diff --git a/adopt.c b/adopt.c
index cceb73e..e48afe7 100644
--- a/adopt.c
+++ b/adopt.c
@@ -12,6 +12,10 @@
 #include <limits.h>
 #include <assert.h>
 
+#if defined(__sun) || defined(__illumos__)
+# include <alloca.h>
+#endif
+
 #include "adopt.h"
 
 #ifdef _WIN32