由单独的源文件中的全局变量引起的C ++分段错误

Question

I'm a c++ noob trouble shooting for 4 hours now. I am getting my first segmentation fault. I think it is coming from the variable data. The program pulls the html (using cURL) from a webpage but seg faults after fetching some HTML. I am calling "curlbar::getThreads();" from main.cpp. The code worked fine when it was all in main.cpp, but when I put it into curlbar, I got the segfault (core dumped) error:

/*
* curlbar.cpp
*
*  Created on: Feb 2, 2014
* 
*/
//get list of threads
#include "headers.h"
class curlbar{
public:
string data;
size_t writeContents(char* buf, size_t size, size_t nmemb, void* up){
        for(unsigned int c = 0; c<size*nmemb; c++){
            this->data.push_back(buf[c]);
        }
        return size*nmemb;
}

static void getThreads(){

    CURL* curl;
    curl_global_init(CURL_GLOBAL_ALL);
    curl = curl_easy_init();

    curl_easy_setopt(curl, CURLOPT_URL, "www.somewebsiteblahblah.com");
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &curlbar::writeContents);
    curl_easy_setopt(curl, CURLOPT_VERBOSE,1L); //tell curl to output its progress
    curl_easy_perform(curl);
    curl_easy_cleanup(curl);
    curl_global_cleanup();
}
};

Is the "string data;" not have enough memory allocated to it? How should I go about fixing this?

Answer 1

I'm fairly sure this is what you're missing. you can't pass a member function as a callback, as the caller has no clue how to appropriately push this as the first parameter.

But you can do this:

class curlbar
{
private:
    // callback version used for curl-write-function
    static size_t writeContents_s(char *buf, size_t size, size_t nmemb, void *up)
    {
        curlbar* pThis = static_cast<curlbar*>(up);
        return pThis->writeContents(buf, size, nmemb);
    }

public:
    std::string data;

    // member version
    size_t writeContents(char* buf, size_t size, size_t nmemb)
    {
        std::copy(buf, buf+(size*nmemb), std::back_inserter(data));
        return size*nmemb;
    }

    void getThreads()
    {
        CURL* curl;
        curl_global_init(CURL_GLOBAL_ALL);
        curl = curl_easy_init();

        curl_easy_setopt(curl, CURLOPT_URL, "www.somewebsiteblahblah.com");
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &curlbar::writeContents_s);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, this); // NOTE ADDITION
        curl_easy_setopt(curl, CURLOPT_VERBOSE,1L); //tell curl to output its progress
        curl_easy_perform(curl);
        curl_easy_cleanup(curl);
        curl_global_cleanup();
    }
};

Given a curlbar obj; object, you invoke it as

curlbar obj;
obj.getThreads();

How It Works

This uses the user-defined data parameter option of an easy-curl handle to establish the piece-o-data that will be send back along with your static writer-callback. That data is provided as the void *up parameter to the callback. So we pass the pointer this and in the static callback use a static_cast to give us an object pointer, pThis . We use that pointer to fire the member function of a similar name, but no longer needing the up param.