libcurl problem with getting data from curl_easy_perform()

To get the data into string, you need to set up a write callback function:

curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, callback_func);

Also, the address of your string variable to receive the data:

curl_easy_setopt(curl, CURLOPT_WRITEDATA, &str)

Callback function would look like this:

size_t callback_func(void *ptr, size_t size, size_t count, void *stream)
{
      /* ptr - your string variable.
      stream - data chuck you received */

     printf("%.*s", size, (char*)stream);
}

Because you won't know the total size of data you'd be receiving so you would need to make pointer re-allocations to get it into a string.

The other answer appears to be wrong in its usage of first and last parameters of callback_func (see the docs). Actual chunk of data you received is in the first parameter, ptr, while the pointer you pass with CURLOPT_WRITEDATA is the last parameter.

I've made a complete compilable example:

#include <stdlib.h>
#include <string.h>
#include <curl/curl.h>

size_t dataSize=0;
size_t curlWriteFunction(void* ptr, size_t size/*always==1*/,
                         size_t nmemb, void* userdata)
{
    char** stringToWrite=(char**)userdata;
    const char* input=(const char*)ptr;
    if(nmemb==0) return 0;
    if(!*stringToWrite)
        *stringToWrite=malloc(nmemb+1);
    else
        *stringToWrite=realloc(*stringToWrite, dataSize+nmemb+1);
    memcpy(*stringToWrite+dataSize, input, nmemb);
    dataSize+=nmemb;
    (*stringToWrite)[dataSize]='\0';
    return nmemb;
}

int main()
{
    char* data=0;
    CURL*const curl=curl_easy_init();
    if(!curl)
    {
        fprintf(stderr, "Failed to init curl");
        return 1;
    }
    curl_easy_setopt(curl, CURLOPT_URL, "https://www.google.com");
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &data);
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &curlWriteFunction);
    if(curl_easy_perform(curl)!=CURLE_OK)
    {
        fprintf(stderr, "Failed to get web page\n");
        return 1;
    }
    curl_easy_cleanup(curl);

    if(!data)
    {
        fprintf(stderr, "Got no data\n");
        return 1;
    }

    printf("Page data:\n\n%s\n", data);
    free(data);
}