diff --git a/bdw.c b/bdw.c
index b77847612..41533e1a2 100644
--- a/bdw.c
+++ b/bdw.c
@@ -240,7 +240,7 @@ int gc_options_parse_and_set(struct gc_options *options, int option,
   return gc_common_options_parse_and_set(&options->common, option, value);
 }
 
-int gc_init(struct gc_options *options, struct gc_stack_addr *stack_base,
+int gc_init(const struct gc_options *options, struct gc_stack_addr *stack_base,
             struct gc_heap **heap, struct gc_mutator **mutator) {
   GC_ASSERT_EQ(gc_allocator_small_granule_size(), GC_INLINE_GRANULE_BYTES);
   GC_ASSERT_EQ(gc_allocator_large_threshold(),
diff --git a/gc-api.h b/gc-api.h
index 183603338..47222706a 100644
--- a/gc-api.h
+++ b/gc-api.h
@@ -22,8 +22,9 @@ GC_API_ void* gc_call_with_stack_addr(void* (*f)(struct gc_stack_addr *,
                                                  void *),
                                       void *data) GC_NEVER_INLINE;
 
-GC_API_ int gc_init(struct gc_options *options, struct gc_stack_addr *base,
-                    struct gc_heap **heap, struct gc_mutator **mutator);
+GC_API_ int gc_init(const struct gc_options *options,
+                    struct gc_stack_addr *base, struct gc_heap **heap,
+                    struct gc_mutator **mutator);
 
 struct gc_mutator_roots;
 GC_API_ void gc_mutator_set_roots(struct gc_mutator *mut,
diff --git a/semi.c b/semi.c
index 1f0bde97b..7c5d8b07d 100644
--- a/semi.c
+++ b/semi.c
@@ -408,7 +408,7 @@ int gc_options_parse_and_set(struct gc_options *options, int option,
   return gc_common_options_parse_and_set(&options->common, option, value);
 }
 
-int gc_init(struct gc_options *options, struct gc_stack_addr *stack_base,
+int gc_init(const struct gc_options *options, struct gc_stack_addr *stack_base,
             struct gc_heap **heap, struct gc_mutator **mut) {
   GC_ASSERT_EQ(gc_allocator_allocation_pointer_offset(),
                offsetof(struct semi_space, hp));
diff --git a/whippet.c b/whippet.c
index aee9086fd..686ac5cda 100644
--- a/whippet.c
+++ b/whippet.c
@@ -2258,7 +2258,7 @@ int gc_options_parse_and_set(struct gc_options *options, int option,
   return gc_common_options_parse_and_set(&options->common, option, value);
 }
 
-static int heap_init(struct gc_heap *heap, struct gc_options *options) {
+static int heap_init(struct gc_heap *heap, const struct gc_options *options) {
   // *heap is already initialized to 0.
 
   pthread_mutex_init(&heap->lock, NULL);
@@ -2316,7 +2316,7 @@ static int mark_space_init(struct mark_space *space, struct gc_heap *heap) {
   return 1;
 }
 
-int gc_init(struct gc_options *options, struct gc_stack_addr *stack_base,
+int gc_init(const struct gc_options *options, struct gc_stack_addr *stack_base,
             struct gc_heap **heap, struct gc_mutator **mut) {
   GC_ASSERT_EQ(gc_allocator_small_granule_size(), GRANULE_SIZE);
   GC_ASSERT_EQ(gc_allocator_large_threshold(), LARGE_OBJECT_THRESHOLD);