libcurl 多线程使用注意事项（补充）——HTTPS,openssl多线程使用加锁

问题

多线程libcurl运行一段时间后出现崩掉，没有确定的点，没有确定的URL。一直查看源代码没有问题，最后通过debug跟踪发现是在访问SSL的时候出现的crash。

才想起来openssl是不支持多线程的，要自己做加锁处理。而且libcurl中并没有支持相关的加锁操作。

解决办法：

在初始化libcurl的时候为openssl创建一个互斥锁函数，一个回调函数传给openss

openssl锁l函数原形 ：void (* func )(int ,int , const char * ,int)

设置方式：CRYPTO_set_locking_callback(void (* func )(int ,int , const char * ,int));

设置这样一个函数还不够，另外还要配置一个锁id回调函数，这个可以参考openssl多线程下的使用相关。

id函数原形：unsigned int (*func)(void)

设置方式：CRYPTO_set_id_callback(unsigned int (*func)(void));

通过这两个设置就可以解决HTTPS多线程请求出现crash的问题。

代码示例：

下面是引用了libcurl示例的一个代码

最关键就是，两个callback的实现，还有初始化锁（init_locks）和释放锁(kill_locks)的位置

#define USE_OPENSSL  
 
#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;
 
  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);
}
#endif
 
#ifdef USE_GNUTLS
#include <gcrypt.h>
#include <errno.h>
 
GCRY_THREAD_OPTION_PTHREAD_IMPL;
 
void init_locks(void)
{
  gcry_control(GCRYCTL_SET_THREAD_CBS);
}
 
#define kill_locks()
#endif
 
/* 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();
  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;
  int error;
  (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();
 
  for(i=0; i< NUMT; i++) {
    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++) {
    error = pthread_join(tid[i], NULL);
    fprintf(stderr, "Thread %d terminated\n", i);
  }
 
  kill_locks();
 
  return 0;
}