //define what we want from getaddrinfo
    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_INET; //IPv4
    hints.ai_socktype = SOCK_DGRAM; //UDP packets

    //call getaddrinfo to fill ret, w/ error chk
    if ((status = getaddrinfo(argv[1], NULL, &hints, &ret)) != 0) {
        printf("getaddrinfo: %s\n", gai_strerror(status));
        return -1;
    }

    //extract IPv4 address from ret
    struct sockaddr_in* ip = (struct sockaddr_in *)ret->ai_addr;

    //convert address from pure numbers to something easier to read
    inet_ntop(ret->ai_family, &(ip->sin_addr), ipv4, INET_ADDRSTRLEN);   

    //kindly inform the user of which hostname they are connecting to
    printf("Route for: %s\n", ipv4);

    //create a socket
    int sock = socket(ret->ai_family, ret->ai_socktype, ret->ai_protocol);

    ttl = 1;
    if ((setsockopt(sock, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl))) != -1) {
        printf("TTL set successfully\n");
    } else {
        printf("Error setting TTL: %s\n", strerror(errno));
    }

    if ((sendto(sock, msg, strlen(msg), 0, ret->ai_addr,
                    ret->ai_addrlen)) != -1) {
        printf("msg sent successfully");
    } else {
      printf("Error sending msg: %s\n", strerror(errno));
    }
    return 0;
}

运行程序给出以下输出：

$ ./routetracer www.google.com
Route for: 173.194.46.82
TTL set successfully
Error sending msg: Invalid argument

Answer 1

正如 Barmar 指出的那样，一个 EINVAL 的原因是不正确的：

  sizeof(struct sockaddr *)

给出指针的大小。见Socket programming: sendto always fails with errno 22 (EINVAL)。

第二个原因似乎是sin_port，getaddrinfo 返回为0。将其更改为 80 say 将清除 EINVAL，如下所示：

  ((struct sockaddr_in *)ret->ai_addr)->sin_port = htons(80); // test

这里的 80 端口不是 HTTP，而是（对于 UDP）是 Google 的实验性 QUIC Chromium。

Wikipedia http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers 声明端口 0 用于 UDP保留，而 TCP 被非正式地保留为“用于指定系统分配（动态）端口的编程技术”。

顺便说一句（并参考原始问题），您可能不需要为变量ip 操心。您将ret->ai_addr 转换为struct sockaddr_in *，然后再次返回 到其原始类型。

而且，正如 Remy Lebeau 所指出的，最好使用 getaddrinfo 的 service 参数。因此，将所有这些放在一起，您的代码可能看起来更像：

  memset(&hints, 0, sizeof(hints));
  hints.ai_family = AF_INET; //IPv4
  hints.ai_socktype = SOCK_DGRAM; //UDP packets
  if ((status = getaddrinfo(argv[1], "80", &hints, &ret)) != 0) {
      printf("getaddrinfo: %s\n", gai_strerror(status));
      return -1;
  }
  assert(ret->ai_family   == AF_INET);     // guaranteed
  assert(ret->ai_socktype == SOCK_DGRAM);  // guaranteed
  assert(((struct sockaddr_in *)ret->ai_addr)->sin_port == htons(80)); // guaranteed
  // ...
  if ((sendto(sock, msg, strlen(msg), 0, ret->ai_addr, ret->ai_addrlen)) != -1) {
  // ...

Answer 2

试试：

if ((sendto(sock, msg, strlen(msg), 0, (struct sockaddr *)ip, 
                sizeof(struct sockaddr_in))) != -1) {

你给它的是指针的大小，而不是结构的大小。并且它必须是特定的结构类型，而不是泛型类型。