diff --git a/Makefile b/Makefile
index 94aa600..440aff4 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 = -DSPARSEMAP_DIAGNOSTIC -DDEBUG -Wall -Wextra -Wpedantic -D_FORTIFY_SOURCE=0 -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 -D_FORTIFY_SOURCE=0 -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/include/sparsemap.h b/include/sparsemap.h
index bf749aa..a08e386 100644
--- a/include/sparsemap.h
+++ b/include/sparsemap.h
@@ -109,6 +109,9 @@ typedef uint64_t sm_bitvec_t;
  * @returns The newly allocated sparsemap reference.
  */
 sparsemap_t *sparsemap(size_t size);
+#ifdef REENTRENT_SPARSEMAP
+sparsemap_t *sparsemap_r(size_t size);
+#endif
 
 /** @brief Allocate a new, empty sparsemap_t that references (wraps) the buffer
  * \b data of \b size bytes to use for storage of bitmap data.
@@ -190,6 +193,9 @@ 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.
  *
diff --git a/src/sparsemap.c b/src/sparsemap.c
index 718c4ef..52575e5 100644
--- a/src/sparsemap.c
+++ b/src/sparsemap.c
@@ -33,6 +33,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#ifdef REENTRENT_SPARSEMAP
+#include <pthread.h>
+#endif
 
 #ifdef SPARSEMAP_DIAGNOSTIC
 #pragma GCC diagnostic push
@@ -109,6 +112,7 @@ enum __SM_CHUNK_INFO {
 };
 
 #define SM_CHUNK_GET_FLAGS(from, at) (((from)) & ((sm_bitvec_t)SM_FLAG_MASK << ((at)*2))) >> ((at)*2)
+gsb #define SM_IS_REENTRENT(from) (((from)) & ((sm_bitvec_t)SM_FLAG_MASK)
 
 typedef struct {
   sm_bitvec_t *m_data;
@@ -966,6 +970,19 @@ sparsemap(size_t size)
   return map;
 }
 
+#ifdef REENTRENT_SPARSEMAP
+sparsemap_t *
+sparsemap_r(size_t size)
+{
+  sparsemap_t *map = sparsemap(size + sizeof(pthread_mutex_t));
+  sparsemap_t *rm = (sparsemap_t *)((uintptr_t)map + sizeof(pthread_mutex_t));
+  pthread_mutex_t *mutex = (pthread_mutex_t *)map;
+  memcpy(rm, map, size);
+  pthread_mutex_init(mutex, NULL);
+  return map;
+}
+#endif
+
 sparsemap_t *
 sparsemap_wrap(uint8_t *data, size_t size)
 {
@@ -1008,13 +1025,24 @@ 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 *rm = (sparsemap_t *)((uintptr_t)map - sizeof(pthread_mutex_t));
+    sparsemap_t *m = (sparsemap_t *)realloc(rm, total_size);
+#else
     sparsemap_t *m = (sparsemap_t *)realloc(map, total_size);
+#endif
     if (!m) {
       return NULL;
     }
+#ifdef REENTRENT_SPARSEMAP
+    sparsemap_t *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);
@@ -1028,6 +1056,18 @@ 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)
+{
+  pthread_mutex_t *mutex = (pthread_mutex_t *)map;
+  sparsemap_t *rm = (sparsemap_t *)((uintptr_t)map + sizeof(pthread_mutex_t));
+  pthread_mutex_lock(mutex);
+  sparsemap_t *ret = sparsemap_set_data_size(rm size, data);
+  pthread_mutex_unlock(mutex);
+  return ret;
+}
+#endif
+
 double
 sparsemap_capacity_remaining(sparsemap_t *map)
 {
diff --git a/tests/test.c b/tests/test.c
index ffc7138..0c87e85 100644
--- a/tests/test.c
+++ b/tests/test.c
@@ -38,6 +38,10 @@ struct user_data {
   int foo;
 };
 
+#ifdef REENTRENT_SPARSEMAP
+#define sparsemap() GSB
+#endif
+
 /* -------------------------- Supporting Functions for Testing */
 
 void