How to download a webpage with C - c

Question

I'm have a bit of a problem with C and XML.
Basically, the code I'm using is:
#include <stdio.h>
#include <curl/curl.h>
#include <libxml/tree.h>
#include <string.h>
#include <stdlib.h>
#define WEBPAGE_URL "http://gdata.youtube.com/feeds/api/videos/mKxLmdBzS10"
typedef struct {
char *contents;
int size;
} data;
/*Curl uses this function to write the contents of a webpage to a file/stdout*/
size_t write_data( void *ptr, size_t size, size_t nmeb, void *stream)
{
data *curl_output = (data *)stream;
int curl_output_size = size * nmeb;
curl_output->contents = (char *) realloc(curl_output->contents, curl_output->size + curl_output_size + 1);
if (curl_output->contents) {
memcpy(curl_output->contents, ptr, curl_output_size); /*Copying the contents*/
curl_output->size += curl_output_size;
curl_output->contents[curl_output->size] = 0;
return curl_output->size;
}
}
int main()
{
data webpage;
webpage.contents = malloc(1);
webpage.size = 1;
CURL *handle = curl_easy_init();
curl_easy_setopt(handle,CURLOPT_URL,WEBPAGE_URL); /*Using the http protocol*/
curl_easy_setopt(handle,CURLOPT_WRITEFUNCTION, write_data); /*Setting up the function meant to copy data*/
curl_easy_setopt(handle,CURLOPT_WRITEDATA, &webpage); /*The data pointer to copy the data*/
curl_easy_perform(handle);
curl_easy_cleanup(handle);
printf("Contents: %s",webpage.contents);
int i;
}
I'm mean't to be getting this XML back: http://gdata.youtube.com/feeds/api/videos/mKxLmdBzS10.
But currently, I only get arbitrary amounts back, sometimes a third sometimes a half and other times just a quater.
Anybody know what I'm doing wrong?

Answer 1

The problem is in your write_data function.
This line copies the new data to the start of your array, rather than the current end.
memcpy(curl_output->contents, ptr, curl_output_size); /*Copying the contents*/
You need to offset your pointer:
memcpy(curl_output->contents + curl_output->size, ptr, curl_output_size); /* Copying the contents */
Additionally, your return value is bad -- it should be return(curl_output_size); to indicate success, via the number of bytes actually processed on the call -- and below the bracket, return(0); to show an error.
You might also find it clarifies things if instead of having curl_output->size and curl_output_size you pick more distinct names...perhaps curl_output->len?