多线程的libcurl的使用

摘要：libcurl在多线程中，采用https访问，经常运行一段时间，会出现crash。

libcurl的在多线程中的使用特别注意的有两点：

1. curl的句柄不能多线程共享。

2. ssl访问时， openssl是线程不安全的。

知道了这两点，就能解决libcurl无故crash的问题了。

第一点：每个线程初始化一个句柄，供这个线程使用。

第二点：需要添加回调函数，进行线程锁。

参考代码如下：

#include <stdio.h>
#include <pthread.h>
#include <curl/curl.h>

#define NUMT 4

/* we have this global to let the callback get easy access to it */
static pthread_mutex_t *lockarray;

#ifdef USE_OPENSSL
#include <openssl/crypto.h>
static void lock_callback(int mode, int type, char *file, int line)
{
(void)file;
(void)line;
if(mode & CRYPTO_LOCK) {
pthread_mutex_lock(&(lockarray[type]));
}
else {
pthread_mutex_unlock(&(lockarray[type]));
}
}

static unsigned long thread_id(void)
{
unsigned long ret;

ret = (unsigned long)pthread_self();
return ret;
}

static void init_locks(void)
{
  int i;
  printf("crypto num is %d\r\n", CRYPTO_num_locks());
  lockarray = (pthread_mutex_t *)OPENSSL_malloc(CRYPTO_num_locks() *
  sizeof(pthread_mutex_t));
  for(i = 0; i<CRYPTO_num_locks(); i++) {
  pthread_mutex_init(&(lockarray[i]), NULL);
}

CRYPTO_set_id_callback((unsigned long (*)())thread_id);
CRYPTO_set_locking_callback((void (*)())lock_callback);
}

static void kill_locks(void)
{
  int i;

  CRYPTO_set_locking_callback(NULL);
  for(i = 0; i<CRYPTO_num_locks(); i++)
    pthread_mutex_destroy(&(lockarray[i]));

  OPENSSL_free(lockarray);
}

/* List of URLs to fetch.*/
const char * const urls[]= {
  "https://www.example.com/",
  "https://www2.example.com/",
  "https://www3.example.com/",
  "https://www4.example.com/",
};

static void *pull_one_url(void *url)
{
  CURL *curl;

  curl = curl_easy_init();//每个线程初始化一个句柄，绝对不能混用。
  curl_easy_setopt(curl, CURLOPT_URL, url);
  /* this example doesn't verify the server's certificate, which means we
     might be downloading stuff from an impostor */
  curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L);
  curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0L);
  curl_easy_perform(curl); /* ignores error */
  curl_easy_cleanup(curl);

  return NULL;
}

int main(int argc, char **argv)
{
  pthread_t tid[NUMT];
  int i;
  (void)argc; /* we don't use any arguments in this example */
  (void)argv;

  /* Must initialize libcurl before any threads are started */
  curl_global_init(CURL_GLOBAL_ALL);

  init_locks();//初始化openssl的回调函数，加载线程锁

  for(i = 0; i< NUMT; i++) {
    int error = pthread_create(&tid[i],
                               NULL, /* default attributes please */
                               pull_one_url,
                               (void *)urls[i]);
    if(0 != error)
      fprintf(stderr, "Couldn't run thread number %d, errno %d\n", i, error);
    else
      fprintf(stderr, "Thread %d, gets %s\n", i, urls[i]);
  }

  /* now wait for all threads to terminate */

  for(i = 0; i< NUMT; i++) {
    pthread_join(tid[i], NULL);
    fprintf(stderr, "Thread %d terminated\n", i);
  }

  kill_locks();//销毁openssl线程锁

  return 0;
}