How to define metadata with libcurl? - c++

Question

I do following command with curl (the command line tool) to upload a file to google drive:
curl --request POST \
'https://www.googleapis.com/upload/drive/v3/files?uploadType=multipart' \
--header 'Authorization: Bearer "..."' \
-F "metadata={name : 'backup.zip'};type=application/json;charset=UTF-8" \
-F "#backup.zip;type=application/zip"
The following programm with libcurl could only use the URL, the header and upload the file using formdata:
#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) {
struct curl_slist *chunk= NULL;
chunk= curl_slist_append(chunk, "Authorization: Bearer \"...\"");
curl_slist_append(chunk, "Accept: application/json");
curl_slist_append(chunk, "Content-Type: application/json");
curl_mimepart *part= NULL;
curl_mime_type(part, "application/octet-stream");
struct curl_httppost* formpost = NULL;
struct curl_httppost* lastptr = NULL;
curl_formadd(&formpost,
&lastptr,
CURLFORM_COPYNAME, "sendfile",
CURLFORM_FILE, "main.c",
CURLFORM_END);
/* Perform the request, res will get the return code */
//curl_easy_setopt(curl, CURLOPT_URL, "https://www.googleapis.com/drive/v3/files");
curl_easy_setopt(curl, CURLOPT_URL, "https://www.googleapis.com/upload/drive/v3/files?uploadType=multipart");
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, chunk);
curl_easy_setopt(curl, CURLOPT_MIMEPOST, part);
curl_easy_setopt(curl, CURLOPT_HTTPPOST, formpost);
curl_easy_perform(curl);
/* always cleanup */
curl_easy_cleanup(curl);
}
curl_global_cleanup();
return 0;
}
How can I specify the -F "metadata={name : 'backup.zip'};type=application/json;charset=UTF-8" in libcurl?
I tried it with CURLFORM_COPYNAME but this doesn't work.

Answer 1

You are setting the HTTP request's top-level Content-Type header to application/json, which is wrong. It needs to be multipart/related instead, per Google's documentation: Send a multipart upload request.
The command-line you showed is inserting a MIME part of type application/json into the POST form data, but you are not doing the same thing in your code.
Also, you should not be using CURLOPT_MIMEPOST and CURLOPT_HTTPPOST together. Use ONE or the OTHER, not BOTH. CURLOPT_HTTPPOST is deprecated, replaced by CURLOPT_MIMEPOST. And you are not even passing the correct input value to CURLOPT_MIMEPOST anyway. It expects a curl_mime*, not a curl_mimepart*.
Try something more like this instead:
#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)
{
/* set custom HTTP headers */
struct curl_slist *header = NULL;
// uncomment this if you are not using libcurl 7.61.0 or later...
//
// header = curl_slist_append(header, "Authorization: Bearer \"...\"");
//
header = curl_slist_append(header, "Content-Type: multipart/related");
header = curl_slist_append(header, "Accept: application/json");
/* set MIME post content */
curl_mime *mime = curl_mime_init(curl);
curl_mimepart *part = curl_mime_addpart(mime);
curl_mime_name(part, "metadata");
curl_mime_type(part, "application/json;charset=UTF-8");
curl_mime_data(part, "{\"name\" : \"backup.zip\"}", CURL_ZERO_TERMINATED);
part = curl_mime_addpart(mime);
curl_mime_type(part, "application/zip");
curl_mime_filedata(part, "backup.zip");
/* Perform the request, res will get the return code */
curl_easy_setopt(curl, CURLOPT_URL, "https://www.googleapis.com/upload/drive/v3/files?uploadType=multipart");
curl_easy_setopt(curl, CURLOPT_POST, 1);
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, header);
curl_easy_setopt(curl, CURLOPT_MIMEPOST, mime);
// comment this out if you are not using libcurl 7.61.0 or later...
//
curl_easy_setopt(curl, CURLOPT_HTTPAUTH, CURLAUTH_BEARER);
curl_easy_setopt(curl, CURLOPT_XOAUTH2_BEARER, "...");
//
res = curl_easy_perform(curl);
/* always cleanup */
curl_slist_free_all(header);
curl_mime_free(mime);
curl_easy_cleanup(curl);
}
curl_global_cleanup();
return 0;
}
Or maybe this (I'm not sure the correct way to specify the HTTP Content-Type when using a MIME body - documentation and examples are very limited on this topic, most of the examples I found are for SMTP, not HTTP):
#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)
{
/* set custom HTTP headers */
struct curl_slist *header = NULL;
// uncomment this if you are not using libcurl 7.61.0 or later...
//
// header = curl_slist_append(header, "Authorization: Bearer \"...\"");
//
header = curl_slist_append(header, "Accept: application/json");
/* set MIME post content */
curl_mime *mime = curl_mime_init(curl);
curl_mime *rel = curl_mime_init(curl);
curl_mimepart *part = curl_mime_addpart(rel);
curl_mime_name(part, "metadata");
curl_mime_type(part, "application/json;charset=UTF-8");
curl_mime_data(part, "{\"name\" : \"backup.zip\"}", CURL_ZERO_TERMINATED);
part = curl_mime_addpart(rel);
curl_mime_type(part, "application/zip");
curl_mime_filedata(part, "backup.zip");
part = curl_mime_addpart(mime);
curl_mime_type(part, "multipart/related");
curl_mime_subparts(part, rel);
/* Perform the request, res will get the return code */
curl_easy_setopt(curl, CURLOPT_URL, "https://www.googleapis.com/upload/drive/v3/files?uploadType=multipart");
curl_easy_setopt(curl, CURLOPT_POST, 1);
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, header);
curl_easy_setopt(curl, CURLOPT_MIMEPOST, mime);
// comment this out if you are not using libcurl 7.61.0 or later...
//
curl_easy_setopt(curl, CURLOPT_HTTPAUTH, CURLAUTH_BEARER);
curl_easy_setopt(curl, CURLOPT_XOAUTH2_BEARER, "...");
//
res = curl_easy_perform(curl);
/* always cleanup */
curl_slist_free_all(header);
curl_mime_free(mime);
curl_easy_cleanup(curl);
}
curl_global_cleanup();
return 0;
}