Sending a file via multipart using libcurl in C++ - c++

Question

I am trying to upload a file via a POST request to a remote location using libcurl and C++. However, I think I am doing something wrong because I am told that the file doesn't arrive on the other side.
I am using the following code:
#include "curl/curl.h"
using namespace std;
size_t WriteCallback(void * buffer, size_t size, size_t count, void * user)
{
((string *) user)->append((char *) buffer, 0, size * count);
return size * count;
}
int main()
{
curl_global_init(CURL_GLOBAL_ALL);
CURL * Curl;
CURLCode res;
Curl = curl_easy_init();
curl_easy_setopt(Curl, CURLOPT_FAILONERROR, 0);
curl_easy_setopt(Curl, CURLOPT_VERBOSE, 1L);
struct curl_httppost * formpost = NULL;
struct curl_httppost * lastptr = NULL;
struct curl_slist * headerlist = NULL;
static const char buf[] = "Expect:";
ImageName = "myimage.jpg"
ImageNameWithPath = "/this/location/right/here/" + ImageName;
curl_formadd(& formpost,
& lastptr,
CURLFORM_COPYNAME, ImageName.c_str();
CURLFORM_FILE, ImageNameWithPath.c_str();
CURLFORM_CONTENTTYPE, "image/jpeg (binary)";
CURLFORM_END);
curl_formadd(& formpost,
& lastptr,
CURLFORM_COPYNAME, ImageName.c_str();
CURLFORM_COPYCONTENTS, ImageNameWithPath.c_str();
CURLFORM_CONTENTTYPE, "image/jpeg (binary)";
CURLFORM_END);
headerlist = curl_slist_append(headerlist, buf);
string MessageBodyLine1 = "Content-Disposition: form-data; name=\"file\"; filename\"" + ImageName + "\"";
headerlist = curl_slist_append(headerlist, MessageBodyLine1.c_str());
string Url = https://www.example.com/ // There is a real URL here
curl_easy_setopt(Curl, CURLOPT_URL, Ulr.c_str());
curl_easy_setopt(Curl, CURLOPT_HTTPHEADER, headerlist);
curl_easy_setopt(Curl, CURLOPT_HTTPPOST, formpost);
string Reponse;
curl_easy_setopt(Curl, CURLOPT_WRITEFUNCTION, WriteCallback);
curl_easy_setopt(Curl, CURLOPT_WRITEDATA, & Response);
res = curl_easy_perform(Curl);
curl_easy_cleanup(Curl);
curl_formfree(formpost);
curl_slist_free_all(headerlist);
curl_global_cleanup();
return 0;
I actually expected the output to look something like this somewhere
POST URL HTTP/1.1
Content-Type: multipart/form-data; boundary=----BoundaryString
----BoundaryString
Content-Disposition: form-data; name="file"; filename="myfile.jpg"
Content-Type: image/jpeg (binary)
and instead I get this:
* Trying IP
* Connected to IP port PORT (#0)
POST URL HTTP/1.1
Host: IP
Accept: IP:PORT
Content-Disposition: form-data; name="file"; filename="myfile.jpg"
Content-Length: totalsize
Content-Type: multipart/form-data; boundary=----BoundaryString
< HTTP/1.1 200 OK
[...]
Now it says OK but I know for a fact that the image does not arrive on the other side. Furthermore my Response string is also empty whereas it should contain a JSON string.
What am I doing wrong? Unfortunately I am stuck with a older libcurl version and thus cannot use the mime format available in curl 7.55.

Answer 1

DO NOT add a Content-Disposition request header to the headerlist of the HTTP request, it does not belong there. It belongs inside each MIME part instead (ie, you should be using CURLFORM_COPYNAME, "file" and CURLFORM_FILE, ImageName.c_str()). I would expect curl_formadd() to handle the Content-Disposition for you, you should not need to create it manually.
Also, image/jpeg (binary) is not a valid Content-Type value, it needs to be just image/jpeg.
And why are you calling curl_formadd() twice for the same file, one with CURLFORM_FILE and the other with CURLFORM_COPYCONTENTS? Especially since your input to CURLFORM_COPYCONTENTS is wrong (it expects a pointer to actual data, not a pointer to a filename string). You should be using only CURLFORM_FILE in this situation.
Try this:
#include "curl/curl.h"
#include <string>
using namespace std;
size_t WriteCallback(void * buffer, size_t size, size_t count, void * user)
{
size_t numBytes = size * count;
static_cast<string*>(user)->append(static_cast<char*>(buffer), 0, numBytes);
return numBytes;
}
int main()
{
curl_global_init(CURL_GLOBAL_ALL);
CURL *Curl = curl_easy_init();
if (!Curl)
return -1;
curl_easy_setopt(Curl, CURLOPT_FAILONERROR, 0);
curl_easy_setopt(Curl, CURLOPT_VERBOSE, 1L);
curl_httppost *formpost = NULL;
curl_httppost *lastptr = NULL;
string ImageName = "myimage.jpg";
string ImageNameWithPath = "/this/location/right/here/" + ImageName;
curl_formadd(&formpost,
&lastptr,
CURLFORM_COPYNAME, "file",
CURLFORM_FILE, ImageNameWithPath.c_str(),
CURLFORM_CONTENTTYPE, "image/jpeg",
CURLFORM_END);
curl_slist *headerlist = curl_slist_append(NULL, "Expect:");
string Url = https://www.example.com/ // There is a real URL here
curl_easy_setopt(Curl, CURLOPT_URL, Url.c_str());
curl_easy_setopt(Curl, CURLOPT_HTTPHEADER, headerlist);
curl_easy_setopt(Curl, CURLOPT_HTTPPOST, formpost);
string Reponse;
curl_easy_setopt(Curl, CURLOPT_WRITEFUNCTION, WriteCallback);
curl_easy_setopt(Curl, CURLOPT_WRITEDATA, &Response);
CURLCode res = curl_easy_perform(Curl);
curl_easy_cleanup(Curl);
curl_formfree(formpost);
curl_slist_free_all(headerlist);
curl_global_cleanup();
return 0;
}