diff --git a/cmake_config.h.in b/cmake_config.h.in
index 8a94219163d..cfd26d77601 100644
--- a/cmake_config.h.in
+++ b/cmake_config.h.in
@@ -5,6 +5,8 @@
  */
 #ifndef _PIO_CONFIG_
 #define _PIO_CONFIG_
+/* Set to avoid warning in intel19 compiler wrt strnlen */
+#define _GNU_SOURCE
 
 /** The major part of the version number. */
 #define PIO_VERSION_MAJOR @VERSION_MAJOR@
diff --git a/configure.ac b/configure.ac
index 567c5e40f6e..6964e7f9639 100644
--- a/configure.ac
+++ b/configure.ac
@@ -156,6 +156,9 @@ AC_DEFINE([INCLUDE_CMAKE_FCI], [1], [defined by CMake build])
 # All builds are on LINUX.
 AC_DEFINE([LINUX], [1], [defined by CMake build])
 
+# Define to solve intel compiler warning.
+AC_DEFINE([_GNU_SOURCE], [1], [solve strnlen declared implicitly warning on intel compiler])
+
 # Check for netCDF library.
 AC_CHECK_LIB([netcdf], [nc_create], [], [AC_MSG_ERROR([Can't find or link to the netcdf library.])])
 
diff --git a/tests/cunit/test_iosystem3.c b/tests/cunit/test_iosystem3.c
index 6470a91c16f..1c91b00c5c4 100644
--- a/tests/cunit/test_iosystem3.c
+++ b/tests/cunit/test_iosystem3.c
@@ -158,7 +158,7 @@ int main(int argc, char **argv)
       int num_flavors; /* Number of PIO netCDF flavors in this build. */
       int flavor[NUM_FLAVORS]; /* iotypes for the supported netCDF IO flavors. */
       int rearranger[NUM_REARRANGERS] = {PIO_REARR_BOX, PIO_REARR_SUBSET};
-      
+
         /* Figure out iotypes. */
         if ((ret = get_iotypes(&num_flavors, flavor)))
             ERR(ret);
@@ -314,7 +314,7 @@ int main(int argc, char **argv)
                     ERR(ret);
 
             } /* next iotype */
-        
+
             /* Finalize PIO systems. */
             if (even_comm != MPI_COMM_NULL)
                 if ((ret = PIOc_free_iosystem(even_iosysid)))