# OpenMP

**OpenMP** is a shared-memory parallelism API for C, C++, and Fortran. Add a `#pragma omp` directive before a loop or block to parallelize it across available cores. The compiler handles thread creation and synchronization; if it doesn't support OpenMP, directives are silently ignored and the program runs serially, making debugging easy.

The execution model is **fork-join**: a single thread forks into a worker team at `#pragma omp parallel`, they execute concurrently, then join back. Thread count defaults to the number of logical cores, configurable via `OMP_NUM_THREADS=N` at runtime or `omp_set_num_threads(n)` in code. Compile with `-fopenmp` and include `<omp.h>` for the `omp_*` runtime functions.

```c
// compile: gcc -O2 -fopenmp -o sum sum.c
// run: OMP_NUM_THREADS=4 ./sum
// description: parallel reduction using a single pragma

#include <omp.h>
#include <stdio.h>

int main(void) {
    long n = 1000000000L, sum = 0;
    double t = omp_get_wtime();
    #pragma omp parallel for reduction(+:sum)
    for (long i = 0; i < n; i++)
        sum += i;
    printf("sum=%ld  time=%.2fs\n", sum, omp_get_wtime() - t);
    return 0;
}
```

On a 4-core machine this runs roughly 4× faster with `-fopenmp` than without. [[amdahls-law|Amdahl's law]] limits speedup when part of the program is inherently serial; reducing overhead like false sharing and load imbalance matters more than just adding threads.

## Concepts

 1. [[openmp-data-sharing|Data sharing]]
 2. [[openmp-parallel-loops|Parallel loops]]
 3. [[openmp-collapse|Collapse]]
 4. [[openmp-reduction|Reduction]]
 5. [[openmp-scheduling|Scheduling]]
 6. [[openmp-simd|SIMD]]
 7. [[openmp-tasks|Tasks]]
 8. [[openmp-single|Single]]
 9. [[openmp-master|Master]]
 10. [[openmp-sections|Sections]]
 11. [[openmp-barrier|Barrier]]
 12. [[openmp-nowait|Nowait]]
 13. [[openmp-critical-sections|Critical sections]]
 14. [[openmp-atomic|Atomic]]
 15. [[openmp-flush|Flush]]
 16. [[openmp-false-sharing|False sharing]]
 17. [[openmp-thread-affinity|Thread affinity]]
 18. [[openmp-time-measurement|Time measurement]]
 19. [[openmp-overview|Overview]]