diff --git a/configure.in b/configure.in
index 8a4e97e..69862e8 100644
--- a/configure.in
+++ b/configure.in
@@ -1048,9 +1048,7 @@ main()
 		ac_cv_func_isnan=yes
 		ac_cv_func_finite=yes
 		ac_cv_func_link=yes
-		ac_cv_func_truncate=yes
 		ac_cv_func_fseeko=yes
-		ac_cv_func_ftello=yes
 		ac_cv_lib_crypt_crypt=no
 		ac_cv_func_getpgrp_void=no
 		ac_cv_func_memcmp_working=yes
@@ -1302,7 +1300,7 @@ else
   AC_LIBOBJ([signbit])
 fi
 AC_CHECK_FUNCS(fmod killpg wait4 waitpid fork spawnv syscall __syscall chroot getcwd eaccess\
-	      truncate ftruncate chsize times utimes utimensat fcntl lockf lstat\
+	      truncate ftruncate ftello chsize times utimes utimensat fcntl lockf lstat\
 	      link symlink readlink readdir_r fsync fdatasync fchown posix_fadvise\
 	      setitimer setruid seteuid setreuid setresuid setproctitle socketpair\
 	      setrgid setegid setregid setresgid issetugid pause lchown lchmod\
diff --git a/include/ruby/win32.h b/include/ruby/win32.h
index 791085f..d822a01 100644
--- a/include/ruby/win32.h
+++ b/include/ruby/win32.h
@@ -381,11 +381,23 @@ scalb(double a, long b)
 //
 
 #define SUFFIX
+
+#if !defined HAVE_FTRUNCATE
 #define ftruncate rb_w32_ftruncate
-extern int       truncate(const char *path, off_t length);
-extern int       ftruncate(int fd, off_t length);
+extern int       rb_w32_ftruncate(int fd, off_t length);
+#endif
+
+#if !defined HAVE_TRUNCATE
+#define truncate rb_w32_truncate
+extern int       rb_w32_truncate(const char *path, off_t length);
+#endif
+
 extern int       fseeko(FILE *stream, off_t offset, int whence);
-extern off_t     ftello(FILE *stream);
+
+#if !defined HAVE_FTELLO
+#define ftello rb_w32_ftello
+extern off_t     rb_w32_ftello(FILE *stream);
+#endif
 
 //
 // stubs
diff --git a/win32/win32.c b/win32/win32.c
index 2e92665..f87cf8b 100644
--- a/win32/win32.c
+++ b/win32/win32.c
@@ -4318,7 +4318,7 @@ rb_chsize(HANDLE h, off_t size)
 }
 
 int
-truncate(const char *path, off_t length)
+rb_w32_truncate(const char *path, off_t length)
 {
     HANDLE h;
     int ret;
@@ -4344,7 +4344,7 @@ truncate(const char *path, off_t length)
 }
 
 int
-ftruncate(int fd, off_t length)
+rb_w32_ftruncate(int fd, off_t length)
 {
     HANDLE h;
 
@@ -4417,7 +4417,7 @@ fseeko(FILE *stream, off_t offset, int whence)
 }
 
 off_t
-ftello(FILE *stream)
+rb_w32_ftello(FILE *stream)
 {
     off_t pos;
     if (fgetpos(stream, (fpos_t *)&pos)) return (off_t)-1;