Define SECP256K1_BUILD in secp256k1.c directly.

This avoids building without it and makes it safer to use a custom
 building environment.  Test harnesses need to #include secp256k1.c
 first now.

Makefile.am

@@ -68,7 +68,7 @@ endif
 endif
 
 libsecp256k1_la_SOURCES = src/secp256k1.c
-libsecp256k1_la_CPPFLAGS = -DSECP256K1_BUILD -I$(top_srcdir)/include -I$(top_srcdir)/src $(SECP_INCLUDES)
+libsecp256k1_la_CPPFLAGS = -I$(top_srcdir)/include -I$(top_srcdir)/src $(SECP_INCLUDES)
 libsecp256k1_la_LIBADD = $(SECP_LIBS) $(COMMON_LIB)
 
 if VALGRIND_ENABLED
@@ -81,22 +81,22 @@ noinst_PROGRAMS += bench_verify bench_sign bench_internal bench_ecmult
 bench_verify_SOURCES = src/bench_verify.c
 bench_verify_LDADD = libsecp256k1.la $(SECP_LIBS) $(SECP_TEST_LIBS) $(COMMON_LIB)
 # SECP_TEST_INCLUDES are only used here for CRYPTO_CPPFLAGS
-bench_verify_CPPFLAGS = -DSECP256K1_BUILD $(SECP_TEST_INCLUDES)
+bench_verify_CPPFLAGS = $(SECP_TEST_INCLUDES)
 bench_sign_SOURCES = src/bench_sign.c
 bench_sign_LDADD = libsecp256k1.la $(SECP_LIBS) $(SECP_TEST_LIBS) $(COMMON_LIB)
 bench_internal_SOURCES = src/bench_internal.c
 bench_internal_LDADD = $(SECP_LIBS) $(COMMON_LIB)
-bench_internal_CPPFLAGS = -DSECP256K1_BUILD $(SECP_INCLUDES)
+bench_internal_CPPFLAGS = $(SECP_INCLUDES)
 bench_ecmult_SOURCES = src/bench_ecmult.c
 bench_ecmult_LDADD = $(SECP_LIBS) $(COMMON_LIB)
-bench_ecmult_CPPFLAGS = -DSECP256K1_BUILD $(SECP_INCLUDES)
+bench_ecmult_CPPFLAGS = $(SECP_INCLUDES)
 endif
 
 TESTS =
 if USE_TESTS
 noinst_PROGRAMS += tests
 tests_SOURCES = src/tests.c
-tests_CPPFLAGS = -DSECP256K1_BUILD -I$(top_srcdir)/src -I$(top_srcdir)/include $(SECP_INCLUDES) $(SECP_TEST_INCLUDES)
+tests_CPPFLAGS = -I$(top_srcdir)/src -I$(top_srcdir)/include $(SECP_INCLUDES) $(SECP_TEST_INCLUDES)
 if VALGRIND_ENABLED
 tests_CPPFLAGS += -DVALGRIND
 noinst_PROGRAMS += valgrind_ctime_test
@@ -114,7 +114,7 @@ endif
 if USE_EXHAUSTIVE_TESTS
 noinst_PROGRAMS += exhaustive_tests
 exhaustive_tests_SOURCES = src/tests_exhaustive.c
-exhaustive_tests_CPPFLAGS = -DSECP256K1_BUILD -I$(top_srcdir)/src $(SECP_INCLUDES)
+exhaustive_tests_CPPFLAGS = -I$(top_srcdir)/src $(SECP_INCLUDES)
 if !ENABLE_COVERAGE
 exhaustive_tests_CPPFLAGS += -DVERIFY
 endif

include/secp256k1.h

@@ -127,6 +127,17 @@ typedef int (*secp256k1_nonce_function)(
 #  define SECP256K1_INLINE inline
 # endif
 
+/** When this header is used at build-time the SECP256K1_BUILD define needs to be set
+ *  to correctly setup export attributes and nullness checks.  This is normally done
+ *  by secp256k1.c but to guard against this header being included before secp256k1.c
+ *  has had a chance to set the define (e.g. via test harnesses that just includes
+ *  secp256k1.c) we set SECP256K1_NO_BUILD when this header is processed without the
+ *  BUILD define so this condition can be caught.
+ */
+#ifndef SECP256K1_BUILD
+# define SECP256K1_NO_BUILD
+#endif
+
 #ifndef SECP256K1_API
 # if defined(_WIN32)
 #  ifdef SECP256K1_BUILD

src/bench_ecmult.c

@@ -5,16 +5,16 @@
  ***********************************************************************/
 #include <stdio.h>
 
-#include "include/secp256k1.h"
+#include "secp256k1.c"
 
+#include "include/secp256k1.h"
 #include "util.h"
 #include "hash_impl.h"
 #include "field_impl.h"
 #include "group_impl.h"
 #include "scalar_impl.h"
 #include "ecmult_impl.h"
 #include "bench.h"
-#include "secp256k1.c"
 
 #define POINTS 32768
 

src/bench_internal.c

@@ -5,8 +5,9 @@
  ***********************************************************************/
 #include <stdio.h>
 
-#include "include/secp256k1.h"
+#include "secp256k1.c"
 
+#include "include/secp256k1.h"
 #include "assumptions.h"
 #include "util.h"
 #include "hash_impl.h"
@@ -16,7 +17,6 @@
 #include "ecmult_const_impl.h"
 #include "ecmult_impl.h"
 #include "bench.h"
-#include "secp256k1.c"
 
 typedef struct {
     secp256k1_scalar scalar[2];

src/secp256k1.c

@@ -4,6 +4,8 @@
  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  ***********************************************************************/
 
+#define SECP256K1_BUILD
+
 #include "include/secp256k1.h"
 #include "include/secp256k1_preallocated.h"
 
@@ -21,6 +23,10 @@
 #include "scratch_impl.h"
 #include "selftest.h"
 
+#ifdef SECP256K1_NO_BUILD
+# error "secp256k1.h processed without SECP256K1_BUILD defined while building secp256k1.c"
+#endif
+
 #if defined(VALGRIND)
 # include <valgrind/memcheck.h>
 #endif

src/tests_exhaustive.c

@@ -20,10 +20,10 @@
 #define EXHAUSTIVE_TEST_ORDER 13
 #endif
 
+#include "secp256k1.c"
 #include "include/secp256k1.h"
 #include "assumptions.h"
 #include "group.h"
-#include "secp256k1.c"
 #include "testrand_impl.h"
 
 static int count = 2;