lib/gis: Add a helper function to determine the number of threads for OpenMP

include/grass/defs/gis.h

@@ -545,6 +545,9 @@ int G_name_is_fully_qualified(const char *, char *, char *);
 char *G_fully_qualified_name(const char *, const char *);
 int G_unqualified_name(const char *, const char *, char *, char *);
 
+/* omp_threads.c */
+int G_setup_threads(char *);
+
 /* open.c */
 int G_open_new(const char *, const char *);
 int G_open_old(const char *, const char *, const char *);

lib/gis/Makefile

@@ -2,8 +2,9 @@ MODULE_TOPDIR = ../..
 
 LIB = GIS
 
-EXTRA_INC = $(ZLIBINCPATH) $(BZIP2INCPATH) $(ZSTDINCPATH) $(PTHREADINCPATH) $(REGEXINCPATH)
-EXTRA_CFLAGS = -DGRASS_VERSION_DATE=\"'$(GRASS_VERSION_DATE)'\"
+LIBES = $(OPENMP_LIBPATH) $(OPENMP_LIB)
+EXTRA_INC = $(ZLIBINCPATH) $(BZIP2INCPATH) $(ZSTDINCPATH) $(PTHREADINCPATH) $(REGEXINCPATH) $(OPENMP_INCPATH)
+EXTRA_CFLAGS = $(OPENMP_CFLAGS) -DGRASS_VERSION_DATE=\"'$(GRASS_VERSION_DATE)'\"
 
 PROJSRC = ellipse.table ellipse.table.solar.system datum.table \
 	datumtransform.table FIPS.code state27 state83 projections

lib/gis/omp_threads.c

@@ -0,0 +1,44 @@
+#if defined(_OPENMP)
+#include <omp.h>
+#endif
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <grass/gis.h>
+#include <grass/glocale.h>
+
+/*! \brief A helper function to setup the number of threads for C modules
+   supported by OpenMP
+   \param nprocs the number of threads specified by the user
+   \return the number of threads set up for OpenMP parallel computing
+ */
+
+int G_setup_threads(char *nprocs)
+{
+    int threads = atoi(nprocs);
+#if defined(_OPENMP)
+    int max_cores = omp_get_num_procs();
+    if (threads > max_cores) {
+        G_warning(
+            _("The number of threads specified is greater than the "
+              "number of processors available (%d). The number of threads "
+              "will be set to %d."),
+            max_cores, max_cores);
+        threads = max_cores;
+    }
+    else if (threads < 1) {
+        threads += max_cores;
+        threads = (threads < 1) ? 1 : threads;
+    }
+    omp_set_num_threads(threads);
+    G_message(_("%d threads are set up for parallel computing."), threads);
+#else
+    if (threads != 1) {
+        G_warning(_("GRASS GIS is not compiled with OpenMP support, parallel "
+                    "computation is disabled. Only one thread will be used."));
+        threads = 1;
+    }
+#endif
+
+    return threads;
+}