Allow usage of external default callbacks

configure.ac

@@ -134,6 +134,11 @@ AC_ARG_ENABLE(module_recovery,
     [enable_module_recovery=$enableval],
     [enable_module_recovery=no])
 
+AC_ARG_ENABLE(external_default_callbacks,
+    AS_HELP_STRING([--enable-external-default-callbacks],[enable external default callback functions (default is no)]),
+    [use_external_default_callbacks=$enableval],
+    [use_external_default_callbacks=no])
+
 AC_ARG_ENABLE(jni,
     AS_HELP_STRING([--enable-jni],[enable libsecp256k1_jni [default=no]]),
     [use_jni=$enableval],
@@ -493,6 +498,10 @@ if test x"$use_external_asm" = x"yes"; then
   AC_DEFINE(USE_EXTERNAL_ASM, 1, [Define this symbol if an external (non-inline) assembly implementation is used])
 fi
 
+if test x"$use_external_default_callbacks" = x"yes"; then
+  AC_DEFINE(USE_EXTERNAL_DEFAULT_CALLBACKS, 1, [Define this symbol if an external implementation of the default callbacks is used])
+fi
+
 if test x"$enable_experimental" = x"yes"; then
   AC_MSG_NOTICE([******])
   AC_MSG_NOTICE([WARNING: experimental build])
@@ -537,6 +546,7 @@ echo
 echo "Build Options:"
 echo "  with endomorphism       = $use_endomorphism"
 echo "  with ecmult precomp     = $set_precomp"
+echo "  with external callbacks = $use_external_default_callbacks"
 echo "  with jni                = $use_jni"
 echo "  with benchmarks         = $use_benchmark"
 echo "  with coverage           = $enable_coverage"

include/secp256k1.h

@@ -247,11 +247,28 @@ SECP256K1_API void secp256k1_context_destroy(
  *  to cause a crash, though its return value and output arguments are
  *  undefined.
  *
+ *  When this function has not been called (or called with fn==NULL), then the
+ *  default handler will be used. The library provides a default handler which
+ *  writes the message to stderr and calls abort. This default handler can be
+ *  replaced at link time if the preprocessor macro
+ *  USE_EXTERNAL_DEFAULT_CALLBACKS is defined, which is the case if the build
+ *  has been configured with --enable-external-default-callbacks. Then the
+ *  following two symbols must be provided to link against:
+ *   - void secp256k1_default_illegal_callback_fn(const char* message, void* data);
+ *   - void secp256k1_default_error_callback_fn(const char* message, void* data);
+ *  The library can call these default handlers even before a proper callback data
+ *  pointer could have been set using secp256k1_context_set_illegal_callback or
+ *  secp256k1_context_set_illegal_callback, e.g., when the creation of a context
+ *  fails. In this case, the corresponding default handler will be called with
+ *  the data pointer argument set to NULL.
+ *
  *  Args: ctx:  an existing context object (cannot be NULL)
  *  In:   fun:  a pointer to a function to call when an illegal argument is
- *              passed to the API, taking a message and an opaque pointer
- *              (NULL restores a default handler that calls abort).
+ *              passed to the API, taking a message and an opaque pointer.
+ *              (NULL restores the default handler.)
  *        data: the opaque pointer to pass to fun above.
+ *
+ *  See also secp256k1_context_set_error_callback.
  */
 SECP256K1_API void secp256k1_context_set_illegal_callback(
     secp256k1_context* ctx,
@@ -271,9 +288,12 @@ SECP256K1_API void secp256k1_context_set_illegal_callback(
  *
  *  Args: ctx:  an existing context object (cannot be NULL)
  *  In:   fun:  a pointer to a function to call when an internal error occurs,
- *              taking a message and an opaque pointer (NULL restores a default
- *              handler that calls abort).
+ *              taking a message and an opaque pointer (NULL restores the
+ *              default handler, see secp256k1_context_set_illegal_callback
+ *              for details).
  *        data: the opaque pointer to pass to fun above.
+ *
+ *  See also secp256k1_context_set_illegal_callback.
  */
 SECP256K1_API void secp256k1_context_set_error_callback(
     secp256k1_context* ctx,

src/secp256k1.c

@@ -33,29 +33,32 @@
     } \
 } while(0)
 
+#ifndef USE_EXTERNAL_DEFAULT_CALLBACKS
 static void default_illegal_callback_fn(const char* str, void* data) {
     (void)data;
     fprintf(stderr, "[libsecp256k1] illegal argument: %s\n", str);
     abort();
 }
+static void default_error_callback_fn(const char* str, void* data) {
+    (void)data;
+    fprintf(stderr, "[libsecp256k1] internal consistency check failed: %s\n", str);
+    abort();
+}
+#else
+void default_illegal_callback_fn(const char* str, void* data);
+void default_error_callback_fn(const char* str, void* data);
+#endif
 
 static const secp256k1_callback default_illegal_callback = {
     default_illegal_callback_fn,
     NULL
 };
 
-static void default_error_callback_fn(const char* str, void* data) {
-    (void)data;
-    fprintf(stderr, "[libsecp256k1] internal consistency check failed: %s\n", str);
-    abort();
-}
-
 static const secp256k1_callback default_error_callback = {
     default_error_callback_fn,
     NULL
 };
 
-
 struct secp256k1_context_struct {
     secp256k1_ecmult_context ecmult_ctx;
     secp256k1_ecmult_gen_context ecmult_gen_ctx;