diff --git a/src/TRestRawMemoryBufferToSignalProcess.cxx b/src/TRestRawMemoryBufferToSignalProcess.cxx
index 444083df..20f459d0 100644
--- a/src/TRestRawMemoryBufferToSignalProcess.cxx
+++ b/src/TRestRawMemoryBufferToSignalProcess.cxx
@@ -117,6 +117,7 @@ using namespace std;
 #endif
 
 #if (defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)) || __APPLE__
+#include <unistd.h>
 // The union is already defined in sys/sem.h
 #else
 union semun {