diff --git a/gebs.h b/gebs.h
index 5bb3a3e..8ffb592 100644
--- a/gebs.h
+++ b/gebs.h
@@ -90,6 +90,10 @@ int main(int argc, char ** argv)
 #include <string.h>
 #include <limits.h>
 
+#ifdef GEBS_ENABLE_PTHREAD_FEATURES
+#   include <pthread.h>
+#endif
+
 #include <unistd.h>
 #include <sys/stat.h>
 #include <sys/types.h>
@@ -440,7 +444,14 @@ void gebs_rebuild_self1_alloc(Gebs_Allocator *alloc, int argc, char ** argv,
 // Scratch arena
 // ----------------------------------------------------------------------------
 
-extern Gebs_Arena gebs_scratch_arena;
+typedef struct {
+    Gebs_Arena value;
+    #ifdef GEBS_ENABLE_PTHREAD_FEATURES
+        pthread_mutex_t lock;
+    #endif
+} Gebs_Scratch_Arena;
+
+extern Gebs_Scratch_Arena gebs_scratch_arena;
 
 void gebs_scratch_arena_reset(void);
 char *gebs_fmt(const char *fmt, ...);
@@ -816,22 +827,25 @@ void gebs_arena_destroy(Gebs_Arena *a)
 // Scratch arena
 // ----------------------------------------------------------------------------
 
-Gebs_Arena gebs_scratch_arena;
+Gebs_Scratch_Arena gebs_scratch_arena;
 
 __attribute__((constructor))
 void gebs_scratch_areana_init(void)
 {
-    gebs_scratch_arena = gebs_arena_get();
+    gebs_scratch_arena.value = gebs_arena_get();
 }
 
 __attribute__((destructor))
 void gebs_scratch_areana_free(void)
 {
-    gebs_arena_destroy(&gebs_scratch_arena);
+    gebs_arena_destroy(&gebs_scratch_arena.value);
 }
 
 char *gebs_fmt(const char *fmt, ...)
 {
+    #ifdef GEBS_ENABLE_PTHREAD_FEATURES
+        pthread_mutex_lock(&gebs_scratch_arena.lock);
+    #endif
     va_list list;
     va_start(list, fmt);
 
@@ -840,23 +854,41 @@ char *gebs_fmt(const char *fmt, ...)
     size_t size = vsnprintf(nil, 0, fmt, list_copy);
     va_end(list_copy);
 
-    char *buf = gebs_malloc(&gebs_scratch_arena, size+1);
+    char *buf = gebs_malloc(&gebs_scratch_arena.value, size+1);
     vsprintf(buf, fmt, list);
     va_end(list);
+    #ifdef GEBS_ENABLE_PTHREAD_FEATURES
+        pthread_mutex_unlock(&gebs_scratch_arena.lock);
+    #endif
     return buf;
 }
 
 void gebs_scratch_arena_reset(void)
 {
-    gebs_arena_reset(&gebs_scratch_arena);
+    #ifdef GEBS_ENABLE_PTHREAD_FEATURES
+        pthread_mutex_lock(&gebs_scratch_arena.lock);
+    #endif
+    gebs_arena_reset(&gebs_scratch_arena.value);
+    #ifdef GEBS_ENABLE_PTHREAD_FEATURES
+        pthread_mutex_unlock(&gebs_scratch_arena.lock);
+    #endif
 }
 
 char *gebs_cwd(void)
 {
-    char *cwd = gebs_malloc(&gebs_scratch_arena, PATH_MAX);
+    #ifdef GEBS_ENABLE_PTHREAD_FEATURES
+        pthread_mutex_lock(&gebs_scratch_arena.lock);
+    #endif
+    char *cwd = gebs_malloc(&gebs_scratch_arena.value, PATH_MAX);
     if (getcwd(cwd, PATH_MAX) == nil) {
+        #ifdef GEBS_ENABLE_PTHREAD_FEATURES
+            pthread_mutex_unlock(&gebs_scratch_arena.lock);
+        #endif
         return nil;
     }
+    #ifdef GEBS_ENABLE_PTHREAD_FEATURES
+        pthread_mutex_unlock(&gebs_scratch_arena.lock);
+    #endif
     return cwd;
 }