From 78cedd81a4dfe1ef1d870df5a5b80751739b35bd Mon Sep 17 00:00:00 2001
From: Mike Vastola <mike@vasto.la>
Date: Wed, 8 Apr 2026 20:51:33 -0400
Subject: [PATCH 1/2] [DRAFT, WIP] Detect presence of flockfile

Just a dry-run at a possible implementation pending clarification on
details.

Fixes fmtlib/fmt#4646
---
 CMakeLists.txt           | 13 +++++++++++++
 include/fmt/format-inl.h |  6 +++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index cc47c96ed8d0..ec224a942e40 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -128,6 +128,7 @@ endif ()
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/support/cmake")
 
 include(CheckCXXCompilerFlag)
+include(CheckCXXSourceCompiles)
 include(JoinPaths)
 
 if (FMT_MASTER_PROJECT AND NOT DEFINED CMAKE_CXX_VISIBILITY_PRESET)
@@ -307,6 +308,18 @@ else ()
   message(WARNING "Feature cxx_std_11 is unknown for the CXX compiler")
 endif ()
 
+check_cxx_source_compiles([[
+    #include <stdio.h>
+    int main() {
+        // Use stdout just to prove we can pass a FILE* to it
+        flockfile(stdout);
+        funlockfile(stdout);
+        return 0;
+    }
+]] HAVE_FLOCKFILE_COMPILE_TEST)
+target_compile_definitions(fmt PRIVATE
+  FMT_HAVE_FLOCKFILE_COMPILE_TEST=$<BOOL:HAVE_FLOCKFILE_COMPILE_TEST>)
+
 # Set FMT_LIB_NAME for pkg-config fmt.pc. We cannot use the OUTPUT_NAME target
 # property because it's not set by default.
 set(FMT_LIB_NAME fmt)
diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index cc11019876bd..b679fd505248 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -1490,7 +1490,11 @@ template <typename F> auto getc_unlocked(F* f) -> decltype(_fgetc_nolock(f)) {
 #endif
 
 #ifndef FMT_USE_FLOCKFILE
-#  define FMT_USE_FLOCKFILE 1
+#  ifdef FMT_HAVE_FLOCKFILE_COMPILE_TEST
+#    define FMT_USE_FLOCKFILE FMT_HAVE_FLOCKFILE_COMPILE_TEST
+#  else
+#    define FMT_USE_FLOCKFILE 1
+#  endif
 #endif
 
 template <typename F = FILE, typename Enable = void>

From 7605e0ba7c9fab062b89ff15678567c6bbeea5f4 Mon Sep 17 00:00:00 2001
From: Mike Vastola <mike@vasto.la>
Date: Thu, 9 Apr 2026 15:17:09 -0400
Subject: [PATCH 2/2] Apply suggested format changes

---
 CMakeLists.txt | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index ec224a942e40..fc671c170fb9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -308,7 +308,8 @@ else ()
   message(WARNING "Feature cxx_std_11 is unknown for the CXX compiler")
 endif ()
 
-check_cxx_source_compiles([[
+check_cxx_source_compiles(
+  [[
     #include <stdio.h>
     int main() {
         // Use stdout just to prove we can pass a FILE* to it
@@ -316,9 +317,11 @@ check_cxx_source_compiles([[
         funlockfile(stdout);
         return 0;
     }
-]] HAVE_FLOCKFILE_COMPILE_TEST)
-target_compile_definitions(fmt PRIVATE
-  FMT_HAVE_FLOCKFILE_COMPILE_TEST=$<BOOL:HAVE_FLOCKFILE_COMPILE_TEST>)
+]]
+  HAVE_FLOCKFILE_COMPILE_TEST)
+target_compile_definitions(
+  fmt
+  PRIVATE FMT_HAVE_FLOCKFILE_COMPILE_TEST=$<BOOL:HAVE_FLOCKFILE_COMPILE_TEST>)
 
 # Set FMT_LIB_NAME for pkg-config fmt.pc. We cannot use the OUTPUT_NAME target
 # property because it's not set by default.