How to extract header information via CURLOPT_HEADERFUNCTION? - c++

Question

I want to extract header information by using the CURLOPT_HEADERFUNCTION in my c++ program.
How can I use CURLOPT_HEADERFUNCTION to read a single response header field? provides the solution on how to get those header information but I want to know why my code is not working and a possible solution with example.
//readHeader function which returns the specific header information
size_t readHeader(char* header, size_t size, size_t nitems, void *userdata) {
Erza oprations; //class which contains string function like startsWith etc
if (oprations.startsWith(header, "Content-Length:")) {
std::string header_in_string = oprations.replaceAll(header, "Content-Length:", "");
long size = atol(header_in_string.c_str());
file_size = size; // file_size is global variable
std::cout << size; // here it is showing correct file size
}
else if (oprations.startsWith(header, "Content-Type:")) {
// do something
}else
// do something
return size * nitems;
}
// part of main function
curl = curl_easy_init();
if (curl) {
fp = fopen(path, "wb");
curl_easy_setopt(curl, CURLOPT_URL, url);
curl_easy_setopt(curl, CURLOPT_CAINFO, "./ca-bundle.crt");
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, false);
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, false);
curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, readHeader);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
res = curl_easy_perform(curl);
curl_easy_cleanup(curl);
fclose(fp);
std::cout << file_size; // showing value 0
Getting correct file size in readHeader function but getting 0 bytes in main function.

Answer 1

As shown in your github depot, oprations (operations !?) is a local variable, and will be released at the end of the readHeader function. A way to process the readHeader function and get the correct file size for a given Erza instance is to pass its pointer to userdata value. The Erza class may be rewritten as :
class Erza : public Endeavour {
//... your class body
public:
bool download (const char *url,const char* path){
curl = curl_easy_init();
if (curl) {
fp = fopen(path, "wb");
curl_easy_setopt(curl, CURLOPT_URL, url);
curl_easy_setopt(curl, CURLOPT_CAINFO, "./ca-bundle.crt");
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, false);
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, false);
curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, readHeader);
curl_easy_setopt(curl, CURLOPT_HEADERDATA, this ); //<-- set this pointer to userdata value used in the callback.
curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
res = curl_easy_perform(curl);
curl_easy_cleanup(curl);
fclose(fp);
return false;
}else
return true;
}
size_t analyseHeader( char* header, size_t size, size_t nitems ){
if (startsWith(header, "Content-Length:")) {
std::string header_in_string = replaceAll(header, "Content-Length:", "");
long size = atol(header_in_string.c_str());
file_size = size; // file_size is a member variable
std::cout << size; // here it is showing correct file size
}
else if (startsWith(header, "Content-Type:")) {
// do something
}else
// do something
return size * nitems;
}
}//Eof class Erza
size_t readHeader(char* header, size_t size, size_t nitems, void *userdata) {
//get the called context (Erza instance pointer set in userdata)
Erza * oprations = (Erza *)userdata;
return oprations->analyseHeader( header, size, nitems );
}