Implementing CURL command using libcurl in C/C++

[Karthik Sharma]

The `CURL` command that I am using is shown below.

    curl -F 'file=@/home/karthik/Workspace/downloadfile.out' http://127.0.0.1:5000/file-upload --verbose

The response from the server is shown below.

    *   Trying 127.0.0.1...
    * Connected to 127.0.0.1 (127.0.0.1) port 5000 (#0)
    > POST /file-upload HTTP/1.1
    > Host: 127.0.0.1:5000
    > User-Agent: curl/7.47.0
    > Accept: */*
    > Content-Length: 663876790
    > Expect: 100-continue
    > Content-Type: multipart/form-data; boundary=------------------------4e96ef0714498bd7
    > 
    < HTTP/1.1 100 Continue
    * HTTP 1.0, assume close after body
    < HTTP/1.0 201 CREATED
    < Content-Type: application/json
    < Content-Length: 46
    < Server: Werkzeug/0.16.0 Python/3.5.2
    < Date: Sat, 14 Dec 2019 07:05:15 GMT
    < 
    {
      "message": "File successfully uploaded"
    }
    * Closing connection 0

I want to implement the same command in C/C++ using libcurl. I am using the following function.

    int FileUploadDownload::upload(const std::string &filename, const std::string &url) {
    
        CURL *curl;
        CURLcode res;
        struct stat file_info;
        curl_off_t speed_upload, total_time;
        FILE *fd;
    
        fd = fopen(filename.c_str(), "rb");
        if(!fd) {
            m_logger->errorf("unable to open file: %s\n",strerror(errno));
            return 1;
        }
        if(fstat(fileno(fd), &file_info) != 0) {
            m_logger->errorf("unable to get file stats: %s\n",strerror(errno));
            return 2;
        }
    
        std::cout << "filename : "<< filename << std::endl;
        std::cout << "url : " << url << std::endl;
    
        curl = curl_easy_init();
        if(curl) {
    
            curl_easy_setopt(curl, CURLOPT_URL,
                             url.c_str());
    
            curl_easy_setopt(curl, CURLOPT_POSTFIELDS, filename.c_str());
            curl_easy_setopt(curl, CURLOPT_POST, 1L);
            curl_easy_setopt(curl, CURLOPT_READDATA, fd);
            curl_easy_setopt(curl, CURLOPT_INFILESIZE_LARGE,
                             (curl_off_t) file_info.st_size);
            curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
    
            res = curl_easy_perform(curl);
            if (res != CURLE_OK) {
                m_logger->errorf("curl_easy_perform() failed: %s\n",curl_easy_strerror(res));
            } else {
                curl_easy_getinfo(curl, CURLINFO_SPEED_UPLOAD, &speed_upload);
                curl_easy_getinfo(curl, CURLINFO_TOTAL_TIME, &total_time);
                m_logger->infof("Speed: %" CURL_FORMAT_CURL_OFF_T " bytes/sec during %"
                                CURL_FORMAT_CURL_OFF_T ".%06ld seconds\n",
                                speed_upload,
                                (total_time / 1000000), (long) (total_time % 1000000));
            }
        }
        return 0;
    }
The below is the result that I get from the server.

    The result that I get is shown below.
       *   Trying 127.0.0.1...
    * Connected to 127.0.0.1 (127.0.0.1) port 5000 (#0)
    > POST /file-upload HTTP/1.1
    > Host: 127.0.0.1:5000
    > User-Agent: curl/7.47.0
    > Accept: */*
    > Content-Length: 550
    > Expect: 100-continue
    > Content-Type: multipart/form-data; boundary=------------------------c8ef4837136fca99
    > 
    < HTTP/1.1 100 Continue
    * HTTP 1.0, assume close after body
    < HTTP/1.0 201 CREATED
    < Content-Type: application/json
    < Content-Length: 46
    < Server: Werkzeug/0.16.0 Python/3.5.2
    < Date: Sat, 14 Dec 2019 07:09:47 GMT
    < 
    {
      "message": "File successfully uploaded"
    }
    * Closing connection 0


My aim is to mimic the curl command above in the C/C++ code below. What am I doing wrong ?