Add a prefix for PIPE output in c++ - c++

Question

How do you add a prefix for the output of command execution with c++
localhost is a Flask web application
std::string exec(const char* cmd) {
std::array<char, 128> buffer;
std::string result;
std::unique_ptr<FILE, decltype(&_pclose)> pipe(_popen(cmd, "r"), _pclose);
if (!pipe) {
throw std::runtime_error("popen() failed!");
}
while (fgets(buffer.data(), buffer.size(), pipe.get()) != nullptr) {
result += buffer.data();
//std::cout << typeid(result).name() << std::endl;
// read form pipe and add to the output string
std::string output = "output=";
output += buffer.data()
std::cout << output << std::endl;
// call report_ to send a post request to the server
report_(output);
}
char* c = const_cast<char*>(result.c_str());
return result;
}
As far as I understand this is a c++ function that returns a string value of the output from the command prompt
int report_(std::string report )
{
CURL* curl;
CURLcode res;
/* In windows, this will init the winsock stuff */
curl_global_init(CURL_GLOBAL_ALL);
/* get a curl handle */
curl = curl_easy_init();
if (curl) {
/* First set the URL that is about to receive our POST. This URL can
just as well be a https:// URL if that is what should receive the
data. */
curl_easy_setopt(curl, CURLOPT_URL, "http://localhost/api/00000000000000000000/report");
/* Now specify the POST data */
// report starts with "output="
std::cout << report << std::endl;
// this is where we add the post data
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, output );
/* Perform the request, res will get the return code */
res = curl_easy_perform(curl);
/* Check for errors */
if (res != CURLE_OK)
fprintf(stderr, "curl_easy_perform() failed: %s\n",
curl_easy_strerror(res));
/* always cleanup */
curl_easy_cleanup(curl);
}
curl_global_cleanup();
return 0;
}
This function reports the output of the exec() function but before you do that you have to add the prefix output= to the output of exec() which takes a string as an argument
The server returns
400 Bad Request: The browser (or proxy) sent a request that this server could not understand. KeyError: 'output'
If you change curl_easy_setopt(curl, CURLOPT_POSTFIELDS, output ); to curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "output=hello world" ); then the server receives the output

Answer 1

This link explains how to add post data to post fields you have to pass a pointer to the data you want to send so using const char*
// this line refers to the pointer of the string needed to be send over
// just replace output with and std::string value and you can send it as post data
// do not use std::string as post data
const char* c = const_cast<char*>(output.c_str());
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, c );