Question

我的C ++程序当前通过管道（popen("curl ...")）调用curl来将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popen()的命令行是：

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（但我可以选择替代方案）和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。有什么建议吗？

[请注意，我通常没有任何网址编码的表单字段，例如：http：//server/handler.php？I = do＆amp; not = use＆amp; these = in＆amp; my = query]

Answer 1

您可以使用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，在提供CURLOPT_POSTFIELDS时，它会自动启用CURLOPT_POST，因此无需在请求中提供CURLOPT_POST。

Answer 2

此处有示例代码：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

匹配Content-Type所需的application/json标头如同op一样？

使用上述两个答案中的CURLOPT_POSTFIELDS以及CURLOPT_POST，Content-Type会自动设置为application/x-www-form-urlencoded。

让我正确设置标头的唯一方法是添加此答案中列出的内容：JSON requests in C using libcurl

Answer 4

此外，您可以使用RAW输入而不是添加额外的反斜杠：

curl_easy_setopt(curl, CURLOPT_POSTFIELDS, R"anydelim( {"hi" : "there"} )anydelim");

带分隔符或没有分隔符。

如何使用libcurl发布JSON缓冲区？

4 个答案: