diff --git a/Makefile b/Makefile
index 94aa600..03bc544 100644
--- a/Makefile
+++ b/Makefile
@@ -5,16 +5,16 @@ SHARED_LIB = libsparsemap.so
 
 #CFLAGS = -Wall -Wextra -Wpedantic -Of -std=c11 -Iinclude/ -fPIC
 #CFLAGS = -Wall -Wextra -Wpedantic -Og -g -std=c11 -Iinclude/ -fPIC
-#CFLAGS = -DSPARSEMAP_DIAGNOSTIC -DDEBUG -Wall -Wextra -Wpedantic -O0 -g -std=c11 -Iinclude/ -fPIC
-CFLAGS = -DSPARSEMAP_DIAGNOSTIC -DDEBUG -Wall -Wextra -Wpedantic -Ofast -g -std=c11 -Iinclude/ -fPIC
+CFLAGS = -DREENTRENT_SPARSEMAP -DSPARSEMAP_DIAGNOSTIC -DDEBUG -Wall -Wextra -Wpedantic -O0 -g -std=c11 -Iinclude/ -fPIC
+#CFLAGS = -DREENTRENT_SPARSEMAP -DSPARSEMAP_DIAGNOSTIC -DDEBUG -Wall -Wextra -Wpedantic -Ofast -g -std=c11 -Iinclude/ -fPIC
 #CFLAGS = -Wall -Wextra -Wpedantic -Og -g -std=c11 -Iinclude/ -fPIC
 #CFLAGS = -Wall -Wextra -Wpedantic -Ofast -g -std=c11 -Iinclude/ -fPIC
 #CFLAGS = -DSPARSEMAP_DIAGNOSTIC -DDEBUG -Wall -Wextra -Wpedantic -Og -g -fsanitize=address,leak,object-size,pointer-compare,pointer-subtract,null,return,bounds,pointer-overflow,undefined -fsanitize-address-use-after-scope -std=c11 -Iinclude/ -fPIC
 #CFLAGS = -Wall -Wextra -Wpedantic -Og -g -fsanitize=all -fhardened -std=c11 -Iinclude/ -fPIC
 
-#TEST_FLAGS = -DDEBUG -Wall -Wextra -Wpedantic -O0 -g -std=c11 -Iinclude/ -Itests/ -fPIC
+TEST_FLAGS = -DREENTRENT_SPARSEMAP -DDEBUG -Wall -Wextra -Wpedantic -O0 -Wno-fortify-source -g -std=c11 -Iinclude/ -Itests/ -fPIC
 #TEST_FLAGS = -Wall -Wextra -Wpedantic -Og -g -std=c11 -Iinclude/ -Itests/ -fPIC
-TEST_FLAGS = -Wall -Wextra -Wpedantic -Ofast -g -std=c11 -Iinclude/ -Itests/ -fPIC
+#TEST_FLAGS = -Wall -Wextra -Wpedantic -Ofast -g -std=c11 -Iinclude/ -Itests/ -fPIC
 #TEST_FLAGS = -DDEBUG -Wall -Wextra -Wpedantic -Og -g -fsanitize=address,leak,object-size,pointer-compare,pointer-subtract,null,return,bounds,pointer-overflow,undefined -fsanitize-address-use-after-scope -std=c11 -Iinclude/ -fPIC
 
 TESTS = tests/test
diff --git a/examples/ex_1.c b/examples/ex_1.c
index dea68f1..2b95971 100644
--- a/examples/ex_1.c
+++ b/examples/ex_1.c
@@ -17,6 +17,9 @@
 
 /* !!! Duplicated here for testing purposes. Keep in sync, or suffer. !!! */
 struct sparsemap {
+#ifdef REENTRENT_SPARSEMAP
+  pthread_mutex_t m_mutex;
+#endif
   size_t m_capacity;
   size_t m_data_used;
   uint8_t *m_data;
diff --git a/examples/soak.c b/examples/soak.c
index 60dd42c..2fe1810 100644
--- a/examples/soak.c
+++ b/examples/soak.c
@@ -805,7 +805,7 @@ main(void)
       size_t len = COUNT;
       // The largest page is at list[1] because this is a reverse sorted list.
       pgno_t pg = list[0] ? list[1] + 1 : 0;
-//      if (toss(6) + 1 < 7) {
+      //      if (toss(6) + 1 < 7) {
       if (true) { // disable shrinking for now...
         MDB_IDL new_list = mdb_midl_alloc(len);
         sparsemap_t *new_map = sparsemap(INITIAL_AMOUNT);
diff --git a/include/sparsemap.h b/include/sparsemap.h
index bf749aa..9a90a52 100644
--- a/include/sparsemap.h
+++ b/include/sparsemap.h
@@ -190,6 +190,9 @@ void sparsemap_clear(sparsemap_t *map);
  * supported.
  */
 sparsemap_t *sparsemap_set_data_size(sparsemap_t *map, size_t size, uint8_t *data);
+#ifdef REENTRENT_SPARSEMAP
+sparsemap_t *sparsemap_set_data_size_r(sparsemap_t *map, size_t size, uint8_t *data);
+#endif
 
 /** @brief Calculate remaining capacity, approaches 0 when full.
  *
@@ -204,6 +207,9 @@ sparsemap_t *sparsemap_set_data_size(sparsemap_t *map, size_t size, uint8_t *dat
  * 100.0 when empty
  */
 double sparsemap_capacity_remaining(sparsemap_t *map);
+#ifdef REENTRENT_SPARSEMAP
+sparsemap_t *sparsemap_capacity_remaining_r(size_t size);
+#endif
 
 /** @brief Returns the capacity of the underlying byte array in bytes.
  *
diff --git a/src/sparsemap.c b/src/sparsemap.c
index 718c4ef..d6302bd 100644
--- a/src/sparsemap.c
+++ b/src/sparsemap.c
@@ -34,6 +34,10 @@
 #include <stdlib.h>
 #include <string.h>
 
+#ifdef REENTRENT_SPARSEMAP
+#include <pthread.h>
+#endif
+
 #ifdef SPARSEMAP_DIAGNOSTIC
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wpedantic"
@@ -114,7 +118,10 @@ typedef struct {
   sm_bitvec_t *m_data;
 } __sm_chunk_t;
 
-struct __attribute__((aligned(8))) sparsemap {
+struct sparsemap {
+#ifdef REENTRENT_SPARSEMAP
+  pthread_mutex_t m_mutex;
+#endif
   size_t m_capacity;  /* The total size of m_data */
   size_t m_data_used; /* The used size of m_data */
   uint8_t *m_data;    /* The serialized bitmap data */
@@ -982,6 +989,9 @@ sparsemap_init(sparsemap_t *map, uint8_t *data, size_t size)
   map->m_data = data;
   map->m_data_used = 0;
   map->m_capacity = size;
+#ifdef REENTRENT_SPARSEMAP
+  pthread_mutex_init(&map->m_mutex);
+#endif
   sparsemap_clear(map);
 }
 
@@ -1008,17 +1018,29 @@ sparsemap_set_data_size(sparsemap_t *map, size_t size, uint8_t *data)
 
     /* Ensure that m_data is 8-byte aligned. */
     size_t total_size = sizeof(sparsemap_t) + data_size;
+#ifdef REENTRENT_SPARSEMAP
+    total_size += sizeof(pthread_mutex_t);
+#endif
     size_t padding = total_size % 8 == 0 ? 0 : 8 - (total_size % 8);
     total_size += padding;
 
+#ifdef REENTRENT_SPARSEMAP
+    sparsemap_t *mem = (sparsemap_t *)((uintptr_t)map - sizeof(pthread_mutex_t));
+    sparsemap_t *m = (sparsemap_t *)realloc(mem, total_size);
+#else
     sparsemap_t *m = (sparsemap_t *)realloc(map, total_size);
+#endif
     if (!m) {
       return NULL;
     }
+#ifdef REENTRENT_SPARSEMAP
+    m = (sparsemap_t *)((uintptr_t)m + sizeof(pthread_mutex_t));
+#endif
     memset(((uint8_t *)m) + sizeof(sparsemap_t) + (m->m_capacity * sizeof(uint8_t)), 0, size - m->m_capacity + padding);
     m->m_capacity = data_size;
     m->m_data = (uint8_t *)(((uintptr_t)m + sizeof(sparsemap_t)) & ~(uintptr_t)7);
-    __sm_when_diag({ __sm_assert(IS_8_BYTE_ALIGNED(m->m_data)); }) return m;
+    __sm_when_diag({ __sm_assert(IS_8_BYTE_ALIGNED(m->m_data)); });
+    return m;
   } else {
     if (data != NULL && data_size > sparsemap_get_capacity(map) && data != map->m_data) {
       map->m_data = data;
@@ -1028,6 +1050,29 @@ sparsemap_set_data_size(sparsemap_t *map, size_t size, uint8_t *data)
   }
 }
 
+#ifdef REENTRENT_SPARSEMAP
+sparsemap_t *
+sparsemap_set_data_size_r(sparsemap_t *map, size_t size, uint8_t *data)
+{
+  sparsemap_t *retval;
+  pthread_mutex_t *mutex;
+
+  if (SM_IS_REENTRENT_MAP(map)) {
+    mutex = SM_REENTRENT_MUTEX(map);
+    pthread_mutex_lock(mutex);
+    sparsemap_t *real_map = SM_REENTRENT_MAP(map);
+    retval = sparsemap_set_data_size(real_map, size, data);
+    /* realloc() could have moved the map, so get these again. */
+    mutex = SM_REENTRENT_MUTEX(retval);
+    retval = SM_REENTRENT_MAP(retval);
+    pthread_mutex_unlock(mutex);
+  } else {
+    retval = sparsemap_set_data_size(map, size, data);
+  }
+  return retval;
+}
+#endif
+
 double
 sparsemap_capacity_remaining(sparsemap_t *map)
 {
diff --git a/tests/test.c b/tests/test.c
index ffc7138..bd95675 100644
--- a/tests/test.c
+++ b/tests/test.c
@@ -29,6 +29,9 @@
 
 /* !!! Duplicated here for testing purposes. Keep in sync, or suffer. !!! */
 struct sparsemap {
+#ifdef REENTRENT_SPARSEMAP
+  pthread_mutex_t m_mutex;
+#endif
   size_t m_capacity;
   size_t m_data_used;
   uint8_t *m_data;
@@ -38,6 +41,11 @@ struct user_data {
   int foo;
 };
 
+#ifdef REENTRENT_SPARSEMAP
+#define sparsemap(size) sparsemap_r(size)
+#define sparsemap_set_data_size(map, size, data) sparsemap_set_data_size_r(map, size, data)
+#endif
+
 /* -------------------------- Supporting Functions for Testing */
 
 void
@@ -1007,6 +1015,34 @@ test_api_span(const MunitParameter params[], void *data)
   return MUNIT_OK;
 }
 
+static void *
+test_api_reentrent_setup(const MunitParameter params[], void *user_data)
+{
+  sparsemap_t *map = sparsemap_r(1024);
+  assert_ptr_not_null(map);
+  return (void *)map;
+}
+static void
+test_api_reentrent_tear_down(void *fixture)
+{
+  sparsemap_t *map = (sparsemap_t *)fixture;
+  free(map);
+  test_api_tear_down(fixture);
+}
+static MunitResult
+test_api_reentrent(const MunitParameter params[], void *data)
+{
+  sparsemap_t *map = (sparsemap_t *)data;
+  (void)params;
+
+  assert_ptr_not_null(map);
+  sparsemap_set_data_size_r(map, 2048, NULL);
+
+  // TODO... moar.
+
+  return MUNIT_OK;
+}
+
 // clang-format off
 static MunitTest api_test_suite[] = {
   { (char *)"/new", test_api_new, NULL, NULL, MUNIT_TEST_OPTION_NONE, NULL },
@@ -1038,6 +1074,7 @@ static MunitTest api_test_suite[] = {
   { (char *)"/rank/true", test_api_rank_true, test_api_rank_true_setup, test_api_rank_true_tear_down, MUNIT_TEST_OPTION_NONE, NULL },
   { (char *)"/rank/false", test_api_rank_false, test_api_rank_false_setup, test_api_rank_false_tear_down, MUNIT_TEST_OPTION_NONE, NULL },
   { (char *)"/span", test_api_span, test_api_span_setup, test_api_span_tear_down, MUNIT_TEST_OPTION_NONE, NULL },
+  { (char *)"/reentrent", test_api_reentrent, test_api_reentrent_setup, test_api_reentrent_tear_down, MUNIT_TEST_OPTION_NONE, NULL },
   { NULL, NULL, NULL, NULL, MUNIT_TEST_OPTION_NONE, NULL }
 };
 // clang-format on