Does local variable in thread function have separe copy according to thread?

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

It's not that each thread has its own copy, it's that each instance of a function invocation has its own copy of all automatic (i.e. local non-static) variables, regardless of whether the instances are in the same thread or different threads. This is true if the instances come into existence due to invocation in different threads, recursive invocation, mutual/indirect recursion, or even invocation from an asynchronous signal handler. Note that while the C standard does not specify threads, the relevant section in the standard is probably 5.2.3 Signals and interrupts:

Functions shall be implemented such that they may be interrupted at any time by a signal, or may be called by a signal handler, or both, with no alteration to earlier, but still active, invocations' control flow (after the interruption), function return values, or objects with automatic storage duration. All such objects shall be maintained outside the function image (the instructions that compose the executable representation of a function) on a per-invocation basis.

This makes it explicit that each invocation must have its own storage for automatic variables.

Answer 2

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.

Answer 3

Local variables are stored in stack memory, which is private to a thread.

Therefore they are not shared between threads: there will be an independent copy of each variable in each thread

Update Whether you would want to share data between threads really boils down to a design question; What are your threads doing? Are their effort co-ordinated or are they simply workers processing a queue.

The main thing to consider is synchronization of shared data. Variables that are shared between threads are variables that can change value unexpectedly (within a single thread) and so need to be treated as such. I would suggest that you err on the side of not sharing, unless you have a specific reason to do so.