最近在做使用openssl鏈接http和https的項目，編譯時出現以下問題。
/usr/local/openssl/lib/libcrypto.a(async.o): In function `async_free_pool_internal':
async.c:(.text+0xe4): undefined reference to `pthread_setspecific'
async.c:(.text+0xf4): undefined reference to `pthread_getspecific'
async.c:(.text+0x104): undefined reference to `pthread_setspecific'
/usr/local/openssl/lib/libcrypto.a(async.o): In function `ASYNC_init_thread.part.1':
async.c:(.text+0x253): undefined reference to `pthread_setspecific'
/usr/local/openssl/lib/libcrypto.a(async.o): In function `async_start_func':
async.c:(.text+0x36f): undefined reference to `pthread_getspecific'
async.c:(.text+0x39e): undefined reference to `pthread_getspecific'
/usr/local/openssl/lib/libcrypto.a(async.o): In function `ASYNC_start_job':
async.c:(.text+0x404): undefined reference to `pthread_getspecific'
async.c:(.text+0x41e): undefined reference to `pthread_getspecific'
async.c:(.text+0x433): undefined reference to `pthread_getspecific'
/usr/local/openssl/lib/libcrypto.a(async.o):async.c:(.text+0x44c): more undefined references to `pthread_getspecific' follow
/usr/local/openssl/lib/libcrypto.a(async.o): In function `ASYNC_start_job':
async.c:(.text+0x7ef): undefined reference to `pthread_setspecific'
async.c:(.text+0x811): undefined reference to `pthread_getspecific'
async.c:(.text+0x82d): undefined reference to `pthread_getspecific'
async.c:(.text+0x83f): undefined reference to `pthread_getspecific'
async.c:(.text+0x870): undefined reference to `pthread_getspecific'
async.c:(.text+0x891): undefined reference to `pthread_getspecific'
/usr/local/openssl/lib/libcrypto.a(async.o):async.c:(.text+0x8a6): more undefined references to `pthread_getspecific' follow
/usr/local/openssl/lib/libcrypto.a(async_posix.o): In function `async_global_init':
async_posix.c:(.text+0xc): undefined reference to `pthread_key_create'
async_posix.c:(.text+0x1e): undefined reference to `pthread_key_create'
/usr/local/openssl/lib/libcrypto.a(async_posix.o): In function `async_local_init':
async_posix.c:(.text+0x3d): undefined reference to `pthread_setspecific'
async_posix.c:(.text+0x50): undefined reference to `pthread_setspecific'
/usr/local/openssl/lib/libcrypto.a(dso_dlfcn.o): In function `dlfcn_globallookup':
dso_dlfcn.c:(.text+0x21): undefined reference to `dlopen'
dso_dlfcn.c:(.text+0x34): undefined reference to `dlsym'
dso_dlfcn.c:(.text+0x3f): undefined reference to `dlclose'
/usr/local/openssl/lib/libcrypto.a(dso_dlfcn.o): In function `dlfcn_bind_func':
dso_dlfcn.c:(.text+0x354): undefined reference to `dlsym'
dso_dlfcn.c:(.text+0x3fb): undefined reference to `dlerror'
/usr/local/openssl/lib/libcrypto.a(dso_dlfcn.o): In function `dlfcn_bind_var':
dso_dlfcn.c:(.text+0x474): undefined reference to `dlsym'
dso_dlfcn.c:(.text+0x52e): undefined reference to `dlerror'
/usr/local/openssl/lib/libcrypto.a(dso_dlfcn.o): In function `dlfcn_load':
dso_dlfcn.c:(.text+0x5a4): undefined reference to `dlopen'
dso_dlfcn.c:(.text+0x60b): undefined reference to `dlclose'
dso_dlfcn.c:(.text+0x638): undefined reference to `dlerror'
/usr/local/openssl/lib/libcrypto.a(dso_dlfcn.o): In function `dlfcn_pathbyaddr':
dso_dlfcn.c:(.text+0x6ce): undefined reference to `dladdr'
dso_dlfcn.c:(.text+0x731): undefined reference to `dlerror'
/usr/local/openssl/lib/libcrypto.a(dso_dlfcn.o): In function `dlfcn_unload':
dso_dlfcn.c:(.text+0x78a): undefined reference to `dlclose'
解決方案：
gcc obj/Debug/main.o -L/usr/local/openssl/lib -lssl -lcrypto -ldl -lpthread -o main
原因分析：
在鏈接這個庫的時候一定要注意2個問題：
1.openssl庫的版本問題，請直接鏈到你需要的openssl庫路徑，比如我的就是/usr/local/openssl/lib,
2.注意-lssl -lcrypto要寫在-ldl -lpthread前面，這四個必須要。
相關知識：
-L ：此選項是指定鏈接庫的文件夾地址
-l（小寫的L）：-lssl 即是尋找/usr/local/openssl/lib目錄下的 libssl.a 這個庫，編譯器會自動尋找 libssl.a或者libssl.so
-I(大寫的i)：指定頭文件的文件夾地址
