diff --git a/nasmlib.c b/nasmlib.c
index fda0c7a5..2dd3a658 100644
--- a/nasmlib.c
+++ b/nasmlib.c
@@ -95,7 +95,7 @@ void nasm_set_malloc_error(efunc error)
 }
 
 #ifdef LOGALLOC
-void *nasm_malloc_log(char *file, int line, size_t size)
+void *nasm_malloc_log(const char *file, int line, size_t size)
 #else
 void *nasm_malloc(size_t size)
 #endif
@@ -112,7 +112,7 @@ void *nasm_malloc(size_t size)
 }
 
 #ifdef LOGALLOC
-void *nasm_zalloc_log(char *file, int line, size_t size)
+void *nasm_zalloc_log(const char *file, int line, size_t size)
 #else
 void *nasm_zalloc(size_t size)
 #endif
@@ -129,7 +129,7 @@ void *nasm_zalloc(size_t size)
 }
 
 #ifdef LOGALLOC
-void *nasm_realloc_log(char *file, int line, void *q, size_t size)
+void *nasm_realloc_log(const char *file, int line, void *q, size_t size)
 #else
 void *nasm_realloc(void *q, size_t size)
 #endif
@@ -149,7 +149,7 @@ void *nasm_realloc(void *q, size_t size)
 }
 
 #ifdef LOGALLOC
-void nasm_free_log(char *file, int line, void *q)
+void nasm_free_log(const char *file, int line, void *q)
 #else
 void nasm_free(void *q)
 #endif
@@ -163,7 +163,7 @@ void nasm_free(void *q)
 }
 
 #ifdef LOGALLOC
-char *nasm_strdup_log(char *file, int line, const char *s)
+char *nasm_strdup_log(const char *file, int line, const char *s)
 #else
 char *nasm_strdup(const char *s)
 #endif
@@ -184,9 +184,9 @@ char *nasm_strdup(const char *s)
 }
 
 #ifdef LOGALLOC
-char *nasm_strndup_log(char *file, int line, char *s, size_t len)
+char *nasm_strndup_log(const char *file, int line, const char *s, size_t len)
 #else
-char *nasm_strndup(char *s, size_t len)
+char *nasm_strndup(const char *s, size_t len)
 #endif
 {
     char *p;
@@ -205,6 +205,13 @@ char *nasm_strndup(char *s, size_t len)
     return p;
 }
 
+noreturn nasm_assert_failed(const char *file, int line, const char *msg)
+{
+    nasm_malloc_error(ERR_FATAL, "assertion %s failed at %s:%d",
+		      msg, file, line);
+    exit(1);
+}
+
 #ifndef nasm_stricmp
 int nasm_stricmp(const char *s1, const char *s2)
 {
diff --git a/nasmlib.h b/nasmlib.h
index 995c9612..6776c91d 100644
--- a/nasmlib.h
+++ b/nasmlib.h
@@ -152,14 +152,14 @@ void *nasm_zalloc(size_t);
 void *nasm_realloc(void *, size_t);
 void nasm_free(void *);
 char *nasm_strdup(const char *);
-char *nasm_strndup(char *, size_t);
+char *nasm_strndup(const char *, size_t);
 #else
-void *nasm_malloc_log(char *, int, size_t);
-void *nasm_zalloc_log(char *, int, size_t);
-void *nasm_realloc_log(char *, int, void *, size_t);
-void nasm_free_log(char *, int, void *);
-char *nasm_strdup_log(char *, int, const char *);
-char *nasm_strndup_log(char *, int, char *, size_t);
+void *nasm_malloc_log(const char *, int, size_t);
+void *nasm_zalloc_log(const char *, int, size_t);
+void *nasm_realloc_log(const char *, int, void *, size_t);
+void nasm_free_log(const char *, int, void *);
+char *nasm_strdup_log(const char *, int, const char *);
+char *nasm_strndup_log(const char *, int, const char *, size_t);
 #define nasm_malloc(x) nasm_malloc_log(__FILE__,__LINE__,x)
 #define nasm_zalloc(x) nasm_zalloc_log(__FILE__,__LINE__,x)
 #define nasm_realloc(x,y) nasm_realloc_log(__FILE__,__LINE__,x,y)
@@ -168,6 +168,13 @@ char *nasm_strndup_log(char *, int, char *, size_t);
 #define nasm_strndup(x,y) nasm_strndup_log(__FILE__,__LINE__,x,y)
 #endif
 
+/*
+ * NASM assert failure
+ */
+noreturn nasm_assert_failed(const char *, int, const char *);
+#define nasm_assert(x) \
+    do { if (!(x)) nasm_assert_failed(__FILE__,__LINE__,#x); } while (0)
+
 /*
  * ANSI doesn't guarantee the presence of `stricmp' or
  * `strcasecmp'.