diff --git a/src/lib/w32/posixio.c b/src/lib/w32/posixio.c
index eb4c57c3..31be7c2d 100644
--- a/src/lib/w32/posixio.c
+++ b/src/lib/w32/posixio.c
@@ -412,6 +412,15 @@ posix_fsync(int fd)
     FILE_FUNCTION(FDMAP_IO_FILE, _commit(handle))
 }
 
+/*
+ * POSIX ftruncate()
+ */
+int
+ftruncate(int fd, off_t length)
+{
+    FILE_FUNCTION(FDMAP_IO_FILE, _chsize(handle, length))
+}
+
 /*
  * POSIX equivalent for fstat().
  * fstat() is used instead of _fstat(),
diff --git a/src/lib/w32/unistd.h b/src/lib/w32/unistd.h
index f382f70a..807be4a5 100644
--- a/src/lib/w32/unistd.h
+++ b/src/lib/w32/unistd.h
@@ -127,6 +127,7 @@ extern int fcntl(int fd, int cmd, ...);
     posix_write((fd), (buffer), (count))
 #define fsync(fd) \
     posix_fsync((fd))
+extern int ftruncate(int fd, off_t length);
 extern int posix_close(int fd);
 extern off_t posix_lseek(int fd, off_t offset, int origin);
 extern ssize_t posix_read(int fd, void *buffer, size_t count);