Add json_get_alloc_funcs() to allow alloc function fetching

This is particularly useful in modular situations where the allocation
functions are either unknown or private. For instance, in such cases,
the caller of json_dumps() has no way to free the returned buffer.

doc/apiref.rst

@@ -1570,6 +1570,11 @@ behavior is needed.
    Jansson's API functions to ensure that all memory operations use
    the same functions.
 
+.. function:: void json_get_alloc_funcs(json_malloc_t *malloc_fn, json_free_t *free_fn)
+
+   Fetch the current malloc_fn and free_fn used. Either parameter
+   may be NULL.
+
 **Examples:**
 
 Circumvent problems with different CRT heaps on Windows by using

src/jansson.def

@@ -66,4 +66,5 @@ EXPORTS
     json_unpack_ex
     json_vunpack_ex
     json_set_alloc_funcs
+    json_get_alloc_funcs
 

src/jansson.h

@@ -289,6 +289,7 @@ typedef void *(*json_malloc_t)(size_t);
 typedef void (*json_free_t)(void *);
 
 void json_set_alloc_funcs(json_malloc_t malloc_fn, json_free_t free_fn);
+void json_get_alloc_funcs(json_malloc_t *malloc_fn, json_free_t *free_fn);
 
 #ifdef __cplusplus
 }

src/memory.c

@@ -59,3 +59,11 @@ void json_set_alloc_funcs(json_malloc_t malloc_fn, json_free_t free_fn)
     do_malloc = malloc_fn;
     do_free = free_fn;
 }
+
+void json_get_alloc_funcs(json_malloc_t *malloc_fn, json_free_t *free_fn)
+{
+    if (malloc_fn)
+        *malloc_fn = do_malloc;
+    if (free_fn)
+        *free_fn = do_free;
+}

test/suites/api/test_memory_funcs.c

@@ -36,10 +36,15 @@ static void my_free(void *ptr)
 
 static void test_simple()
 {
+    json_malloc_t mfunc = NULL;
+    json_free_t ffunc = NULL;
+
     json_set_alloc_funcs(my_malloc, my_free);
+    json_get_alloc_funcs(&mfunc, &ffunc);
     create_and_free_complex_object();
 
-    if(malloc_called != 1 || free_called != 1)
+    if (malloc_called != 1 || free_called != 1
+        || mfunc != my_malloc || ffunc != my_free)
         fail("Custom allocation failed");
 }