Undefined reference to 'pthread _ create'

As a beginner in Linux, I am only getting started with thread programming. According to an example in the gun/Linux programming guide, the following error occurs:
Undefined reference to 'pthread _ create'
Undefined reference to 'pthread _ join'

Cause:
 The pthread library is not the default library in Linux. You must use the static library libpthread. A for connection. Therefore, you must use pthread_create () to create a thread and call the pthread_atfork () function to create a fork for processing.ProgramYou need to link the library.

Solution:
Add the-lpthread parameter to the compilation.
GCC thread. C-o thread-lpthread
Thread. C is your source file. Do not forget to add the header file # include <pthread. h>