diff --git a/include/libfile.h b/include/libfile.h
index de4f42d..dd0b00f 100644
--- a/include/libfile.h
+++ b/include/libfile.h
@@ -1,7 +1,7 @@
 #ifndef __LIBFILE_H
 #define __LIBFILE_H
 
-int write_full(int fd, void *buf, size_t size);
+int write_full(int fd, const void *buf, size_t size);
 int read_full(int fd, void *buf, size_t size);
 
 char *read_file_line(const char *fmt, ...);
diff --git a/lib/libfile.c b/lib/libfile.c
index 62e9b8e..cba2f02 100644
--- a/lib/libfile.c
+++ b/lib/libfile.c
@@ -28,7 +28,7 @@
  * Like write, but guarantees to write the full buffer out, else
  * it returns with an error.
  */
-int write_full(int fd, void *buf, size_t size)
+int write_full(int fd, const void *buf, size_t size)
 {
 	size_t insize = size;
 	int now;