How do u get the response header from libcURL with c++ - c++

Question

ok so basically i want the response headers after i sendt my post request.
this is my send post code
pDataInfo->recvHeadBuff = (char*)VirtualAlloc(NULL, 4096, MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE);
InitPost(pDataInfo);
curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 5L);
curl_easy_setopt(curl, CURLOPT_URL, pDataInfo->URL);
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, pDataInfo->PostData.c_str());
curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);
curl_easy_setopt(curl, CURLOPT_HEADERDATA, pDataInfo->recvHeadBuff);
curl_easy_perform(curl);
curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &pDataInfo->repCode);
k so right here im trying to create a buffer that im going to use for my userdata*
callback func:
static size_t header_callback(char* buffer, size_t size,
size_t nitems, void* userdata)
{
strcpy((char*)userdata, buffer);
std::cout << (char*)userdata; // This prints the headers correctly
return nitems * size;
}
now i thought that my buffer would be filled with the characters from the header buffer but when i try to cout the buffer again from outside this callback function i get nothing.
basically need help with a way to use userdata and buffer to get my variable out of this func, ik its noobxd.

Answer 1

Your CURLOPT_HEADERFUNCTION callback is assuming the provided buffer is null-terminated, but it is not. The documentation even says so:
This function gets called by libcurl as soon as it has received header data. The header callback will be called once for each header and only complete header lines are passed on to the callback. Parsing headers is very easy using this. buffer points to the delivered data, and the size of that data is nitems; size is always 1. Do not assume that the header line is null-terminated!
You are also assuming the full header will fit inside the 4K recvHeadBuff that you are allocating, but that is not guaranteed either, so you are risking a buffer overflow.
Try something more like this instead:
std::string headers;
InitPost(pDataInfo);
curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 5L);
curl_easy_setopt(curl, CURLOPT_URL, pDataInfo->URL);
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, pDataInfo->PostData.c_str());
curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);
curl_easy_setopt(curl, CURLOPT_HEADERDATA, &headers);
curl_easy_perform(curl);
curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &pDataInfo->repCode);
// use headers as needed...
DWORD size = (headers.size() + 1) * sizeof(char);
pDataInfo->recvHeadBuff = (char*) VirtualAlloc(NULL, size, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
if (pDataInfo->recvHeadBuff)
CopyMemory(pDataInfo->recvHeadBuff, headers.c_str(), size);
...
static size_t header_callback(char* buffer, size_t size,
size_t nitems, void* userdata)
{
std::string *headers = (std::string*) userdata;
headers->append(buffer, nitems * size);
return nitems * size;
}
Alternatively, you should change pDataInfo->recvHeadBuff to use the same string type that pDataInfo->PostData is using (I'm assuming std::string) rather than being a char*, and then you can do this instead:
InitPost(pDataInfo);
curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 5L);
curl_easy_setopt(curl, CURLOPT_URL, pDataInfo->URL);
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, pDataInfo->PostData.c_str());
curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);
curl_easy_setopt(curl, CURLOPT_HEADERDATA, &(pDataInfo->recvHeaders));
curl_easy_perform(curl);
curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &pDataInfo->repCode);
...
static size_t header_callback(char* buffer, size_t size,
size_t nitems, void* userdata)
{
std::string *headers = (std::string*) userdata;
headers->append(buffer, nitems * size);
return nitems * size;
}