write access violation with c++ libcurl [closed] - c++

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I'm trying to create c++ code that download data from some URLs, but it's throwing a write access violation:
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#ifndef WIN32
#endif
#include <curl/curl.h>
#include <string>
static const char* urls[] = {
"http://www.example.com",
"http://www.example1.com",
};
#define MAX_PARALLEL 10 /* number of simultaneous transfers */
#define NUM_URLS sizeof(urls)/sizeof(char *)
static size_t write_cb(void* ptr, size_t size, size_t nmemb, void* buffer)
{
((std::string*)buffer)->append((char*)ptr, nmemb);
return nmemb;
}
static void add_transfer(CURLM* cm, int i, int* left)
{
CURL* eh = curl_easy_init();
curl_easy_setopt(eh, CURLOPT_WRITEFUNCTION, write_cb);
curl_easy_setopt(eh, CURLOPT_URL, urls[i]);
curl_easy_setopt(eh, CURLOPT_PRIVATE, urls[i]);
curl_easy_setopt(eh, CURLOPT_WRITEDATA, &write_cb);
curl_easy_setopt(eh, CURLOPT_VERBOSE, 1L);
curl_multi_add_handle(cm, eh);
(*left)++;
}
int main(void)
{
CURLM* cm;
unsigned int transfers = 0;
int msgs_left = -1;
int left = 0;
curl_global_init(CURL_GLOBAL_ALL);
cm = curl_multi_init();
/* Limit the amount of simultaneous connections curl should allow: */
curl_multi_setopt(cm, CURLMOPT_MAXCONNECTS, (long)MAX_PARALLEL);
for (transfers = 0; transfers < MAX_PARALLEL && transfers < NUM_URLS;
transfers++)
add_transfer(cm, transfers, &left);
do {
int still_alive = 1;
curl_multi_perform(cm, &still_alive);
CURLMsg* msg;
int queued;
CURLMcode mc = curl_multi_perform(cm, &still_alive);
if (cm)
/* wait for activity, timeout or "nothing" */
mc = curl_multi_poll(cm, NULL, 0, 1000, NULL);
if (mc)
break;
do {
msg = curl_multi_info_read(cm, &queued);
if (msg) {
if (msg->msg == CURLMSG_DONE) {
/* a transfer ended */
fprintf(stderr, "Transfer completed\n");
}
}
} while (msg);
if (left)
curl_multi_wait(cm, NULL, 0, 1000, NULL);
} while (left);
curl_multi_cleanup(cm);
curl_global_cleanup();
return EXIT_SUCCESS;
}
Its crashing on the line:
_Mypair._Myval2._Mysize = _Old_size + _Count;
The full error message is:
Exception thrown: write access violation.
this was 0x7FF7941D39D0.
How can I make this code download each Url data without any error?

Answer 1

In your write_cb() callback, you are expecting the buffer parameter to point at a std::string object, but in add_transfer() you are setting CURLOPT_WRITEDATA to point at write_cb itself rather than at a std::string object.
Try something more like this instead:
struct url_info {
const char* url;
std::string data;
};
static url_info urls[] = {
{"http://www.example.com", ""},
{"http://www.example1.com", ""}
};
static const int NUM_URLS = sizeof(urls)/sizeof(urls[0]);
static size_t write_cb(void* ptr, size_t size, size_t nmemb, void* buffer)
{
size_t result = size * nmemb;
static_cast<std::string*>(buffer)->append(static_cast<char*>(ptr), result);
return result;
}
static void add_transfer(CURLM* cm, int i, int* left)
{
...
curl_easy_setopt(eh, CURLOPT_URL, urls[i].url);
curl_easy_setopt(eh, CURLOPT_WRITEDATA, &urls[i].data);
curl_easy_setopt(eh, CURLOPT_PRIVATE, &urls[i]);
...
}
int main(void)
{
...
while ((msg = curl_multi_info_read(cm, &queued)) != NULL) {
if (msg->msg == CURLMSG_DONE) {
/* a transfer ended */
url_info *info;
curl_easy_getinfo(msg->easy_handle, CURLINFO_PRIVATE, reinterpret_cast<char**>(&info));
// use info->url and info->data as needed...
std::cerr << "Transfer completed from " << info->url << ", bytes received: " << info->data.size() << "\n";
}
}
...
}