build: Introduce SECP256K1_STATIC macro for Windows users

It is a non-Libtool-specific way to explicitly specify the user's
intention to consume a static `libseck256k1`.

This change allows to get rid of MSVC linker warnings LNK4217 and
LNK4286. Also, it makes possible to merge the `SECP256K1_API` and
`SECP256K1_API_VAR` into one.

Author: hebasto

CHANGELOG.md

@@ -14,6 +14,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
    - Document `doc/ellswift.md` which explains the mathematical background of the scheme.
    - The [paper](https://eprint.iacr.org/2022/759) on which the scheme is based.
 
+#### Changed
+ - When consuming libsecp256k1 as a static library on Windows, the user must now define the `SECP256K1_STATIC` macro before including `secp256k1.h`.
+
 ## [0.3.2] - 2023-05-13
 We strongly recommend updating to 0.3.2 if you use or plan to use GCC >=13 to compile libsecp256k1. When in doubt, check the GCC version using `gcc -v`.
 

Makefile.am

@@ -153,7 +153,7 @@ endif
 if USE_EXAMPLES
 noinst_PROGRAMS += ecdsa_example
 ecdsa_example_SOURCES = examples/ecdsa.c
-ecdsa_example_CPPFLAGS = -I$(top_srcdir)/include
+ecdsa_example_CPPFLAGS = -I$(top_srcdir)/include -DSECP256K1_STATIC
 ecdsa_example_LDADD = libsecp256k1.la
 ecdsa_example_LDFLAGS = -static
 if BUILD_WINDOWS
@@ -163,7 +163,7 @@ TESTS += ecdsa_example
 if ENABLE_MODULE_ECDH
 noinst_PROGRAMS += ecdh_example
 ecdh_example_SOURCES = examples/ecdh.c
-ecdh_example_CPPFLAGS = -I$(top_srcdir)/include
+ecdh_example_CPPFLAGS = -I$(top_srcdir)/include -DSECP256K1_STATIC
 ecdh_example_LDADD = libsecp256k1.la
 ecdh_example_LDFLAGS = -static
 if BUILD_WINDOWS
@@ -174,7 +174,7 @@ endif
 if ENABLE_MODULE_SCHNORRSIG
 noinst_PROGRAMS += schnorr_example
 schnorr_example_SOURCES = examples/schnorr.c
-schnorr_example_CPPFLAGS = -I$(top_srcdir)/include
+schnorr_example_CPPFLAGS = -I$(top_srcdir)/include -DSECP256K1_STATIC
 schnorr_example_LDADD = libsecp256k1.la
 schnorr_example_LDFLAGS = -static
 if BUILD_WINDOWS

configure.ac

@@ -127,12 +127,6 @@ AC_DEFUN([SECP_TRY_APPEND_DEFAULT_CFLAGS], [
       SECP_TRY_APPEND_CFLAGS([-wd4267], $1) # Disable warning C4267 "'var' : conversion from 'size_t' to 'type', possible loss of data".
       # Eliminate deprecation warnings for the older, less secure functions.
       CPPFLAGS="-D_CRT_SECURE_NO_WARNINGS $CPPFLAGS"
-      # We pass -ignore:4217 to the MSVC linker to suppress warning 4217 when
-      # importing variables from a statically linked secp256k1.
-      # (See the libtool manual, section "Windows DLLs" for background.)
-      # Unfortunately, libtool tries to be too clever and strips "-Xlinker arg"
-      # into "arg", so this will be " -Xlinker -ignore:4217" after stripping.
-      LDFLAGS="-Xlinker -Xlinker -Xlinker -ignore:4217 $LDFLAGS" 
     fi
 ])
 SECP_TRY_APPEND_DEFAULT_CFLAGS(SECP_CFLAGS)

examples/CMakeLists.txt

@@ -6,9 +6,6 @@ target_link_libraries(example INTERFACE
   secp256k1
   $<$<PLATFORM_ID:Windows>:bcrypt>
 )
-if(NOT BUILD_SHARED_LIBS AND MSVC)
-  target_link_options(example INTERFACE /IGNORE:4217)
-endif()
 
 add_executable(ecdsa_example ecdsa.c)
 target_link_libraries(ecdsa_example example)

include/secp256k1.h

@@ -141,10 +141,10 @@ typedef int (*secp256k1_nonce_function)(
 #   define SECP256K1_API            __declspec (dllexport)
 #   define SECP256K1_API_VAR extern __declspec (dllexport)
 #  endif
-# elif defined _MSC_VER
-#  define SECP256K1_API
-#  define SECP256K1_API_VAR  extern __declspec (dllimport)
-# elif defined DLL_EXPORT
+  /* The user must define SECP256K1_STATIC when consuming libsecp256k1 as a static
+   * library on Windows. */
+# elif !defined(SECP256K1_STATIC)
+   /* Consuming libsecp256k1 as a DLL. */
 #  define SECP256K1_API             __declspec (dllimport)
 #  define SECP256K1_API_VAR  extern __declspec (dllimport)
 # endif

src/CMakeLists.txt

@@ -24,6 +24,7 @@ endif()
 # This matches libtool's usage of DLL_EXPORT
 if(WIN32)
   set_target_properties(secp256k1 PROPERTIES DEFINE_SYMBOL "DLL_EXPORT")
+  target_compile_definitions(secp256k1 INTERFACE $<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:SECP256K1_STATIC>)
 endif()
 
 # Object libs don't know if they're being built for a shared or static lib.