diff --git a/libshvchainpack/c/include/shv/chainpack/compat.h b/libshvchainpack/c/include/shv/chainpack/compat.h
index b702fa70f..6608846ef 100644
--- a/libshvchainpack/c/include/shv/chainpack/compat.h
+++ b/libshvchainpack/c/include/shv/chainpack/compat.h
@@ -1,4 +1,11 @@
 #pragma once
+#ifdef __has_include
+	#if __has_include(<unistd.h>)
+		#include <unistd.h>
+	#endif
+#endif
+
 #ifndef __ssize_t_defined
-#define ssize_t int64_t
+#include <stdint.h>
+typedef intmax_t ssize_t;
 #endif