Report reason and circumstances of fork failure

utest/test_post_fork.c

@@ -33,6 +33,7 @@ USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <errno.h>
 #include <cblas.h>
 #ifdef USE_OPENMP
 #include <omp.h>
@@ -114,7 +115,11 @@ exit(0);
 
         fork_pid = fork();
         if (fork_pid == -1) {
-            CTEST_ERR("Failed to fork process.");
+            perror("fork");
+            CTEST_ERR("Failed to fork subprocesses in a loop.");
+#ifdef USE_OPENMP
+            CTEST_ERR("Number of OpenMP threads was %d in this attempt.",i);
+#endif
         } else if (fork_pid == 0) {
             // Just pretend to do something, e.g. call `uname`, then exit
             exit(0);