[SOLVED] Is it necessary to mutex lock here?(C++)

vendtagain · 08-12-2011, 11:27 PM

I am trying to better understand threads and mutexes.
I have two threads. If both threads only Read from a variable (and do not change it), is a mutex needed?

In that situation, is a mutex needed to prevent undefined behavior?

A simple example:

Code:

struct point
{
float x,y,z;
};

point ppp;

void firstfunc()
{
       --ppp.x;
       --ppp.y;
       --ppp.z;
}
void secondfunc()
{
       ++ppp.x;
       ++ppp.y;
       ++ppp.z;
}

main()
{
thread t1(firstfunc);
thread t2(secondfunc):
t1.join();
t2.join();
}

ta0kira · 08-13-2011, 12:04 AM

Quote:

Originally Posted by vendtagain

If both threads only Read from a variable (and do not change it), is a mutex needed?

This doesn't appear to correspond to the code in your post. Both threads modify ppp. You shouldn't need a mutex in situations where nothing in the program changes the data being read, but in the example you posted you do need a mutex.
Kevin Barry

vendtagain · 08-14-2011, 12:20 AM

Oops, I changed the question a few times before posting and forgot to update the code, I did mean read only in both threads. Thanks.