for (OpenMP)
Causes the work done in a for loop inside a parallel region to be divided among threads.
#pragma omp [parallel] for [clauses]
for_statement
Remarks
where,
clause (optional)
Zero or more clauses. See the Remarks section for a list of the clauses supported by for.for_statement
A for loop. Undefined behavior will result if user code in the for loop changes the index variable.
Remarks
The for directive supports the following OpenMP clauses:
If parallel is also specified, clause can be any clause accepted by the parallel or for directives, except nowait.
For more information, see 2.4.1 for Construct.
Example
// omp_for.cpp
// compile with: /openmp
#include <stdio.h>
#include <math.h>
#include <omp.h>
#define NUM_THREADS 4
#define NUM_START 1
#define NUM_END 10
int main() {
int i, nRet = 0, nSum = 0, nStart = NUM_START, nEnd = NUM_END;
int nThreads = 0, nTmp = nStart + nEnd;
unsigned uTmp = (unsigned((abs(nStart - nEnd) + 1)) *
unsigned(abs(nTmp))) / 2;
int nSumCalc = uTmp;
if (nTmp < 0)
nSumCalc = -nSumCalc;
omp_set_num_threads(NUM_THREADS);
#pragma omp parallel default(none) private(i) shared(nSum, nThreads, nStart, nEnd)
{
#pragma omp master
nThreads = omp_get_num_threads();
#pragma omp for
for (i=nStart; i<=nEnd; ++i) {
#pragma omp atomic
nSum += i;
}
}
if (nThreads == NUM_THREADS) {
printf_s("%d OpenMP threads were used.\n", NUM_THREADS);
nRet = 0;
}
else {
printf_s("Expected %d OpenMP threads, but %d were used.\n",
NUM_THREADS, nThreads);
nRet = 1;
}
if (nSum != nSumCalc) {
printf_s("The sum of %d through %d should be %d, "
"but %d was reported!\n",
NUM_START, NUM_END, nSumCalc, nSum);
nRet = 1;
}
else
printf_s("The sum of %d through %d is %d\n",
NUM_START, NUM_END, nSum);
}
4 OpenMP threads were used. The sum of 1 through 10 is 55