diff --git a/src/sim/syscall_emul.hh b/src/sim/syscall_emul.hh
index ceeacd39bf..5ba487c7cd 100644
--- a/src/sim/syscall_emul.hh
+++ b/src/sim/syscall_emul.hh
@@ -116,6 +116,10 @@
 #include "sim/syscall_emul_buf.hh"
 #include "sim/syscall_return.hh"
 
+#if defined(__APPLE__) && defined(__MACH__) && !defined(CMSG_ALIGN)
+#define CMSG_ALIGN(len) (((len) + sizeof(size_t) - 1) & ~(sizeof(size_t) - 1))
+#endif
+
 //////////////////////////////////////////////////////////////////////
 //
 // The following emulation functions are generic enough that they