Question

I tried to use a C++11 lambda expression with CURLOPT_WRITEFUNCTION, but the program crashes at runtime with an access violation. I’m not sure how to look further into this due to lack of C++11 knowledge, but maybe someone else has an idea how to make this work.

The function:

#ifndef CURL_GET_H
#define CURL_GET_H

#include <curl/curl.h>
#include <curl/easy.h>
#include <vector>
#include <string>

std::vector<std::string> curl_get(const char* url)
{
    CURL *curl;
    CURLcode res;

    std::vector<std::string> content;
    auto curl_callback = [](void *ptr, size_t size, size_t nmemb, void *stream) -> size_t {
        // does nothing at the moment due to testing...
        return size * nmemb;
    };

    curl = curl_easy_init();
    if (curl)
    {
        curl_easy_setopt(curl, CURLOPT_URL, "http://localhost/aaa.txt");
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, curl_callback);
        res = curl_easy_perform(curl);
        curl_easy_cleanup(curl);
    }

    return content;
}

#endif // CURL_GET_H

The error:

Unbehandelte Ausnahme bei 0x000000cc in lpip_dl.exe: 0xC0000005: Zugriffsverletzung bei Position 0x00000000000000cc.

(Access violation at position 0x00000000000000cc)

Happens when curl wants to use the callback:

wrote = data->set.fwrite_func(ptr, 1, len, data->set.out);

Answer 1

libcurl is plain C library, you need to set a callback that can be called from a such. This means funny C++ things need to be "C’ified" first to work. Like into an old-style function pointer.

This is also addressed in the libcurl FAQ entry "Using C++ non-static functions for callbacks?"

See also: C-style Callbacks in C++11