libcurl does not send json-Data - flask

Question

I try to pass some json-Code from c++ to a Python Flask Rest Api. But unfortunatelly this does not work and a don't see my mistake :(
This is my c-Code:
#include <stdio.h>
#include <curl/curl.h>
#include <string>
using namespace std;
int main(void)
{
CURL *curl;
CURLcode res;
curl_global_init(CURL_GLOBAL_ALL);
curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "localhost:5000/todo/api/v1.0/tasks/debug");
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "{\"title\" : \"The Title\"}");
struct curl_slist *headers = NULL;
curl_slist_append(headers, "Accept: application/json");
curl_slist_append(headers, "Content-Type: application/json");
curl_slist_append(headers, "charsets: utf-8");
curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "POST");
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
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;
}
This is the flask-function:
#POST DEBUG
#app.route("/todo/api/v1.0/tasks/debug", methods=['POST'])
def echo():
print(request.json)
return "ReturnString \n"
The output of the flask-Server looks like this:
None
127.0.0.1 - - [12/Jul/2017 12:49:15] "POST /todo/api/v1.0/tasks/debug HTTP/1.1" 200 -
So it seems to me that the json-Data is not passed to the flask-function.
I tried basically the same with text and it worked.
When I try the same thing with a curl-Call from the command-line, it works.
Curl-Command:
curl -i -H "Content-Type: application/json" -X POST -d '{"title":"Read a book"}' http://localhost:5000/todo/api/v1.0/tasks/debug
Output from Flask-Server:
{'title': 'Read a book'}
127.0.0.1 - - [12/Jul/2017 13:11:54] "POST /todo/api/v1.0/tasks/debug HTTP/1.1" 200 -
Any Help ?

Answer 1

After I ran your code and looked at the HTTP request with Wireshark, I found that the Content-Type header was not set to application/json. I then compared your code to the one produced by curl ... --libcurl example.c and found that request headers have to be added like so:
struct curl_slist *headers = NULL;
headers = curl_slist_append(headers, "Accept: application/json");
headers = curl_slist_append(headers, "Content-Type: application/json");
headers = curl_slist_append(headers, "charsets: utf-8");