如何使用libcurl发布JSON缓冲区？

Question

我的C++程序当前通过pipe(popen("curl ..."))调用curl 将JSON数据文件POST到Web服务器.由于需要将JSON保存到文件并在子shell中调用curl,因此这具有明显的性能限制.我想重写它以使用libcurl,但我不清楚如何做到这一点.我传递给的命令行popen()是:

curl -s -S -D /dev/null -H "Content-Type: application/json" -X POST -d file-of-json http://server/handler.php

在我需要发布之前,JSON数据(大约3K)位于RAM中的缓冲区中.我期望使用libcurl的CURLOPT_READFUNCTION将缓冲区假脱机到libcurl(但我对其他选项开放),并使用CURLOPT_WRITEFUNCTION来捕获服务器的回复,类似于我从popen的管道读取回复的方式.

所有这一切似乎都很简单.令人困惑的是我需要CURLOPT_POST,CURLOPT_HTTPPOST,CURLOPT_POSTFIELDS,CURLOPT_HTTPHEADER的哪种组合.我已经阅读了很多关于这个主题的帖子(没有双关语),没有一个与我的场景完全匹配.有什么建议？

[请注意,我通常没有任何URL编码的表单字段,如下所示:http://server/handler.php？I = do¬ = use&these = in&my = query]

Answer 1

你可以使用CURLOPT_POSTFIELDS:

CURL *curl = curl_easy_init();

curl_easy_setopt(curl, CURLOPT_URL, "http://example.com/api/endpoint");
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "{\"hi\" : \"there\"}");

curl_easy_perform(curl);

由于CURLOPT_POSTFIELDS不以任何方式修改有效负载,因此POST POST JSON数据非常方便.另请注意,CURLOPT_POSTFIELDS提供时,它会自动启用,CURLOPT_POST因此无需CURLOPT_POST在请求中提供.

Answer 2

这里有一个示例代码:http://curl.haxx.se/libcurl/c/post-callback.html

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.
 *
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at http://curl.haxx.se/docs/copyright.html.
 *
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 ***************************************************************************/ 
/* An example source code that issues a HTTP POST and we provide the actual
 * data through a read callback.
 */ 
#include 
#include 
#include 

const char data[]="this is what we post to the silly web server";

struct WriteThis {
  const char *readptr;
  int sizeleft;
};

static size_t read_callback(void *ptr, size_t size, size_t nmemb, void *userp)
{
  struct WriteThis *pooh = (struct WriteThis *)userp;

  if(size*nmemb sizeleft) {
    *(char *)ptr = pooh->readptr[0]; /* copy one single byte */ 
    pooh->readptr++;                 /* advance pointer */ 
    pooh->sizeleft--;                /* less data left */ 
    return 1;                        /* we return 1 byte at a time! */ 
  }

  return 0;                          /* no more data left to deliver */ 
}

int main(void)
{
  CURL *curl;
  CURLcode res;

  struct WriteThis pooh;

  pooh.readptr = data;
  pooh.sizeleft = strlen(data);

  curl = curl_easy_init();
  if(curl) {
    /* First set the URL that is about to receive our POST. */ 
    curl_easy_setopt(curl, CURLOPT_URL, "http://example.com/index.cgi");

    /* Now specify we want to POST data */ 
    curl_easy_setopt(curl, CURLOPT_POST, 1L);

    /* we want to use our own read function */ 
    curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);

    /* pointer to pass to our read function */ 
    curl_easy_setopt(curl, CURLOPT_READDATA, &pooh);

    /* get verbose debug output please */ 
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

    /*
      If you use POST to a HTTP 1.1 server, you can send data without knowing
      the size before starting the POST if you use chunked encoding. You
      enable this by adding a header like "Transfer-Encoding: chunked" with
      CURLOPT_HTTPHEADER. With HTTP 1.0 or without chunked transfer, you must
      specify the size in the request.
    */ 
#ifdef USE_CHUNKED
    {
      struct curl_slist *chunk = NULL;

      chunk = curl_slist_append(chunk, "Transfer-Encoding: chunked");
      res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, chunk);
      /* use curl_slist_free_all() after the *perform() call to free this
         list again */ 
    }
#else
    /* Set the expected POST size. If you want to POST large amounts of data,
       consider CURLOPT_POSTFIELDSIZE_LARGE */ 
    curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, (curl_off_t)pooh.sizeleft);
#endif

#ifdef DISABLE_EXPECT
    /*
      Using POST with HTTP 1.1 implies the use of a "Expect: 100-continue"
      header.  You can disable this header with CURLOPT_HTTPHEADER as usual.
      NOTE: if you want chunked transfer too, you need to combine these two
      since you can only set one list of headers with CURLOPT_HTTPHEADER. */ 

    /* A less good option would be to enforce HTTP 1.0, but that might also
       have other implications. */ 
    {
      struct curl_slist *chunk = NULL;

      chunk = curl_slist_append(chunk, "Expect:");
      res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, chunk);
      /* use curl_slist_free_all() after the *perform() call to free this
         list again */ 
    }
#endif

    /* Perform the request, res will get the return code */ 
    res = curl_easy_perform(curl);

    /* always cleanup */ 
    curl_easy_cleanup(curl);
  }
  return 0;
}

Answer 3

就像操作所要求的那样，Content-Type匹配所需的标头怎么样？application/json

使用CURLOPT_POSTFIELDS上面的两个答案以及CURLOPT_POST，Content-Type自动设置为application/x-www-form-urlencoded。

我正确设置标头的唯一方法是添加此答案中概述的内容：JSON requests in C using libcurl