realloc(): invalid next size

Question

I'm having a problem with the realloc function. I'm using C only (so no vector) with LibCurl. The problem I'm having is that I'm getting the following error (realloc(): invalid next size) on the 12th iteration of the write_data function (the function I pass to Curl as a callback, it is called each time libcurl has some data to pass back (data is passed in chunks) ).

Trace:

-Removed-

Source:

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

char * Data; //stores the data
size_t  RunningSize;

int write_data( char *ptr, size_t size, size_t nmemb, void *stream )
{
    size_t ThisSize = (size * nmemb); //Stores the size of the data to be stored
    size_t DataLen = strlen( Data ); //length of the data so far

    RunningSize = (RunningSize + ThisSize ); //update running size (used as new size)

   Data = realloc( Data, RunningSize ); //get new mem location (on the 12th iteration, this fails)
   strcat( Data, ptr); //add data in ptr to Data

    return ThisSize; //the function must return the size of the data it received so cURL knows things went ok.
}

int main( )
{
  CURL *curl;
  CURLcode res;
  const char * UserAgent = "";

  Data = malloc(1); //so realloc will work
  RunningSize += 1;

  curl = curl_easy_init();
  if(curl)
  {
    curl_easy_setopt( curl, CURLOPT_NOBODY, 0 );
    curl_easy_setopt( curl, CURLOPT_URL, "http://www.google.co.uk/" );
    curl_easy_setopt( curl, CURLOPT_WRITEFUNCTION, write_data);
    curl_easy_setopt( curl, CURLOPT_USERAGENT, UserAgent );
    curl_easy_setopt( curl, CURLOPT_HEADER, 1 );

    //preform request.
    res = curl_easy_perform(curl);

    //output the data (debugging purposes)
    puts( Data );

    //cleanup
    curl_easy_cleanup(curl);
    free(Data);
  }

  return 0;
}

Thanks in advance,

Solution

The data passed in to write_data() isn't necessarily nul-terminated; that's why it tells you the number of bytes.

This means you can't use strcat(). Using it is running off the end of the array and corrupting the data structures used by malloc / realloc, hence the error.

Your write_data() should use memcpy() instead, like so:

int write_data( char *ptr, size_t size, size_t nmemb, void *stream )
{
    size_t ThisSize = (size * nmemb); //Stores the size of the data to be stored
    size_t DataLen = RunningSize; //length of the data so far

    RunningSize = (RunningSize + ThisSize ); //update running size (used as new size)

    Data = realloc( Data, RunningSize ); //get new mem location (on the 12th iteration, this fails)
    memcpy((char *)Data + DataLen, ptr, ThisSize); //add data in ptr to Data

    return ThisSize; //the function must return the size of the data it received so cURL knows things went ok.
}

You will also need to initialise RunningSize to 0, not 1. You can initialise Data to NULL - passing NULL to realloc() is allowed (and makes it behave just like malloc()).