diff --git a/repos/demo/include/mini_c/stdio.h b/repos/demo/include/mini_c/stdio.h
index a98be2c15..f59a2330a 100644
--- a/repos/demo/include/mini_c/stdio.h
+++ b/repos/demo/include/mini_c/stdio.h
@@ -17,7 +17,7 @@
 #include <stdarg.h>
 #include <stddef.h>
 
-#define FILE int
+#define FILE void
 
 #define EOF (-1)
 
diff --git a/repos/demo/src/lib/mini_c/abort.cc b/repos/demo/src/lib/mini_c/abort.cc
index d873e8713..84b8d9d47 100644
--- a/repos/demo/src/lib/mini_c/abort.cc
+++ b/repos/demo/src/lib/mini_c/abort.cc
@@ -14,9 +14,8 @@
 #include <base/log.h>
 #include <base/sleep.h>
 
-extern "C" void *abort(void)
+extern "C" void abort(void)
 {
 	Genode::warning("abort called");
 	Genode::sleep_forever();
-	return 0;
 }
diff --git a/repos/demo/src/lib/mini_c/malloc_free.cc b/repos/demo/src/lib/mini_c/malloc_free.cc
index f6cec833c..9ff31cb6b 100644
--- a/repos/demo/src/lib/mini_c/malloc_free.cc
+++ b/repos/demo/src/lib/mini_c/malloc_free.cc
@@ -32,7 +32,7 @@ static Allocator &alloc()
 void mini_c_init(Allocator &alloc) { _alloc_ptr = &alloc; }
 
 
-extern "C" void *malloc(unsigned size)
+extern "C" void *malloc(size_t size)
 {
 	/*
 	 * We store the size of the allocation at the very
@@ -50,7 +50,7 @@ extern "C" void *malloc(unsigned size)
 }
 
 
-extern "C" void *calloc(unsigned nmemb, unsigned size)
+extern "C" void *calloc(size_t nmemb, size_t size)
 {
 	void *addr = malloc(nmemb*size);
 	memset(addr, 0, nmemb*size);