diff --git a/unittest/c-library/test_library_properties.cpp b/unittest/c-library/test_library_properties.cpp
index 754c6df77f..192c6897e9 100644
--- a/unittest/c-library/test_library_properties.cpp
+++ b/unittest/c-library/test_library_properties.cpp
@@ -3,6 +3,7 @@
 #include "lammps.h"
 #include "library.h"
 #include "lmptype.h"
+#include "platform.h"
 #include <string>
 
 #include "gmock/gmock.h"
@@ -13,6 +14,7 @@
 #define STRINGIFY(val) XSTR(val)
 #define XSTR(val) #val
 
+using ::LAMMPS_NS::platform::path_join;
 using ::LAMMPS_NS::tagint;
 using ::testing::HasSubstr;
 using ::testing::StartsWith;
@@ -82,7 +84,7 @@ TEST_F(LibraryProperties, get_mpi_comm)
 TEST_F(LibraryProperties, natoms)
 {
     if (!lammps_has_style(lmp, "atom", "full")) GTEST_SKIP();
-    std::string input = INPUT_DIR + PATH_SEP + "in.fourmol";
+    std::string input = path_join(INPUT_DIR, "in.fourmol");
     if (!verbose) ::testing::internal::CaptureStdout();
     lammps_file(lmp, input.c_str());
     if (!verbose) ::testing::internal::GetCapturedStdout();
@@ -92,7 +94,7 @@ TEST_F(LibraryProperties, natoms)
 TEST_F(LibraryProperties, thermo)
 {
     if (!lammps_has_style(lmp, "atom", "full")) GTEST_SKIP();
-    std::string input = INPUT_DIR + PATH_SEP + "in.fourmol";
+    std::string input = path_join(INPUT_DIR, "in.fourmol");
     ::testing::internal::CaptureStdout();
     lammps_file(lmp, input.c_str());
     lammps_command(lmp, "run 2 post no");
@@ -108,7 +110,7 @@ TEST_F(LibraryProperties, thermo)
 TEST_F(LibraryProperties, box)
 {
     if (!lammps_has_style(lmp, "atom", "full")) GTEST_SKIP();
-    std::string input = INPUT_DIR + PATH_SEP + "in.fourmol";
+    std::string input = path_join(INPUT_DIR, "in.fourmol");
     ::testing::internal::CaptureStdout();
     lammps_file(lmp, input.c_str());
     lammps_command(lmp, "run 2 post no");
@@ -248,7 +250,7 @@ TEST_F(LibraryProperties, setting)
     EXPECT_EQ(lammps_extract_setting(lmp, "UNKNOWN"), -1);
 
     if (lammps_has_style(lmp, "atom", "full")) {
-        std::string input = INPUT_DIR + PATH_SEP + "in.fourmol";
+        std::string input = path_join(INPUT_DIR, "in.fourmol");
         if (!verbose) ::testing::internal::CaptureStdout();
         lammps_file(lmp, input.c_str());
         lammps_command(lmp, "run 2 post no");
@@ -289,7 +291,7 @@ TEST_F(LibraryProperties, global)
 {
     if (!lammps_has_style(lmp, "atom", "full")) GTEST_SKIP();
 
-    std::string input = INPUT_DIR + PATH_SEP + "in.fourmol";
+    std::string input = path_join(INPUT_DIR, "in.fourmol");
     if (!verbose) ::testing::internal::CaptureStdout();
     lammps_file(lmp, input.c_str());
     lammps_command(lmp, "run 2 post no");
diff --git a/unittest/c-library/test_library_scatter_gather.cpp b/unittest/c-library/test_library_scatter_gather.cpp
index 7ee1cfc9eb..6144516ac2 100644
--- a/unittest/c-library/test_library_scatter_gather.cpp
+++ b/unittest/c-library/test_library_scatter_gather.cpp
@@ -3,6 +3,7 @@
 #include "lammps.h"
 #include "library.h"
 #include "lmptype.h"
+#include "platform.h"
 #include <string>
 
 #include "gmock/gmock.h"
@@ -13,6 +14,7 @@
 #define STRINGIFY(val) XSTR(val)
 #define XSTR(val) #val
 
+using ::LAMMPS_NS::platform::path_join;
 using ::LAMMPS_NS::bigint;
 using ::LAMMPS_NS::tagint;
 using ::testing::HasSubstr;
@@ -55,7 +57,7 @@ protected:
 TEST_F(GatherProperties, gather_bonds_newton_on)
 {
     if (!lammps_has_style(lmp, "atom", "full")) GTEST_SKIP();
-    std::string input = INPUT_DIR + PATH_SEP + "in.fourmol";
+    std::string input = path_join(INPUT_DIR, "in.fourmol");
     if (!verbose) ::testing::internal::CaptureStdout();
     lammps_command(lmp, "newton on on");
     lammps_file(lmp, input.c_str());
@@ -95,7 +97,7 @@ TEST_F(GatherProperties, gather_bonds_newton_on)
 TEST_F(GatherProperties, gather_bonds_newton_off)
 {
     if (!lammps_has_style(lmp, "atom", "full")) GTEST_SKIP();
-    std::string input = INPUT_DIR + PATH_SEP + "in.fourmol";
+    std::string input = path_join(INPUT_DIR, "in.fourmol");
     if (!verbose) ::testing::internal::CaptureStdout();
     lammps_command(lmp, "newton off off");
     lammps_file(lmp, input.c_str());
diff --git a/unittest/c-library/test_main.h b/unittest/c-library/test_main.h
index 567b913d27..9a719e4b88 100644
--- a/unittest/c-library/test_main.h
+++ b/unittest/c-library/test_main.h
@@ -25,9 +25,3 @@ extern bool verbose;
     } while (0);
 
 #endif
-
-#if defined _WIN32
-static const char PATH_SEP = '\\';
-#else
-static const char PATH_SEP = '/';
-#endif
diff --git a/unittest/force-styles/test_angle_style.cpp b/unittest/force-styles/test_angle_style.cpp
index ca0573840e..002ea58a24 100644
--- a/unittest/force-styles/test_angle_style.cpp
+++ b/unittest/force-styles/test_angle_style.cpp
@@ -108,7 +108,7 @@ LAMMPS *init_lammps(int argc, char **argv, const TestConfig &cfg, const bool new
         command(pre_command);
     }
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     parse_input_script(input_file);
 
     command("angle_style " + cfg.angle_style);
@@ -194,7 +194,7 @@ void data_lammps(LAMMPS *lmp, const TestConfig &cfg)
     command("variable angle_style index '" + cfg.angle_style + "'");
     command("variable data_file index " + cfg.basename + ".data");
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     parse_input_script(input_file);
 
     for (auto &angle_coeff : cfg.angle_coeff) {
diff --git a/unittest/force-styles/test_bond_style.cpp b/unittest/force-styles/test_bond_style.cpp
index a8f5ba3044..b913c121e1 100644
--- a/unittest/force-styles/test_bond_style.cpp
+++ b/unittest/force-styles/test_bond_style.cpp
@@ -108,7 +108,7 @@ LAMMPS *init_lammps(int argc, char **argv, const TestConfig &cfg, const bool new
         command(pre_command);
     }
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     parse_input_script(input_file);
 
     command("bond_style " + cfg.bond_style);
@@ -194,7 +194,7 @@ void data_lammps(LAMMPS *lmp, const TestConfig &cfg)
     command("variable bond_style index '" + cfg.bond_style + "'");
     command("variable data_file index " + cfg.basename + ".data");
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     parse_input_script(input_file);
 
     for (auto &bond_coeff : cfg.bond_coeff) {
diff --git a/unittest/force-styles/test_dihedral_style.cpp b/unittest/force-styles/test_dihedral_style.cpp
index e26b066485..c98f97d260 100644
--- a/unittest/force-styles/test_dihedral_style.cpp
+++ b/unittest/force-styles/test_dihedral_style.cpp
@@ -108,7 +108,7 @@ LAMMPS *init_lammps(int argc, char **argv, const TestConfig &cfg, const bool new
         command(pre_command);
     }
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     parse_input_script(input_file);
 
     command("dihedral_style " + cfg.dihedral_style);
@@ -203,7 +203,7 @@ void data_lammps(LAMMPS *lmp, const TestConfig &cfg)
         command("variable pair_style index 'lj/charmmfsw/coul/charmmfsh 7.0 8.0'");
     }
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     parse_input_script(input_file);
 
     for (auto &dihedral_coeff : cfg.dihedral_coeff) {
diff --git a/unittest/force-styles/test_fix_timestep.cpp b/unittest/force-styles/test_fix_timestep.cpp
index 8c6ffeb617..6a8ccdb948 100644
--- a/unittest/force-styles/test_fix_timestep.cpp
+++ b/unittest/force-styles/test_fix_timestep.cpp
@@ -99,7 +99,7 @@ LAMMPS *init_lammps(int argc, char **argv, const TestConfig &cfg, const bool use
     for (auto &pre_command : cfg.pre_commands)
         command(pre_command);
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     lmp->input->file(input_file.c_str());
 
     if (use_respa) command("run_style respa 2 1 bond 1 pair 2");
diff --git a/unittest/force-styles/test_improper_style.cpp b/unittest/force-styles/test_improper_style.cpp
index 749f8be53f..cef703d614 100644
--- a/unittest/force-styles/test_improper_style.cpp
+++ b/unittest/force-styles/test_improper_style.cpp
@@ -108,7 +108,7 @@ LAMMPS *init_lammps(int argc, char **argv, const TestConfig &cfg, const bool new
         command(pre_command);
     }
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     parse_input_script(input_file);
 
     command("improper_style " + cfg.improper_style);
@@ -194,7 +194,7 @@ void data_lammps(LAMMPS *lmp, const TestConfig &cfg)
     command("variable improper_style index '" + cfg.improper_style + "'");
     command("variable data_file index " + cfg.basename + ".data");
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     parse_input_script(input_file);
 
     for (auto &improper_coeff : cfg.improper_coeff) {
diff --git a/unittest/force-styles/test_main.h b/unittest/force-styles/test_main.h
index 4fe0422318..f940f7dd3c 100644
--- a/unittest/force-styles/test_main.h
+++ b/unittest/force-styles/test_main.h
@@ -36,10 +36,4 @@ void write_yaml_header(class YamlWriter *writer, TestConfig *cfg, const char *ve
 
 void EXPECT_STRESS(const std::string & name, double * stress, const stress_t & expected_stress, double epsilon);
 
-#if defined _WIN32
-static const char PATH_SEP = '\\';
-#else
-static const char PATH_SEP = '/';
-#endif
-
 #endif
diff --git a/unittest/force-styles/test_pair_style.cpp b/unittest/force-styles/test_pair_style.cpp
index 205f709c92..3a23b655e9 100644
--- a/unittest/force-styles/test_pair_style.cpp
+++ b/unittest/force-styles/test_pair_style.cpp
@@ -109,7 +109,7 @@ LAMMPS *init_lammps(int argc, char **argv, const TestConfig &cfg, const bool new
         command(pre_command);
     }
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     parse_input_script(input_file);
 
     command("pair_style " + cfg.pair_style);
@@ -207,7 +207,7 @@ void data_lammps(LAMMPS *lmp, const TestConfig &cfg)
     command("variable pair_style index '" + cfg.pair_style + "'");
     command("variable data_file index " + cfg.basename + ".data");
 
-    std::string input_file = INPUT_FOLDER + PATH_SEP + cfg.input_file;
+    std::string input_file = platform::path_join(INPUT_FOLDER, cfg.input_file);
     parse_input_script(input_file);
 
     for (auto &pair_coeff : cfg.pair_coeff) {