diff --git a/include/nasmlib.h b/include/nasmlib.h
index a505a1296..ea0a880b9 100644
--- a/include/nasmlib.h
+++ b/include/nasmlib.h
@@ -368,9 +368,10 @@ static inline const char *nasm_digit_chars(bool ucase)
 int32_t seg_alloc(void);
 
 /*
- * Add/replace or remove an extension to the end of a filename
+ * Add/replace or remove an extension to the end of a filename;
+ * returns a newly allocated buffer with the modified filename.
  */
-const char *filename_set_extension(const char *inname, const char *extension);
+char *filename_set_extension(const char *inname, const char *extension);
 
 /*
  * Utility macros...
diff --git a/nasmlib/path.c b/nasmlib/path.c
index 0a0a67331..1d7ad8777 100644
--- a/nasmlib/path.c
+++ b/nasmlib/path.c
@@ -186,7 +186,7 @@ char *nasm_catfile(const char *dir, const char *file)
  * string buffer. If the extension is not an empty string, the first
  * character is replaced by the appropriate extsep for the filesystem.
  */
-const char *filename_set_extension(const char *inname, const char *extension)
+char *filename_set_extension(const char *inname, const char *extension)
 {
     const char *nulp, *ep, *p;
     char *outname, *q;