build, core: check for experimental std::filesystem and link the additional library if needed

build-support/custom-modules/ns3-compiler-workarounds.cmake

@@ -48,3 +48,29 @@ if(${MISSING_OSTREAM_NULLPTR_OPERATOR})
     ${CMAKE_CURRENT_SOURCE_DIR}/build-support/compiler-workarounds/ostream-operator-nullptr.h
   )
 endif()
+
+# Some compilers (e.g. GCC < 9.1 and Clang < 9) do not link
+# std::filesystem/std::experimental::filesystem by default.
+# If the sample program can be linked, it means it is
+# indeed linked by default. Otherwise, we link it manually.
+# https://en.cppreference.com/w/cpp/filesystem
+check_cxx_source_compiles(
+  "
+  # ifdef __cpp_lib_filesystem
+    #include <filesystem>
+    namespace fs = std::filesystem;
+  #else
+    #include <experimental/filesystem>
+    namespace fs = std::experimental::filesystem;
+  #endif
+  int main()
+  {
+    std::string path = \"/\";
+    return !fs::exists (path);
+  }
+  "
+  FILESYSTEM_LIBRARY_IS_LINKED
+)
+if(NOT FILESYSTEM_LIBRARY_IS_LINKED)
+  link_libraries(-lstdc++fs)
+endif()

build-support/macros-and-definitions.cmake

@@ -221,6 +221,12 @@ if(${CLANG} AND APPLE)
   set(STATIC_LINK_FLAGS)
 endif()
 
+# Include CMake files used for compiler checks
+include(CheckIncludeFile) # Used to check a single C header at a time
+include(CheckIncludeFileCXX) # Used to check a single C++ header at a time
+include(CheckIncludeFiles) # Used to check multiple headers at once
+include(CheckFunctionExists)
+
 macro(SUBDIRLIST result curdir)
   file(GLOB children RELATIVE ${curdir} ${curdir}/*)
   set(dirlist "")
@@ -1078,11 +1084,6 @@ macro(process_options)
     set(INT64X64_USE_CAIRO TRUE)
   endif()
 
-  include(CheckIncludeFile) # Used to check a single C header at a time
-  include(CheckIncludeFileCXX) # Used to check a single C++ header at a time
-  include(CheckIncludeFiles) # Used to check multiple headers at once
-  include(CheckFunctionExists)
-
   # Check for required headers and functions, set flags if they're found or warn
   # if they're not found
   check_include_file("stdint.h" "HAVE_STDINT_H")

src/core/model/system-path.cc

@@ -25,12 +25,28 @@
 #include <algorithm>
 #include <cstdlib> // getenv
 #include <cstring> // strlen
-#include <filesystem>
 #include <tuple>
 #include <sstream>
 #include <ctime>
 #include <regex>
 
+// Some compilers such as GCC < 8 (Ubuntu 18.04
+// ships with GCC 7) do not ship with the
+// std::filesystem header,  but with the
+// std::experimental::filesystem header.
+// Since Clang reuses these headers and the libstdc++
+// from GCC, we need to either use the experimental
+// version or require a more up-to-date GCC.
+// we use the "fs" namespace to prevent collisions
+// with musl libc.
+# ifdef __cpp_lib_filesystem
+    #include <filesystem>
+    namespace fs = std::filesystem;
+#else
+    #include <experimental/filesystem>
+    namespace fs = std::experimental::filesystem;
+#endif
+
 #ifdef __APPLE__
 #include <mach-o/dyld.h>
 #endif /* __APPLE__ */
@@ -84,13 +100,13 @@ ReadFilesNoThrow (std::string path)
 {
   NS_LOG_FUNCTION (path);
   std::list<std::string> files;
-  if (!std::filesystem::exists (path))
+  if (!fs::exists (path))
     {
       return std::make_tuple (files, true);
     }
-  for (auto &it : std::filesystem::directory_iterator (path))
+  for (auto &it : fs::directory_iterator (path))
     {
-      if (!it.is_directory ())
+      if (!fs::is_directory(it.path()))
         {
           files.push_back (it.path ().filename ().string ());
         }
@@ -341,9 +357,9 @@ MakeDirectories (std::string path)
   NS_LOG_FUNCTION (path);
 
   std::error_code ec;
-  if (!std::filesystem::exists (path))
+  if (!fs::exists (path))
     {
-      std::filesystem::create_directories (path, ec);
+      fs::create_directories (path, ec);
     }
 
   if (ec.value ())