线程使用libcurl的巨坑踩了一天！！！_在多线程下使用 curl 踩坑

用libcurl一段时间遇到莫名其妙的程序崩溃的情况，开始觉得是线程栈溢出导致的段错误，专门增加了线程栈的大小，还是有问题。线程也是分离的。用valgrind定位到问题可能出现在curl的调用上。

排查的时候也发现了libcurl一些额外的坑，现做个总结笔记。

1.、

线程使用libcurl访问时，设置了超时时间，而libcurl库不会为这个超时信号做任何处理，信号产生而没有信号句柄处理，可能导致程序退出。用以下选项禁止访问超时的时候抛出超时信号。

curl_setopt(curl, CURLOPT_NOSIGNAL,1L);

2、

正常使用流程是先调用curl_global_init初始化资源，而这个函数不是线程安全的。curl_easy_init发现没有做初始化时会自动初始化，多个线程同时进行curl_easy_init时会导致异常。curl官方建议在主线程中先做初始化。

curl_global_init(CURL_GLOBAL_ALL); 

3、

用valgrind调试，做压力测试多线程运行时，valgrind弹出类似访问冲突的错误，把错误定位到libcurl的互斥访问的处理上。

大致是进行访问设计到ssl的东西的时候，需要做相应的加锁处理，libcurl源码没有加这块处理，不过给了个针对这种情况加锁的例子，总的来说curl的示例代码很重要很有用。

https://curl.haxx.se/libcurl/c/example.html

#define USE_OPENSSL  





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#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; }

未完待续。。。