diff --git a/mmap.h b/mmap.h
index 53ea10e..aaab28c 100644
--- a/mmap.h
+++ b/mmap.h
@@ -10,7 +10,7 @@ extern "C" {
 
 /* open file for reading, mmap whole file, close file, write length of
  * map in filesize and return pointer to map. */
-char* mmap_read(const char *filename,size_t* filesize);
+const char* mmap_read(const char *filename,size_t* filesize);
 
 /* open file for writing, mmap whole file privately (copy on write),
  * close file, write length of map in filesize and return pointer to
diff --git a/mmap/mmap_read.c b/mmap/mmap_read.c
index 61ddb37..14dd6d9 100644
--- a/mmap/mmap_read.c
+++ b/mmap/mmap_read.c
@@ -8,7 +8,7 @@
 #include "open.h"
 #include "mmap.h"
 
-extern char* mmap_read(const char* filename,size_t * filesize) {
+extern const char* mmap_read(const char* filename,size_t * filesize) {
 #ifdef __MINGW32__
   HANDLE fd,m;
   char* map;