如何修复'curl_easy_perform（56）失败：从对等方接收数据时失败'错误在C ++中

Question

我是c ++的新手，想发送电子邮件，然后我找到了一个用于在c ++中发送电子邮件的库（curl-lib）。

这是针对linux的，过去我尝试过curl-lib站点中可用的代码。但是它给出错误“ curl_easy_perform（56）失败：从对等方接收数据时失败”。我不知道是什么问题

    #include <stdio.h>
    #include <string.h>
    #include <curl/curl.h>
    #include <iostream>
    #include <stdlib.h> 
    #include <cstring>
    #include <unistd.h> 

    using namespace std;
    /*
    * For an SMTP example using the multi interface please see smtp-multi.c.
    */ 

    /* The libcurl options want plain addresses, the viewable headers in the mail
    * can very well get a full name as well.
    */ 
    #define FROM_ADDR    "test@example.com"
    #define TO_ADDR      "test@example.de"


    #define FROM_MAIL "Sender Person " FROM_ADDR
    #define TO_MAIL   "A Receiver " TO_ADDR


    static const char *payload_text[] = {
    "To: " TO_MAIL "\r\n",
    "From: " FROM_MAIL "\r\n",
    "Message-ID: <dcd7cb36-11db-487a-9f3a-e652a9458efd@"

    "Subject: SMTP example message\r\n",
    "\r\n", /* empty line to divide headers from body, see RFC5322 */ 
    "The body of the message starts here.\r\n",
    "\r\n",
    NULL
    };

    struct upload_status {
    int lines_read;
    };

    static size_t payload_source(void *ptr, size_t size, size_t nmemb, void *userp)
    {
    struct upload_status *upload_ctx = (struct upload_status *)userp;
    const char *data;

    if((size == 0) || (nmemb == 0) || ((size*nmemb) < 1)) {
    return 0;
    }

    data = payload_text[upload_ctx->lines_read];

    if(data) {
    size_t len = strlen(data);
    memcpy(ptr, data, len);
    upload_ctx->lines_read++;

    return len;
    }

    return 0;
    }

    int main(void)
    {
    CURL *curl;
    CURLcode res = CURLE_OK;
    struct curl_slist *recipients = NULL;
    struct upload_status upload_ctx;

    upload_ctx.lines_read = 0;

    curl = curl_easy_init();
    if(curl) {

    /* This is the URL for your mailserver */ 
    curl_easy_setopt(curl, CURLOPT_URL, "smtp://mail.example.com");
     curl_easy_setopt(curl, CURLOPT_PORT, 587);
    /* Note that this option isn't strictly required, omitting it will result
     * in libcurl sending the MAIL FROM command with empty sender data. All
     * autoresponses should have an empty reverse-path, and should be directed
     * to the address in the reverse-path which triggered them. Otherwise,
     * they could cause an endless loop. See RFC 5321 Section 4.5.5 for more
     * details.
     */ 
    curl_easy_setopt(curl, CURLOPT_MAIL_FROM, FROM_ADDR);
    curl_easy_setopt(curl, CURLOPT_USERNAME, "test");
    curl_easy_setopt(curl, CURLOPT_PASSWORD, "test");
    /* Add two recipients, in this particular case they correspond to the
     * To: and Cc: addressees in the header, but they could be any kind of
     * recipient. */ 
    recipients = curl_slist_append(recipients, TO_ADDR);
    //recipients = curl_slist_append(recipients, CC_ADDR);
    curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);

    /* We're using a callback function to specify the payload (the headers and
     * body of the message). You could just use the CURLOPT_READDATA option to
     * specify a FILE pointer to read from. */ 
    curl_easy_setopt(curl, CURLOPT_READFUNCTION, payload_source);
    //curl_easy_setopt(curl, CURLOPT_READDATA, &upload_ctx);


    /* Send the message */ 
    res = curl_easy_perform(curl);
    cout<<res<<endl;
    /* Check for errors */ 
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n",
              curl_easy_strerror(res));

    /* Free the list of recipients */ 
    curl_slist_free_all(recipients);

    /* curl won't send the QUIT command until you call cleanup, so you should
     * be able to re-use this connection for additional messages (setting
     * CURLOPT_MAIL_FROM and CURLOPT_MAIL_RCPT as required, and calling
     * curl_easy_perform() again. It may not be a good idea to keep the
     * connection open for a very long time though (more than a few minutes
     * may result in the server timing out the connection), and you do want to
     * clean up in the end.
     */ 
    curl_easy_cleanup(curl);
    }

    return (int)res;
    }