C++ pthread, two threads read a global variable

Question

If there are two threads that just read a global variable, is it necessary to use mutex to lock and unlock the global variable?

Answer 1

If the threads are only reading the variable and nobody is writing to it (not one of the threads, not someone else), then you're perfectly fine without locks. If any concurrent modification could happen, then everyone (including pure readers) must be synchronised somehow - by a mutex, a read/write lock or in some other way.

Answer 2

In general, exclusive access is required to prevent one from seeing an inconsistent state. For a reader thread, this means avoiding partial reads.

What does that mean ? Imagine that you have a value stored on two (atomic) integers, for example coordinates.

int i = 3;
int j = 4;

Now, we are going to read i and j whilst they undergo modification, more precisely when a Writer thread want to move in a diagonal fashion from (3, 4) to (4, 5):

Reader     Writer
  |          |
  |        i = 4
  |          |
i = 4     <pause>
j = 4        |
  |        j = 5
  |          |

This is called a partial read: the Reader thread has gotten information that the object is at (4, 4) even though it was never there. I'll let you think what occurs if those coordinates are used to compute the trajectory of a plane...

Avoiding partial reads however is fairly easy: mutations should be seen atomically.

• if there is no mutation (for the duration of the reads) then you can just read
• if there are mutations, you need a mutual exclusion mechanism (such as a reader/writer mutex)

And thus, to answer your question, if the only accesses are read accesses, the no synchronization is required. But if you sometimes (even infrequently) modify the information read, then you need some mechanism.

Answer 3

If they're just reading, then you don't need locking.