curl 库的使用

#include 
#include 
#include 

#include 
#include 
#include 

struct MemoryStruct {
  char *memory;
  size_t size;
};

static void *myrealloc(void *ptr, size_t size);

static void *myrealloc(void *ptr, size_t size)
{
  /* There might be a realloc() out there that doesn't like reallocing
     NULL pointers, so we take care of it here */
  if(ptr)
    return realloc(ptr, size);
  else
    return malloc(size);
}

static size_t
WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
{
  size_t realsize = size * nmemb;
  struct MemoryStruct *mem = (struct MemoryStruct *)data;

  mem->memory = myrealloc(mem->memory, mem->size + realsize + 1);
  if (mem->memory) {
    memcpy(&(mem->memory[mem->size]), ptr, realsize);
    mem->size += realsize;
    mem->memory[mem->size] = 0;
  }
  return realsize;
}

int main(int argc, char **argv)
{
  CURL *curl_handle;

  struct MemoryStruct chunk;

  chunk.memory=NULL; /* we expect realloc(NULL, size) to work */
  chunk.size = 0;    /* no data at this point */

  curl_global_init(CURL_GLOBAL_ALL);

  /* init the curl session */
  curl_handle = curl_easy_init();

  /* specify URL to get */
  curl_easy_setopt(curl_handle, CURLOPT_URL, "http://cool.haxx.se/");

  /* send all data to this function  */
  curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);

  /* we pass our 'chunk' struct to the callback function */
  curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, (void *)&chunk);

  /* some servers don't like requests that are made without a user-agent
     field, so we provide one */
  curl_easy_setopt(curl_handle, CURLOPT_USERAGENT, "libcurl-agent/1.0");

  /* get it! */
  curl_easy_perform(curl_handle);

  /* cleanup curl stuff */
  curl_easy_cleanup(curl_handle);

  /*
   * Now, our chunk.memory points to a memory block that is chunk.size
   * bytes big and contains the remote file.
   *
   * Do something nice with it!
   *
   * You should be aware of the fact that at this point we might have an
   * allocated data block, and nothing has yet deallocated that data. So when
   * you're done with it, you should free() it as a nice application.
   */

  if(chunk.memory)
    free(chunk.memory);

  /* we're done with libcurl, so clean it up */
  curl_global_cleanup();

  return 0;
}

1.先贴上官方的example.如上

2.多线程中的使用:

在程序最开始的地方 curl_global_init， 然后再创建各个线程，在各个线程中用 curl_easy_init来创建curl 的 handle,然后针对这个handle用 curl_easy_setopt(handle, xxx, xxx) 设置传输相关的各种参数，最后用 cur_easy_perform(handle)来执行动作，传输结束后，最后调用 curl_easy_cleanup(handle) 来释放，然后结束线程.在程序的主线程中，等所有线程都结束后，在程序退出前，调用 curl_global_cleanup来结束 libcurl

1. 如果不加限定的取消线程curl相关函数中会有内存泄露的问题

    解决方法：设置线程当进入某个函数时该函数不可取消

2. 对于libcurl来说，如果不可取消，将会导致下载过程一直持续下去，为了避免线程已经取消，而curl函数不退出的情况：

    解决方法：配置curl参数，当下载速度过低时，就放弃继续执行

3. 连接未建立时(即完全无网络的情况下), 1的解决方法将导致函数一直执行完才能退出，一般来说是到CURLOPT_TIMEOUT时间才会退出函数

    解决方法：CURLOP_CONNECTTIMEOUT配置连接超时函数，一旦连接超时，就退出函数

3.一些设置

curl_easy_setopt(m_pCurl, CURLOPT_HTTPHEADER, chunk);    
                curl_easy_setopt(m_pCurl, CURLOPT_TIMEOUT, 120);
                curl_easy_setopt(m_pCurl, CURLOPT_URL, pURL);
                curl_easy_setopt(m_pCurl, CURLOPT_WRITEFUNCTION, Url_IconWrite);
                curl_easy_setopt(m_pCurl, CURLOPT_WRITEDATA, &m_HtmlBuff);