diff --git a/libmaxsi/Makefile b/libmaxsi/Makefile
index d2314ee6..2cda163e 100644
--- a/libmaxsi/Makefile
+++ b/libmaxsi/Makefile
@@ -67,6 +67,7 @@ string.o \
 error.o \
 format.o \
 access.o \
+_assert.o \
 chdir.o \
 chmod.o \
 close.o \
diff --git a/libmaxsi/_assert.cpp b/libmaxsi/_assert.cpp
new file mode 100644
index 00000000..90b30306
--- /dev/null
+++ b/libmaxsi/_assert.cpp
@@ -0,0 +1,35 @@
+/******************************************************************************
+
+	Copyright(C) Jonas 'Sortie' Termansen 2012.
+
+	This file is part of LibMaxsi.
+
+	LibMaxsi is free software: you can redistribute it and/or modify it under
+	the terms of the GNU Lesser General Public License as published by the Free
+	Software Foundation, either version 3 of the License, or (at your option)
+	any later version.
+
+	LibMaxsi is distributed in the hope that it will be useful, but WITHOUT ANY
+	WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+	FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for
+	more details.
+
+	You should have received a copy of the GNU Lesser General Public License
+	along with LibMaxsi. If not, see <http://www.gnu.org/licenses/>.
+
+	_assert.cpp
+	Reports the occurence of an assertion failure.
+
+******************************************************************************/
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+void _assert(const char* filename, unsigned int line, const char* functionname,
+             const char* expression)
+{
+	fprintf(stderr, "Assertion failure: %s:%u: %s: %s\n", filename, line,
+	        functionname, expression);
+	abort();
+}
diff --git a/libmaxsi/include/assert.h b/libmaxsi/include/assert.h
index 321e58ea..fbd7927a 100644
--- a/libmaxsi/include/assert.h
+++ b/libmaxsi/include/assert.h
@@ -1,6 +1,6 @@
 /******************************************************************************
 
-	COPYRIGHT(C) JONAS 'SORTIE' TERMANSEN 2012.
+	Copyright(C) Jonas 'Sortie' Termansen 2012.
 
 	This file is part of LibMaxsi.
 
@@ -27,18 +27,40 @@
 
 #include <features.h>
 
+/* stdlib.h is not needed, but GCC fixincludes thinks it is, so fool it. */
+#if 0
+#include <stdlib.h>
+#endif
+
 __BEGIN_DECLS
 
-#ifdef assert
-#undef assert
-#endif
-
-#ifndef NDEBUG
-/* #warning The assert macro is not implemented */
-#endif
-
-#define assert(ignore)((void) 0)
+/* The actual implementation of assert. */
+void _assert(const char* filename, unsigned int line, const char* functionname,
+             const char* expression) __attribute__ ((noreturn));
 
 __END_DECLS
 
 #endif
+
+/* Rid ourselves of any previous declaration of assert. */
+#ifdef assert
+#undef assert
+#endif
+
+/* Redefine the assert macro on each <assert.h> inclusion. */
+#ifdef NDEBUG
+
+#define assert(ignore) ((void) 0)
+
+#else /* !NDEBUG */
+
+/* Use __builtin_expect to tell the compiler that we don't expect a failure to
+   happen and thus it can do better branch prediction. Naturally we don't
+   optimize for the case where the program is about to abort(). */
+#define assert(invariant) \
+	if ( __builtin_expect(!(invariant), 0) ) \
+	{ \
+		_assert(__FILE__, __LINE__, __PRETTY_FUNCTION__, #invariant); \
+	}
+
+#endif /* !NDEBUG */