Merge pull request #431 from lu1and10/omp-warning

Enforce nthreads = 1 for single threaded lib

Author: lu1and10

src/finufft.cpp

@@ -591,11 +591,16 @@ int FINUFFT_MAKEPLAN(int type, int dim, BIGINT* n_modes, int iflag,
   p->fftSign = (iflag>=0) ? 1 : -1;         // clean up flag input
 
   // choose overall # threads...
-  int nthr = MY_OMP_GET_MAX_THREADS();      // use as many as OMP gives us
-  if (p->opts.nthreads>0)
-    nthr = p->opts.nthreads;                // user override (no limit or check)
+  int maxnthr = MY_OMP_GET_MAX_THREADS();
+  int nthr = maxnthr;                       // use as many as OMP gives us
+  if (p->opts.nthreads>0) {
+    nthr = min(maxnthr,p->opts.nthreads);   // user override up to max avail
+    if (p->opts.nthreads > maxnthr)         // if no OMP, maxnthr=1
+      fprintf(stderr,"%s warning: user requested %d threads, but only %d threads available; enforcing nthreads=%d.\n",__func__,p->opts.nthreads,maxnthr,nthr);
+  }
   p->opts.nthreads = nthr;                  // store actual # thr planned for
-
+  // (this sets all downstream spread/interp, 1dkernel, and FFT thread counts)
+  
   // choose batchSize for types 1,2 or 3... (uses int ceil(b/a)=1+(b-1)/a trick)
   if (p->opts.maxbatchsize==0) {            // logic to auto-set best batchsize
     p->nbatch = 1+(ntrans-1)/nthr;          // min # batches poss