Missing symbols when compiling for Windows with mingw-w64

[rbberger]

For Windows, the fmt/chrono.h header uses _get_timezone and _get_dstbias, which seem to be only defined when using the UCRT.

Compiling with mingw-w64 causes a link error:

/usr/lib/gcc/x86_64-w64-mingw32/11.2.0/../../../../x86_64-w64-mingw32/bin/ld: CMakeFiles/test_me.dir/objects.a(test_me.cpp.obj):test_me.cpp:(.text+0x73c): undefined reference to `__imp__get_timezone'
/usr/lib/gcc/x86_64-w64-mingw32/11.2.0/../../../../x86_64-w64-mingw32/bin/ld: CMakeFiles/test_me.dir/objects.a(test_me.cpp.obj):test_me.cpp:(.text+0x196c): undefined reference to `__imp__get_dstbias'
collect2: error: ld returned 1 exit status

Minimal example to reproduce:

#include <fmt/chrono.h>

int main() {
  std::time_t now = std::time(nullptr);
  fmt::print("Printed on {:%a %b %d %H:%M:%S %Y}\n", fmt::localtime(now));
  return 0;
}

cmake_minimum_required(VERSION 3.10)
project(test_me)
enable_language(CXX)

add_subdirectory(fmt)

add_executable(test_me test_me.cpp)
target_link_libraries(test_me fmt)

Suggested workaround/bugfix:

diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index 31fee070..908999ab 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -1082,7 +1082,7 @@ template <typename OutputIt, typename Char> class tm_writer {
   }
   template <typename T, FMT_ENABLE_IF(!has_member_data_tm_gmtoff<T>::value)>
   void format_utc_offset_impl(const T& tm) {
-#if defined(_WIN32)
+#if defined(_WIN32) && defined(_UCRT)
 #  if FMT_USE_TZSET
     tzset_once();
 #  endif

[vitaut]

Could you submit a PR with the proposed fix since you are in the best position to test it?

[rbberger]

Sure.