windows: cross-compile Fluent Bit for Windows with the MinGW-w64 (UCRT64) toolchain

.github/workflows/pr-compile-check.yaml

@@ -155,3 +155,49 @@ jobs:
           cmake -DFLB_PREFER_SYSTEM_LIB_ZSTD=ON -DFLB_PREFER_SYSTEM_LIB_KAFKA=ON ../
           make -j $nparallel
         working-directory: build
+
+  # Sanity check for Windows cross-compilation from Linux with the
+  # MinGW-w64 UCRT64 toolchain (no MSVC or Windows runner involved).
+  # See "Cross-compiling for Windows with MinGW" in DEVELOPER_GUIDE.md.
+  pr-compile-mingw-cross:
+    runs-on: ubuntu-latest
+    container: fedora:44
+    timeout-minutes: 60
+    steps:
+      - name: Install toolchain and build dependencies
+        run: |
+          dnf install -y bison file flex gcc git make ninja-build \
+              ucrt64-gcc ucrt64-gcc-c++ ucrt64-openssl ucrt64-zlib
+
+      - name: Install cmake
+        uses: jwlawson/actions-setup-cmake@0d6a7d60b009d01c9e7523be22153ff8f19460d3 # v2.2.0
+        with:
+          cmake-version: "3.31.6"
+
+      - name: Checkout Fluent Bit code
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+
+      - name: Cross-compile a static libyaml
+        run: |
+          mkdir -p /tmp/libyaml-src
+          curl -sfL https://github.com/yaml/libyaml/releases/download/0.2.5/yaml-0.2.5.tar.gz \
+            | tar xz --strip-components 1 -C /tmp/libyaml-src
+          cd /tmp/libyaml-src
+          ./configure --host=x86_64-w64-mingw32ucrt --prefix=/opt/libyaml \
+              --enable-static --disable-shared CFLAGS="-DYAML_DECLARE_STATIC"
+          make -j 8 -C src
+          make -C src install
+          make -C include install
+
+      # No plugin selection here: the build takes the standard Windows
+      # defaults from cmake/windows-setup.cmake, the same profile the MSVC
+      # Windows CI builds.
+      - name: Cross-compile Fluent Bit for Windows
+        run: |
+          cmake -G Ninja \
+              -DCMAKE_TOOLCHAIN_FILE=/usr/share/mingw/toolchain-ucrt64.cmake \
+              -DFLB_LIBYAML_DIR=/opt/libyaml \
+              ../
+          ninja
+          file bin/fluent-bit.exe
+        working-directory: build

CMakeLists.txt

@@ -85,6 +85,20 @@ if (MSVC)
   add_compile_options(/MT)
 else()
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall")
+  if (MINGW)
+    # MinGW-w64 headers only declare the POSIX thread-safe time functions
+    # (gmtime_r and friends) when the feature test macro is set.
+    add_definitions(-D_POSIX_THREAD_SAFE_FUNCTIONS=200112L)
+
+    # The Winsock APIs take 'char *' buffers where POSIX takes 'void *',
+    # which trips -Wincompatible-pointer-types across the code base. MSVC
+    # reports the same mismatches as warnings; keep GCC (>= 14 makes them
+    # errors by default) at warning level too unless the strict option is
+    # enabled explicitly.
+    if(NOT FLB_COMPILER_STRICT_POINTER_TYPES)
+      add_compile_options(-Wno-error=incompatible-pointer-types)
+    endif()
+  endif()
   if (CMAKE_SYSTEM_NAME STREQUAL "Linux")
     # Prevent the toolchain from emitting an executable stack on Linux targets,
     # which triggers kernel warnings (e.g. "started with executable stack") and
@@ -570,7 +584,7 @@ endif()
 
 # Harden release binary against security vulnerabilities
 if(FLB_SECURITY STREQUAL "On" OR (FLB_SECURITY STREQUAL "ReleaseOnly" AND FLB_RELEASE))
-  if (NOT MSVC)
+  if (NOT WIN32)
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wl,-z,relro,-z,now")
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wl,-z,noexecstack")
     if(NOT FLB_SMALL)
@@ -865,8 +879,10 @@ if(FLB_TLS)
   option(ENABLE_PROGRAMS OFF)
   option(INSTALL_MBEDTLS_HEADERS OFF)
 
-  # Link OpenSSL statically on Windows.
-  if (FLB_SYSTEM_WINDOWS)
+  # Link OpenSSL statically on MSVC Windows builds only.
+  # MinGW cross-compiles use dynamic OpenSSL to avoid GNU ld link-order issues
+  # with static archives that have cascading system library dependencies.
+  if (FLB_SYSTEM_WINDOWS AND MSVC)
     set(OPENSSL_USE_STATIC_LIBS ON)
     set(OPENSSL_MSVC_STATIC_RT  ON)
   endif()
@@ -1138,7 +1154,9 @@ check_c_source_compiles("
       close(sock);
       return 0;
   }" FLB_HAVE_UNIX_SOCKET)
-if(FLB_HAVE_UNIX_SOCKET)
+if(FLB_HAVE_UNIX_SOCKET AND NOT FLB_SYSTEM_WINDOWS)
+  # MinGW headers can satisfy the sys/un.h probe, but the Windows unix
+  # socket paths are untested; keep feature parity with MSVC builds.
   FLB_DEFINITION(FLB_HAVE_UNIX_SOCKET)
 endif()
 
@@ -1170,7 +1188,10 @@ if(FLB_CONFIG_YAML)
       set(LIBYAML_LIBRARY_DIRS "${FLB_LIBYAML_DIR}/lib")
       set(LIBYAML_INCLUDEDIR "${FLB_LIBYAML_DIR}/include")
       message(STATUS "specified libyaml dir: ${FLB_LIBYAML_DIR}")
-      if (MSVC)
+      if (FLB_SYSTEM_WINDOWS)
+        # FLB_LIBYAML_DIR on Windows carries a static libyaml (the MSVC CI
+        # uses vcpkg static triplets, MinGW cross-builds a static libyaml),
+        # so keep <yaml.h> from declaring dllimport symbols.
         FLB_DEFINITION(YAML_DECLARE_STATIC)
       endif ()
       set(FLB_HAVE_LIBYAML 1)
@@ -1233,7 +1254,9 @@ if (FLB_BENCHMARKS)
 endif()
 
 # Build tools/xxd-c
-add_subdirectory(tools/xxd-c)
+if(NOT CMAKE_CROSSCOMPILING)
+  add_subdirectory(tools/xxd-c)
+endif()
 
 # Static configuration generator (using xxd-c)
 if(FLB_STATIC_CONF)
@@ -1256,13 +1279,29 @@ if(FLB_PROXY_GO)
   FLB_DEFINITION(FLB_HAVE_PROXY_GO)
 endif()
 
+if("${GNU_HOST}" STREQUAL "" AND CMAKE_CROSSCOMPILING
+   AND CMAKE_C_COMPILER_ID STREQUAL "GNU")
+    # Derive the autoconf host triple from the cross compiler so the
+    # external autotools sub-builds (libbacktrace, jemalloc) configure for
+    # the target instead of silently building host objects.
+    get_filename_component(FLB_C_COMPILER_NAME "${CMAKE_C_COMPILER}" NAME)
+    string(REGEX REPLACE "-gcc(-[0-9.]+)?$" "" GNU_HOST "${FLB_C_COMPILER_NAME}")
+    if(GNU_HOST STREQUAL FLB_C_COMPILER_NAME)
+        # Compiler name carries no triple prefix; leave GNU_HOST unset.
+        set(GNU_HOST "")
+    endif()
+endif()
+
 if("${GNU_HOST}" STREQUAL "")
     set(AUTOCONF_HOST_OPT "")
 else()
     set(AUTOCONF_HOST_OPT "--host=${GNU_HOST}")
 endif()
 
-if(CMAKE_GENERATOR MATCHES "Ninja" AND NOT FLB_SYSTEM_WINDOWS)
+# The $(MAKE) escape only exists in the Make-family generators; under Ninja
+# the external autotools sub-builds always run with plain make on the build
+# host, including when cross-compiling for a Windows target.
+if(CMAKE_GENERATOR MATCHES "Ninja")
   set(EXTERNAL_BUILD_TOOL "make")
 else()
   set(EXTERNAL_BUILD_TOOL "$(MAKE)")
@@ -1332,6 +1371,12 @@ if(FLB_REGEX)
   endif()
   add_subdirectory(${FLB_PATH_LIB_ONIGMO} EXCLUDE_FROM_ALL)
   target_include_directories(onigmo-static INTERFACE ${FLB_PATH_ROOT_SOURCE}/${FLB_PATH_LIB_ONIGMO})
+  if(CMAKE_CROSSCOMPILING AND WIN32)
+    # Onigmo's configure_file(config.h.cmake config.h) misdetects when cross
+    # compiling; overwrite it with the prebuilt Windows config it ships.
+    file(COPY ${FLB_PATH_ROOT_SOURCE}/${FLB_PATH_LIB_ONIGMO}/win32/config.h
+         DESTINATION ${FLB_PATH_ROOT_SOURCE}/${FLB_PATH_LIB_ONIGMO}/)
+  endif()
 endif()
 
 # tutf8e (UTF8 Encoding)

DEVELOPER_GUIDE.md

@@ -676,6 +676,21 @@ At time of writing this workflow *does not run any of the Fluent Bit test suite*
 - so it only checks that the target branch can be compiled. To run tests, a local
 build will be required.
 
+#### Cross-compiling from Linux with MinGW
+
+Fluent Bit can also be cross-compiled for Windows from a Linux host using the
+MinGW-w64 UCRT64 toolchain, without a Windows machine or MSVC. The
+[`.github/workflows/pr-compile-check.yaml`](.github/workflows/pr-compile-check.yaml)
+`pr-compile-mingw-cross` job is the reference recipe: it runs in a Fedora
+container, installs the `ucrt64-gcc` cross toolchain, cross-builds a static
+libyaml, and then configures with
+`-DCMAKE_TOOLCHAIN_FILE=/usr/share/mingw/toolchain-ucrt64.cmake`.
+
+Cross-builds produce `bin/fluent-bit.exe` but cannot run the test suites (the
+binaries are Windows executables). Note the toolchain must target UCRT
+(`x86_64-w64-mingw32ucrt`); the older MSVCRT-based MinGW toolchains are not
+supported.
+
 #### Using Docker for Windows
 
 For Windows users with Hyper-V capable machines the simplest way to build a

include/fluent-bit/flb_compat.h

@@ -40,7 +40,7 @@
 #define WIN32_LEAN_AND_MEAN
 #include <winsock2.h>
 #include <windows.h>
-#include <Wincrypt.h> /* flb_io_tls.c */
+#include <wincrypt.h> /* flb_io_tls.c */
 
 #include <monkey/mk_core/mk_sleep.h>
 #include <fluent-bit/flb_dlfcn_win32.h>
@@ -56,23 +56,30 @@
 /* monkey exposes a broken vsnprintf macro. Undo it  */
 #undef vsnprintf
 
-/*
- * Windows prefer to add an underscore to each POSIX function.
- * To suppress compiler warnings, we need these trivial macros.
- */
-#define timezone _timezone
-#define tzname _tzname
-#define strcasecmp _stricmp
-#define strncasecmp _strnicmp
 #define timegm _mkgmtime
 
+
 static inline int getpagesize(void)
 {
     SYSTEM_INFO info;
     GetSystemInfo(&info);
     return info.dwPageSize;
 }
 
+static inline char* realpath(char *path, char *buf)
+{
+    if (buf != NULL) {
+        return NULL;  /* Read BUGS in realpath(3) */
+    }
+    return _fullpath(NULL, path, 0);
+}
+
+#ifndef __MINGW32__
+#define timezone _timezone
+#define tzname _tzname
+#define strcasecmp _stricmp
+#define strncasecmp _strnicmp
+
 static inline struct tm *gmtime_r(const time_t *timep, struct tm *result)
 {
     if (gmtime_s(result, timep)) {
@@ -90,10 +97,6 @@ static inline char *ctime_r(const time_t *timep, char *result)
     return strcpy(result, tmp);
 }
 
-/*
- * We can't just define localtime_r here, since mk_core/mk_utils.c is
- * exposing a symbol with the same name inadvertently.
- */
 static struct tm *flb_localtime_r(time_t *timep, struct tm *result)
 {
     if (localtime_s(result, timep)) {
@@ -103,34 +106,41 @@ static struct tm *flb_localtime_r(time_t *timep, struct tm *result)
 }
 #define localtime_r flb_localtime_r
 
+static inline int usleep(LONGLONG usec)
+{
+    // Convert into 100ns unit.
+    return nanosleep(usec * 10);
+}
+#else /* __MINGW32__ */
+/*
+ * MinGW-w64 headers provide the POSIX functions replaced above, but the
+ * real unistd.h they ship has no BSD random()/srandom(); map them to
+ * rand()/srand() like monkey's unistd drop-in does for MSVC.
+ */
+#define random rand
+#define srandom srand
+#endif /* !__MINGW32__ */
+
 static inline char* basename(const char *path)
 {
     char drive[_MAX_DRIVE];
     char dir[_MAX_DIR];
     char fname[_MAX_FNAME];
     char ext[_MAX_EXT];
+#ifdef _MSC_VER
     static __declspec(thread) char buf[_MAX_PATH];
+#else
+    /* MinGW GCC ignores __declspec(thread); use GCC's own TLS keyword so
+     * the buffer keeps the thread-safety the MSVC build has. */
+    static __thread char buf[_MAX_PATH];
+#endif
 
     _splitpath_s(path, drive, _MAX_DRIVE, dir, _MAX_DIR,
                        fname, _MAX_FNAME, ext, _MAX_EXT);
 
     _makepath_s(buf, _MAX_PATH, "", "", fname, ext);
     return buf;
 }
-
-static inline char* realpath(char *path, char *buf)
-{
-    if (buf != NULL) {
-        return NULL;  /* Read BUGS in realpath(3) */
-    }
-    return _fullpath(NULL, path, 0);
-}
-
-static inline int usleep(LONGLONG usec)
-{
-    // Convert into 100ns unit.
-    return nanosleep(usec * 10);
-}
 #else
 #include <netdb.h>
 #include <netinet/in.h>

include/fluent-bit/flb_coro.h

@@ -38,6 +38,11 @@
 
 #include <stdlib.h>
 #include <limits.h>
+
+/* MinGW-w64's winpthreads does not define PTHREAD_STACK_MIN */
+#ifndef PTHREAD_STACK_MIN
+#define PTHREAD_STACK_MIN 16384
+#endif
 #include <libco.h>
 
 #ifdef FLB_HAVE_VALGRIND

include/fluent-bit/flb_langinfo.h

@@ -26,7 +26,7 @@
 #ifndef FLB_LANGINFO_H
 #define FLB_LANGINFO_H
 
-#ifndef _MSC_VER
+#ifndef _WIN32
 #include <langinfo.h>
 #else
 

include/fluent-bit/flb_pthread.h

@@ -21,9 +21,10 @@
 #define FLB_PTHREAD_H
 
 #include <fluent-bit/flb_info.h>
-#ifdef FLB_SYSTEM_WINDOWS
+#if defined(FLB_SYSTEM_WINDOWS) && !defined(__MINGW32__)
 #include <monkey/mk_core/external/winpthreads.h>
 #else
+/* MinGW-w64 ships a real pthread implementation (winpthreads) */
 #include <pthread.h>
 #endif
 

include/fluent-bit/flb_thread_pool.h

@@ -27,9 +27,10 @@
 #define FLB_THREAD_POOL_STOPPED   2
 
 #include <fluent-bit/flb_info.h>
-#ifdef FLB_SYSTEM_WINDOWS
+#if defined(FLB_SYSTEM_WINDOWS) && !defined(__MINGW32__)
 #include <monkey/mk_core/external/winpthreads.h>
 #else
+/* MinGW-w64 ships a real pthread implementation (winpthreads) */
 #include <pthread.h>
 #endif
 

include/fluent-bit/flb_thread_storage.h

@@ -22,9 +22,10 @@
 
 #include <fluent-bit/flb_info.h>
 
-#ifdef FLB_SYSTEM_WINDOWS
+#if defined(FLB_SYSTEM_WINDOWS) && !defined(__MINGW32__)
 #include <monkey/mk_core/external/winpthreads.h>
 #else
+/* MinGW-w64 ships a real pthread implementation (winpthreads) */
 #include <pthread.h>
 #endif
 

lib/chunkio/include/chunkio/chunkio_compat.h

@@ -42,8 +42,10 @@
 #define S_ISREG(m) (((m) & S_IFMT) == S_IFREG)
 #define strerror_r(errno,buf,len) strerror_s(buf,len,errno)
 
+#ifndef __MINGW32__
 typedef SSIZE_T ssize_t;
 typedef unsigned mode_t;
+#endif
 
 static inline char* dirname(const char *path)
 {

lib/chunkio/src/CMakeLists.txt

@@ -23,7 +23,7 @@ if(${CMAKE_SYSTEM_NAME} MATCHES "Windows")
     )
   set(libs
     ${libs}
-    Shell32.lib)
+    shell32)
 else()
   set(src
     ${src}

lib/chunkio/src/cio_os.c

@@ -28,7 +28,7 @@
 #include <chunkio/chunkio_compat.h>
 
 #ifdef _WIN32
-#include <Shlobj.h>
+#include <shlobj.h>
 #endif
 
 /* Check if a path is a directory */