--- a/src/libc_fcns.h
+++ b/src/libc_fcns.h
@@ -0,0 +1,8 @@
+#ifndef _ZZUF_LIBC_FCNS_H
+#define _ZZUF_LIBC_FCNS_H
+
+/* Prototypes for internal libc functions zzuf hooks into */
+extern int __fseeko64(FILE *fp, off64_t offset, int whence);
+extern int _IO_getc(FILE *__fp);
+
+#endif
--- a/src/libzzuf/lib-stream.c
+++ b/src/libzzuf/lib-stream.c
@@ -57,6 +57,7 @@
 #include "debug.h"
 #include "fuzz.h"
 #include "fd.h"
+#include "libc_fcns.h"
 
 #if defined HAVE_FPOS64_T
 #   define FPOS64_T fpos64_t
--- a/src/zzat.c
+++ b/src/zzat.c
@@ -51,6 +51,8 @@
 
 #include "util/getopt.h"
 
+#include "libc_fcns.h"
+
 static int run(char const *sequence, char const *file);
 static void output(char const *buf, size_t len);