#include <curl/curl.h>
#include <assert.h>

static size_t read_callback(char* buffer, size_t size, size_t nitems, void* userdata);

int main(int argc, char* argv[])
{
  CURL* curl = curl_easy_init();
  assert(curl != NULL);
  const char* url = argc > 1 ? argv[1] : "http://localhost:8080/";
  curl_easy_setopt(curl, CURLOPT_URL, url);
  curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

  /* 1st request is a POST with no content */
  curl_easy_setopt(curl, CURLOPT_POST, 1L);
  curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, 0L);
  curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "");
  CURLcode result = curl_easy_perform(curl);
  assert(result == CURLE_OK);

  /* 2nd request is a POST with "chunked" transfer encoding */
  curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);
  curl_easy_setopt(curl, CURLOPT_POSTFIELDS, NULL);
  struct curl_slist* slist = curl_slist_append(NULL, "Transfer-Encoding: chunked");
  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, slist);
  result = curl_easy_perform(curl);
  assert(result == CURLE_OK);

  curl_slist_free_all(slist);
  curl_easy_cleanup(curl);

  return 0;
}

static size_t read_callback(char* buffer, size_t size, size_t nitems, void* userdata)
{
  static int done = 0;
  if (done) {
    return 0;
  }
  done = 1;
  assert(size * nitems >= 2);
  buffer[0] = 'H';
  buffer[1] = 'i';
  return 2;
}

./curltest http://httpbin.org/post
* About to connect() to httpbin.org port 80 (#0)
*   Trying 34.205.150.168...
* Connected to httpbin.org (34.205.150.168) port 80 (#0)
> POST /post HTTP/1.1
Host: httpbin.org
Accept: */*
Content-Length: 0
Content-Type: application/x-www-form-urlencoded

< HTTP/1.1 200 OK
< (...response truncated for conciseness...)
* Connection #0 to host httpbin.org left intact
* Found bundle for host httpbin.org: 0x1e56ab0
* Re-using existing connection! (#0) with host httpbin.org
* Connected to httpbin.org (34.205.150.168) port 80 (#0)
> POST /post HTTP/1.1
Host: httpbin.org
Accept: */*
Transfer-Encoding: chunked
Content-Type: application/x-www-form-urlencoded

  curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, 1L);
  curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "X");

# curl -V
curl 7.29.0 (x86_64-redhat-linux-gnu) libcurl/7.87.0 OpenSSL/1.0.2k-fips zlib/1.2.7 nghttp2/1.31.1
Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtsp smb smbs smtp smtps telnet tftp
Features: AsynchDNS Debug TrackMemory IPv6 Largefile NTLM NTLM_WB SSL libz unix-sockets

# uname -a
Linux blade-162-115 3.10.0-1160.15.2.el7.x86_64 #1 SMP Wed Feb 3 15:06:38 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux