Question

I have declared some local variable in one function like this:

void* thread_function (void* parameter)
{
   struct parameter * thread_data = (struct parameter *)parameter;
   char buffer[20];
   int temp;
}

Here if I have created two threads then in one thread if buffer & temp is updated so will it effect other thread ?

i mean if there are two thread then does there will be two copy of all local variable?

EDIT : then in which case i need to used thread specific data.? i mean pthread_setspecific & all such stuff

Answer 1

These variables are allocated on the stack, and each thread has its own stack: these variables are private to each thread (they are not shared). (See this answer for more details.)

If you assign thread_data to a global pointer, for example, other threads will be able to access thread_data via the global pointer.

Thread specific data (e.g. pthread_setspecific) is used to create variables that are global, but still specific to each thread (not shared): They are thread-specific global variables.

You need to use thread specific variables when you want global variables, but don’t want to share them between threads.