Race conditions can occur even if only one thread is running a time. What matters is when one thread can interrupt another thread.

Consider two threads doing i+=1 in C. 

         i+=1

         CPU
----------+----------
 thread 1 | thread 2
==========+==========
 ...      |             T
 load i   |             i
 add  1   |             m
----------+----------   e
          | ...         |
          | load i      |
          | add  1      v
          | save i
          | ...
----------+----------
 save i   |
 ...      |

[download]

Even though both threads have run, and even though only one thread executed at a time, i was only incremented by one.

So my question remains. Is .= atomic (i.e. uninterruptable, safe) or does it introduce a race condition (like C's i+=1, requiring the use of locks)?

Update: Answered in Threads: why locking is required when using shared variables.

In reply to Re^4: using parallel processing to concatenate a string, where order of concatenation doesn't matter by ikegami
in thread using parallel processing to concatenate a string, where order of concatenation doesn't matter by tphyahoo

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