In most cases the usage of multiple threads requires that some data are accessed from more than one thread. For simplicity let’s assume there are 3 global variables which can be accessed from different threads:
// Global data definition with initial values
int g_someCounter = 0;
double g_someValue = 0.0;
bool g_ready = false;Assume that the data are changed by some worker thread, which signals the end of his work by setting the ready flag.
// Thread changing global data
g_someCounter = 47;
g_someValue = 3.14;
g_ready = true;Some other thread may want to process the data if they are available:
// Thread using global data
if (g_ready)
{
myCounter += g_someCounter;
myValue = g_someValue * 2;
}This code has at least the following problems:
- possible: changed execution order
compiler may generate optimized code which first sets the ready flag and then changes the other values. As a consequence the consuming thread may work with wrong (or half written) data. - possible: caching strategy
changes to data may only be visible to one thread - data race
one thread writes, other thread reads or writes same memory location (e.g. double value g_someValue), result depends on random thread execution - race condition
same problem on a higher level, two threads work on shared data, result depends on random thread execution
For more infos about “race condition” and “data race” see blog.regehr.org