diff --git a/src/include/port.h b/src/include/port.h
index d2409a43e33..818b7c7baef 100644
--- a/src/include/port.h
+++ b/src/include/port.h
@@ -306,6 +306,33 @@ extern bool rmtree(const char *path, bool rmtopdir);
 
 #if defined(WIN32) && !defined(__CYGWIN__)
 
+/*
+ * We want the 64-bit variant of lseek().
+ *
+ * For Visual Studio, this must be after <io.h> to avoid messing up its
+ * lseek() and _lseeki64() function declarations.
+ *
+ * For MinGW there is already a macro, so we have to undefine it (depending on
+ * _FILE_OFFSET_BITS, it may point at its own lseek64, but we don't want to
+ * count on that being set).
+ */
+#undef lseek
+#define lseek(a,b,c) _lseeki64((a),(b),(c))
+
+/*
+ * We want the 64-bit variant of chsize().  It sets errno and also returns it,
+ * so convert non-zero result to -1 to match POSIX.
+ *
+ * Prevent MinGW from declaring functions, and undefine its macro before we
+ * define our own.
+ */
+#ifndef _MSC_VER
+#define FTRUNCATE_DEFINED
+#include <unistd.h>
+#undef ftruncate
+#endif
+#define ftruncate(a,b) (_chsize_s((a),(b)) == 0 ? 0 : -1)
+
 /*
  * open() and fopen() replacements to allow deletion of open files and
  * passing of other special options.
diff --git a/src/include/port/win32_port.h b/src/include/port/win32_port.h
index d9794fa8727..7900272e8d1 100644
--- a/src/include/port/win32_port.h
+++ b/src/include/port/win32_port.h
@@ -79,8 +79,6 @@
 /* Must be here to avoid conflicting with prototype in windows.h */
 #define mkdir(a,b)	mkdir(a)
 
-#define ftruncate(a,b)	chsize(a,b)
-
 /* Windows doesn't have fsync() as such, use _commit() */
 #define fsync(fd) _commit(fd)
 
diff --git a/src/pl/plperl/plperl_system.h b/src/pl/plperl/plperl_system.h
index f2e7f1873dc..57355ac1ed4 100644
--- a/src/pl/plperl/plperl_system.h
+++ b/src/pl/plperl/plperl_system.h
@@ -103,6 +103,7 @@
 #undef fstat
 #undef kill
 #undef listen
+#undef lseek
 #undef lstat
 #undef mkdir
 #undef open