libcurl不发送json-Data

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

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

  curl_global_init(CURL_GLOBAL_DEFAULT);

  curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "http://example.com/api");
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "{\"key\": \"value\"}");
    curl_easy_setopt(curl, CURLOPT_HTTPHEADER, "Content-Type: application/json");

    res = curl_easy_perform(curl);
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));

    curl_easy_cleanup(curl);
  }

  curl_global_cleanup();

  return 0;
}