openMP increment add int among threads? - c++

Question

Hye
I'm trying to multithread the function below. I fail to get the counter to be properly shared among OpenMP threads, I tried atomic and int, atomic seem to not be working, neither do INT. Not sure, I'm lost, how can I solve this?
std::vector<myStruct> _myData(100);
int counter;
counter =0
int index;
#pragma omp parallel for private(index)
for (index = 0; index < 500; ++index) {
if (data[index].type == "xx") {
myStruct s;
s.innerData = data[index].rawData
processDataA(s); // processDataA(myStruct &data)
processDataB(s);
_myData[counter++] = s; // each thread should have unique int not going over 100 of initially allocated items in _myData
}
}
Edit. Update bad syntax/missing parts

Answer 1

If you cannot use OpenMP atomic capture, I would try:
std::vector<myStruct> _myData(100);
int counter = 0;
#pragma omp parallel for schedule(dynamic)
for (int index = 0; index < 500; ++index) {
if (data[index].type == "xx") {
myStruct s;
s.innerData = data[index].rawData
processDataA(s);
processDataB(s);
int temp;
#pragma omp critical
temp = counter++;
assert(temp < _myData.size());
_myData[temp] = s;
}
}
Or:
#pragma omp parallel for schedule(dynamic,c)
and experiment with chunk size c.
However, atomics would be likely more efficient than critical sections. There should be some form of atomics supported by your compiler.
Note that your solution is kind of fragile, since it works only if the condition inside the loop is evaluated to true less than 101x. That's why I added assertion into the code. Maybe a better solution:
std::vector<myStruct> _myData;
size_t size = 0;
#pragma omp parallel for reduction(+,size)
for (int index = 0; index < data.size(); ++index)
if (data[index].type == "xx") size++;
v.resize(size);
...
Then, you don't need to care about the vector size and also don't waste memory space.