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

… OpenMP (#3929)

* determine the number of threads for OpenMP in opt parser

* add OpenMP library paths

* solve segfault when nprocs option is not specified in the command

* only change nprocs for C module

* update the description of the function, which does not return any value

* add helper function, left parser unchanged

* return threads rather than change the answer of nprocs

* modify code based on review comments

* remove threads > logic cores check, update based on review comments

include/grass/defs/gis.h

@@ -548,6 +548,9 @@ 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_set_omp_num_threads(struct Option *);
+
 /* 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 +1,45 @@
+#if defined(_OPENMP)
+#include <omp.h>
+#endif
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <grass/gis.h>
+#include <grass/glocale.h>
+
+/*! \brief Set the number of threads for OpenMP
+    The intended usage is at the beginning of a C tool when parameters are
+    processed, namely the G_OPT_M_NPROCS standard option.
+
+    \param opt A nprocs Option struct to specify the number of threads
+    \return the number of threads set up for OpenMP parallel computing
+*/
+
+int G_set_omp_num_threads(struct Option *opt)
+{
+    /* make sure Option is not null */
+    if (opt == NULL)
+        G_fatal_error(_("Option is NULL."));
+    else if (opt->key == NULL)
+        G_fatal_error(_("Option key is NULL."));
+
+    int threads = atoi(opt->answer);
+#if defined(_OPENMP)
+    int num_logic_procs = omp_get_num_procs();
+    if (threads < 1) {
+        threads += num_logic_procs;
+        threads = (threads < 1) ? 1 : threads;
+    }
+    omp_set_num_threads(threads);
+    G_verbose_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;
+}