diff --git a/libc/Android.mk b/libc/Android.mk
index bcb4afc..683dfa9 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -461,7 +461,6 @@
     upstream-openbsd/lib/libc/stdio/fputwc.c \
     upstream-openbsd/lib/libc/stdio/fputws.c \
     upstream-openbsd/lib/libc/stdio/fscanf.c \
-    upstream-openbsd/lib/libc/stdio/funopen.c \
     upstream-openbsd/lib/libc/stdio/fvwrite.c \
     upstream-openbsd/lib/libc/stdio/fwalk.c \
     upstream-openbsd/lib/libc/stdio/fwide.c \
diff --git a/libc/include/stdio.h b/libc/include/stdio.h
index a227904..b618091 100644
--- a/libc/include/stdio.h
+++ b/libc/include/stdio.h
@@ -170,21 +170,43 @@
 
 int fseek(FILE*, long, int);
 long ftell(FILE*);
+
 #if defined(__USE_FILE_OFFSET64)
 int fgetpos(FILE*, fpos_t*) __RENAME(fgetpos64);
 int fsetpos(FILE*, const fpos_t*) __RENAME(fsetpos64);
 int fseeko(FILE*, off_t, int) __RENAME(fseeko64);
 off_t ftello(FILE*) __RENAME(ftello64);
+#  if defined(__USE_BSD)
+FILE* funopen(const void*,
+              int (*)(void*, char*, int),
+              int (*)(void*, const char*, int),
+              fpos_t (*)(void*, fpos_t, int),
+              int (*)(void*)) __RENAME(funopen64);
+#  endif
 #else
 int fgetpos(FILE*, fpos_t*);
 int fsetpos(FILE*, const fpos_t*);
 int fseeko(FILE*, off_t, int);
 off_t ftello(FILE*);
+#  if defined(__USE_BSD)
+FILE* funopen(const void*,
+              int (*)(void*, char*, int),
+              int (*)(void*, const char*, int),
+              fpos_t (*)(void*, fpos_t, int),
+              int (*)(void*));
+#  endif
 #endif
 int fgetpos64(FILE*, fpos64_t*);
 int fsetpos64(FILE*, const fpos64_t*);
 int fseeko64(FILE*, off64_t, int);
 off64_t ftello64(FILE*);
+#if defined(__USE_BSD)
+FILE* funopen64(const void*,
+                int (*)(void*, char*, int),
+                int (*)(void*, const char*, int),
+                fpos64_t (*)(void*, fpos64_t, int),
+                int (*)(void*));
+#endif
 
 #if __ISO_C_VISIBLE >= 1999 || __BSD_VISIBLE
 int	 snprintf(char * __restrict, size_t, const char * __restrict, ...)
@@ -254,18 +276,8 @@
 int ferror_unlocked(FILE*);
 int fileno_unlocked(FILE*);
 
-/*
- * Stdio function-access interface.
- * TODO: __USE_FILE_OFFSET64
- */
-FILE	*funopen(const void *,
-		int (*)(void *, char *, int),
-		int (*)(void *, const char *, int),
-		fpos_t (*)(void *, fpos_t, int),
-		int (*)(void *));
-
-#define	fropen(cookie, fn) funopen(cookie, fn, 0, 0, 0)
-#define	fwopen(cookie, fn) funopen(cookie, 0, fn, 0, 0)
+#define fropen(cookie, fn) funopen(cookie, fn, 0, 0, 0)
+#define fwopen(cookie, fn) funopen(cookie, 0, fn, 0, 0)
 #endif /* __BSD_VISIBLE */
 
 extern char* __fgets_chk(char*, int, FILE*, size_t);
diff --git a/libc/libc.arm.brillo.map b/libc/libc.arm.brillo.map
index d8d5033..8280d91 100644
--- a/libc/libc.arm.brillo.map
+++ b/libc/libc.arm.brillo.map
@@ -1233,6 +1233,7 @@
     freeifaddrs;
     fseeko64;
     fsetpos64;
+    funopen64;
     getgrgid_r;
     getgrnam_r;
     getifaddrs;
diff --git a/libc/libc.arm.map b/libc/libc.arm.map
index df8030f..536273d 100644
--- a/libc/libc.arm.map
+++ b/libc/libc.arm.map
@@ -1233,6 +1233,7 @@
     freeifaddrs;
     fseeko64;
     fsetpos64;
+    funopen64;
     getgrgid_r;
     getgrnam_r;
     getifaddrs;
diff --git a/libc/libc.arm64.map b/libc/libc.arm64.map
index 838a978..057e167 100644
--- a/libc/libc.arm64.map
+++ b/libc/libc.arm64.map
@@ -1157,6 +1157,7 @@
     freeifaddrs;
     fseeko64;
     fsetpos64;
+    funopen64;
     getgrgid_r;
     getgrnam_r;
     getifaddrs;
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 9317f81..32ef22a 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1260,6 +1260,7 @@
     freeifaddrs;
     fseeko64;
     fsetpos64;
+    funopen64;
     getgrgid_r;
     getgrnam_r;
     getifaddrs;
diff --git a/libc/libc.mips.brillo.map b/libc/libc.mips.brillo.map
index 683fb24..ef3244a 100644
--- a/libc/libc.mips.brillo.map
+++ b/libc/libc.mips.brillo.map
@@ -1218,6 +1218,7 @@
     freeifaddrs;
     fseeko64;
     fsetpos64;
+    funopen64;
     getgrgid_r;
     getgrnam_r;
     getifaddrs;
diff --git a/libc/libc.mips.map b/libc/libc.mips.map
index f72d84f..9ab0c65 100644
--- a/libc/libc.mips.map
+++ b/libc/libc.mips.map
@@ -1218,6 +1218,7 @@
     freeifaddrs;
     fseeko64;
     fsetpos64;
+    funopen64;
     getgrgid_r;
     getgrnam_r;
     getifaddrs;
diff --git a/libc/libc.mips64.map b/libc/libc.mips64.map
index 838a978..057e167 100644
--- a/libc/libc.mips64.map
+++ b/libc/libc.mips64.map
@@ -1157,6 +1157,7 @@
     freeifaddrs;
     fseeko64;
     fsetpos64;
+    funopen64;
     getgrgid_r;
     getgrnam_r;
     getifaddrs;
diff --git a/libc/libc.x86.brillo.map b/libc/libc.x86.brillo.map
index 090d5c5..c6277dc 100644
--- a/libc/libc.x86.brillo.map
+++ b/libc/libc.x86.brillo.map
@@ -1217,6 +1217,7 @@
     freeifaddrs;
     fseeko64;
     fsetpos64;
+    funopen64;
     getgrgid_r;
     getgrnam_r;
     getifaddrs;
diff --git a/libc/libc.x86.map b/libc/libc.x86.map
index f3bda0b..6497bf8 100644
--- a/libc/libc.x86.map
+++ b/libc/libc.x86.map
@@ -1217,6 +1217,7 @@
     freeifaddrs;
     fseeko64;
     fsetpos64;
+    funopen64;
     getgrgid_r;
     getgrnam_r;
     getifaddrs;
diff --git a/libc/libc.x86_64.map b/libc/libc.x86_64.map
index 838a978..057e167 100644
--- a/libc/libc.x86_64.map
+++ b/libc/libc.x86_64.map
@@ -1157,6 +1157,7 @@
     freeifaddrs;
     fseeko64;
     fsetpos64;
+    funopen64;
     getgrgid_r;
     getgrnam_r;
     getifaddrs;
diff --git a/libc/stdio/local.h b/libc/stdio/local.h
index 524e6de..7fe339a 100644
--- a/libc/stdio/local.h
+++ b/libc/stdio/local.h
@@ -72,7 +72,6 @@
 
 	// Function pointers used by `funopen`.
 	// Note that `_seek` is ignored if `_seek64` (in __sfileext) is set.
-	// TODO: implement `funopen64`.
 	// TODO: NetBSD has `funopen2` which corrects the `int`s to `size_t`s.
 	// TODO: glibc has `fopencookie` which passes the function pointers in a struct.
 	void* _cookie;	/* cookie passed to io functions */
diff --git a/libc/stdio/stdio.cpp b/libc/stdio/stdio.cpp
index 23b6971..2139621 100644
--- a/libc/stdio/stdio.cpp
+++ b/libc/stdio/stdio.cpp
@@ -538,3 +538,56 @@
   *pos = ftello64(fp);
   return (*pos == -1);
 }
+
+static FILE* __funopen(const void* cookie,
+                       int (*read_fn)(void*, char*, int),
+                       int (*write_fn)(void*, const char*, int),
+                       int (*close_fn)(void*)) {
+  if (read_fn == nullptr && write_fn == nullptr) {
+    errno = EINVAL;
+    return nullptr;
+  }
+
+  FILE* fp = __sfp();
+  if (fp == nullptr) return nullptr;
+
+  if (read_fn != nullptr && write_fn != nullptr) {
+    fp->_flags = __SRW;
+  } else if (read_fn != nullptr) {
+    fp->_flags = __SRD;
+  } else if (write_fn != nullptr) {
+    fp->_flags = __SWR;
+  }
+
+  fp->_file = -1;
+  fp->_cookie = const_cast<void*>(cookie); // The funopen(3) API is incoherent.
+  fp->_read = read_fn;
+  fp->_write = write_fn;
+  fp->_close = close_fn;
+
+  return fp;
+}
+
+FILE* funopen(const void* cookie,
+              int (*read_fn)(void*, char*, int),
+              int (*write_fn)(void*, const char*, int),
+              fpos_t (*seek_fn)(void*, fpos_t, int),
+              int (*close_fn)(void*)) {
+  FILE* fp = __funopen(cookie, read_fn, write_fn, close_fn);
+  if (fp != nullptr) {
+    fp->_seek = seek_fn;
+  }
+  return fp;
+}
+
+FILE* funopen64(const void* cookie,
+                int (*read_fn)(void*, char*, int),
+                int (*write_fn)(void*, const char*, int),
+                fpos64_t (*seek_fn)(void*, fpos64_t, int),
+                int (*close_fn)(void*)) {
+  FILE* fp = __funopen(cookie, read_fn, write_fn, close_fn);
+  if (fp != nullptr) {
+    _EXT(fp)->_seek64 = seek_fn;
+  }
+  return fp;
+}
diff --git a/libc/upstream-openbsd/lib/libc/stdio/funopen.c b/libc/upstream-openbsd/lib/libc/stdio/funopen.c
deleted file mode 100644
index b85ee96..0000000
--- a/libc/upstream-openbsd/lib/libc/stdio/funopen.c
+++ /dev/null
@@ -1,68 +0,0 @@
-/*	$OpenBSD: funopen.c,v 1.8 2005/08/08 08:05:36 espie Exp $ */
-/*-
- * Copyright (c) 1990, 1993
- *	The Regents of the University of California.  All rights reserved.
- *
- * This code is derived from software contributed to Berkeley by
- * Chris Torek.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- * 1. Redistributions of source code must retain the above copyright
- *    notice, this list of conditions and the following disclaimer.
- * 2. Redistributions in binary form must reproduce the above copyright
- *    notice, this list of conditions and the following disclaimer in the
- *    documentation and/or other materials provided with the distribution.
- * 3. Neither the name of the University nor the names of its contributors
- *    may be used to endorse or promote products derived from this software
- *    without specific prior written permission.
- *
- * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
- * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
- * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
- * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
- * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
- * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
- * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
- * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
- * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
- * SUCH DAMAGE.
- */
-
-#include <stdio.h>
-#include <errno.h>
-#include "local.h"
-
-FILE *
-funopen(const void *cookie, int (*readfn)(void *, char *, int),
-	int (*writefn)(void *, const char *, int),
-	fpos_t (*seekfn)(void *, fpos_t, int), int (*closefn)(void *))
-{
-	FILE *fp;
-	int flags;
-
-	if (readfn == NULL) {
-		if (writefn == NULL) {		/* illegal */
-			errno = EINVAL;
-			return (NULL);
-		} else
-			flags = __SWR;		/* write only */
-	} else {
-		if (writefn == NULL)
-			flags = __SRD;		/* read only */
-		else
-			flags = __SRW;		/* read-write */
-	}
-	if ((fp = __sfp()) == NULL)
-		return (NULL);
-	fp->_flags = flags;
-	fp->_file = -1;
-	fp->_cookie = (void *)cookie;		/* SAFE: cookie not modified */
-	fp->_read = readfn;
-	fp->_write = writefn;
-	fp->_seek = seekfn;
-	fp->_close = closefn;
-	return (fp);
-}