diff --git a/configure.ac b/configure.ac
index a165d0879..41761c5b9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2092,6 +2092,12 @@ AC_CHECK_DECLS([O_NONBLOCK], , ,
 #endif
 	])
 
+AC_CHECK_DECLS([ftruncate], , ,
+	[
+#include <sys/types.h>
+#include <unistd.h>
+	])
+
 AC_CHECK_DECLS([readv, writev], , , [
 #include <sys/types.h>
 #include <sys/uio.h>
diff --git a/openbsd-compat/openbsd-compat.h b/openbsd-compat/openbsd-compat.h
index c202e1429..0cecfb621 100644
--- a/openbsd-compat/openbsd-compat.h
+++ b/openbsd-compat/openbsd-compat.h
@@ -65,6 +65,10 @@ int bindresvport_sa(int sd, struct sockaddr *sa);
 void closefrom(int);
 #endif
 
+#if defined(HAVE_DECL_FTRUNCATE) && HAVE_DECL_FTRUNCATE == 0
+int ftruncate(int filedes, off_t length);
+#endif
+
 #ifndef HAVE_GETLINE
 #include <stdio.h>
 ssize_t getline(char **, size_t *, FILE *);