Question

I read that threads share the memory address space of it’s parent thread.
If that is true , why can’t a thread function access a local variable belonging to it’s parent thread ?

void* PrintVar(void* arg){
   printf( "%d\n", a);
}

int main(int argc, char*argv[]) {
   int a;
   a = 10;
   pthread_t thr;
   pthread_create( &thr, NULL, PrintVar, NULL );

}

If the thread shares the address space , then the function PrintVar should have been able to print the value of variable a , right ?

I read this piece of info on http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html

Threads in the same process share:
Process instructions
Most data
open files (descriptors)
signals and signal handlers
current working directory
User and group id

If that is true, then why does int a not qualify as a shared variable ?

I’d like to see an example code where the file descriptors are shared

Answer 1

you couldn’t do that even if this were not a thread, because a is out of scope.

put a in the global scope, like so:

int a;
void* PrintVar(void* arg){
   printf( "%d\n", a);
}

int main(int argc, char*argv[]) {
   a = 10;
   pthread_t thr;
   pthread_create( &thr, NULL, PrintVar, NULL );

}

This is actually not an issue of threads. consider the following code:

void PrintVar(){
   printf( "%d\n", a);
}

int main(int argc, char*argv[]) {
   int a;
   a = 10;
   PrintVar();
}

This obviously won’t work, because the variable name a declared in main is not visible in PrintVar, because it’s in the local scope of another block. This is a compile-time problem, the compiler just doesn’t know what you mean by a when you mention it in PrintVar.

But there is also another threading issue. when the main thread of a process exit, all other threads are terminated (specifically, when any thread calls _exit, then all threads are terminated, and _start calls _exit after main returns). but your main returns immediately after invoking the other thread. To prevent this, you should call pthread_join which will wait for a thread to exit before returning. that’ll look like this

int a;
void* PrintVar(void* arg){
   printf( "%d\n", a);
}

int main(int argc, char*argv[]) {
   void *dummy;
   a = 10;

   pthread_t thr;
   pthread_create( &thr, NULL, PrintVar, NULL );
   pthread_join( thr, &dummy);
}