From 7a30481760f1f7bfb7928d0b22d49a5ae263386f Mon Sep 17 00:00:00 2001
From: Jeremy Drake <github@jdrake.com>
Date: Tue, 3 Jun 2025 10:35:46 -0700
Subject: [PATCH] dllmain: exclude from Cygwin builds

On Cygwin, it is unsafe to call POSIX functions from DllMain, which
OPENSSL_thread_stop does.  Additionally, it should be unnecessary as
OpenSSL uses pthread_key_create to register a thread destructor to do
thread cleanup in a POSIX way.

Reported-by: Yuyi Wang

Ref: https://cygwin.com/pipermail/cygwin/2025-June/258235.html

Fixes #17262
Closes https://github.com/curl/curl/pull/17528
---
 configure.ac       |  1 -
 lib/CMakeLists.txt |  9 +--------
 lib/Makefile.am    |  9 ---------
 lib/dllmain.c      | 13 ++-----------
 4 files changed, 3 insertions(+), 29 deletions(-)

diff --git a/configure.ac b/configure.ac
index 9529dc95ff..9a5dd60f6c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -637,7 +637,6 @@ curl_cv_cygwin='no'
 case $host_os in
   cygwin*|msys*) curl_cv_cygwin='yes';;
 esac
-AM_CONDITIONAL(DOING_CYGWIN, test "x$curl_cv_cygwin" = xyes)
 
 AM_CONDITIONAL([HAVE_WINDRES],
   [test "$curl_cv_native_windows" = "yes" && test -n "${RC}"])
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 12840952b3..9ab9e01857 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -179,15 +179,8 @@ if(BUILD_SHARED_LIBS)
   list(APPEND libcurl_export ${LIB_SHARED})
   add_library(${LIB_SHARED} SHARED ${LIB_SOURCE})
   add_library(${PROJECT_NAME}::${LIB_SHARED} ALIAS ${LIB_SHARED})
-  if(WIN32 OR CYGWIN)
-    if(CYGWIN)
-      # For Cygwin always compile dllmain.c as a separate unit since it
-      # includes windows.h, which should not be included in other units.
-      set_source_files_properties("dllmain.c" PROPERTIES SKIP_UNITY_BUILD_INCLUSION ON)
-    endif()
-    set_property(TARGET ${LIB_SHARED} APPEND PROPERTY SOURCES "dllmain.c")
-  endif()
   if(WIN32)
+    set_property(TARGET ${LIB_SHARED} APPEND PROPERTY SOURCES "dllmain.c")
     set_property(TARGET ${LIB_SHARED} APPEND PROPERTY SOURCES "libcurl.rc")
     if(CURL_HIDES_PRIVATE_SYMBOLS)
       set_property(TARGET ${LIB_SHARED} APPEND PROPERTY SOURCES "${PROJECT_SOURCE_DIR}/lib/libcurl.def")
diff --git a/lib/Makefile.am b/lib/Makefile.am
index 8dac236606..463b72ed8e 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -74,10 +74,6 @@ include Makefile.inc
 
 if DOING_NATIVE_WINDOWS
 CSOURCES += dllmain.c
-else
-if DOING_CYGWIN
-CSOURCES += dllmain.c
-endif
 endif
 
 if USE_UNITY
@@ -89,11 +85,6 @@ if CURLDEBUG
 # applying to it.
 curl_EXCLUDE += memdebug.c
 endif
-# For Cygwin always compile dllmain.c as a separate unit since it
-# includes windows.h, which should not be included in other units.
-if DOING_CYGWIN
-curl_EXCLUDE += dllmain.c
-endif
 libcurl_unity.c: $(top_srcdir)/scripts/mk-unity.pl $(CSOURCES)
 	@PERL@ $(top_srcdir)/scripts/mk-unity.pl $(srcdir) $(CSOURCES) --exclude $(curl_EXCLUDE) > libcurl_unity.c
 
diff --git a/lib/dllmain.c b/lib/dllmain.c
index 41e97b37eb..33076e0571 100644
--- a/lib/dllmain.c
+++ b/lib/dllmain.c
@@ -28,22 +28,13 @@
 #include <openssl/crypto.h>
 #endif
 
-/* The fourth-to-last include */
-#ifdef __CYGWIN__
-#define WIN32_LEAN_AND_MEAN
-#include <windows.h>
-#ifdef _WIN32
-#undef _WIN32
-#endif
-#endif
-
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
 #include "curl_memory.h"
 #include "memdebug.h"
 
-/* DllMain() must only be defined for Windows and Cygwin DLL builds. */
-#if (defined(_WIN32) || defined(__CYGWIN__)) && !defined(CURL_STATICLIB)
+/* DllMain() must only be defined for Windows DLL builds. */
+#if defined(_WIN32) && !defined(CURL_STATICLIB)
 
 #if defined(USE_OPENSSL) && \
     !defined(OPENSSL_IS_AWSLC) && \
-- 
2.47.2