realloc()：下一个大小无效

Question

我在使用 realloc 函数时遇到问题。我只使用 C（所以没有向量）和 LibCurl。我遇到的问题是，在 write_data 函数的第 12 次迭代中出现以下错误 (realloc(): invalid next size)（我作为回调传递给 Curl 的函数，每次 libcurl 都会调用它）一些要传回的数据（数据以块的形式传递）。

跟踪：-

已删除-

来源：

#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;
}

提前致谢，

Answer 1

传递给 write_data() 的数据不一定以 null 结尾；这就是为什么它告诉您字节数。

这意味着您无法使用 strcat()。使用它会超出数组的末尾并破坏 malloc / realloc 使用的数据结构，因此会出现错误。

您的 write_data() 应该使用 memcpy() 代替，如下所示：

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.
}

您还需要将 RunningSize 初始化为 0，而不是 1。您可以将 Data 初始化为 NULL - 允许将 NULL 传递给 realloc() （并使其行为类似于 >malloc())。