Index: libc/config/linux/api.td
===================================================================
--- libc/config/linux/api.td
+++ libc/config/linux/api.td
@@ -18,6 +18,12 @@
   }];
 }
 
+def FILE : TypeDecl<"FILE"> {
+  let Decl = [{
+    typedef struct {} FILE;
+  }];
+}
+
 def AssertMacro : MacroDef<"assert"> {
   let Defn = [{
     #undef assert
@@ -159,10 +165,11 @@
 def StdIOAPI : PublicAPI<"stdio.h"> {
   let TypeDeclarations = [
     SizeT,
+    FILE,
   ];
 
   let Functions = [
-    "snprintf",
+    "fwrite",
   ];
 }
 
Index: libc/include/CMakeLists.txt
===================================================================
--- libc/include/CMakeLists.txt
+++ libc/include/CMakeLists.txt
@@ -75,6 +75,14 @@
     ../config/${LIBC_TARGET_OS}/signal.h.in
 )
 
+add_gen_header(
+  stdio_h
+  DEF_FILE stdio.h.def
+  GEN_HDR stdio.h
+  DEPENDS
+    llvm_libc_common_h
+)
+
 add_gen_header(
   stdlib_h
   DEF_FILE stdlib.h.def
Index: libc/include/stdio.h.def
===================================================================
--- /dev/null
+++ libc/include/stdio.h.def
@@ -0,0 +1,16 @@
+//===-- C standard library header stdio.h ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_STDIO_H
+#define LLVM_LIBC_STDIO_H
+
+#include <__llvm-libc-common.h>
+
+%%public_api()
+
+#endif // LLVM_LIBC_STDIO_H
Index: libc/spec/stdc.td
===================================================================
--- libc/spec/stdc.td
+++ libc/spec/stdc.td
@@ -18,6 +18,10 @@
 
   NamedType SigHandlerT = NamedType<"__sighandler_t">;
 
+  NamedType FILE = NamedType<"FILE">;
+  PtrType FILEPtr = PtrType<FILE>;
+  RestrictedPtrType FILERestrictedPtr = RestrictedPtrType<FILE>;
+
   HeaderSpec Assert = HeaderSpec<
       "assert.h",
       [
@@ -190,16 +194,17 @@
       [], // Macros
       [ // Types
           SizeTType,
+          FILE,
       ],
       [], // Enumerations
       [
           FunctionSpec<
-              "snprintf",
-              RetValSpec<IntType>,
-              [ArgSpec<CharPtr>,
-               ArgSpec<SizeTType>,
-               ArgSpec<ConstCharRestrictedPtr>,
-               ArgSpec<VarArgType>]
+              "fwrite",
+              RetValSpec<SizeTType>,
+              [ArgSpec<ConstVoidRestrictedPtr>,
+              ArgSpec<SizeTType>,
+              ArgSpec<SizeTType>,
+              ArgSpec<FILERestrictedPtr>]
           >,
       ]
   >;
Index: libc/src/CMakeLists.txt
===================================================================
--- libc/src/CMakeLists.txt
+++ libc/src/CMakeLists.txt
@@ -2,6 +2,7 @@
 add_subdirectory(errno)
 add_subdirectory(math)
 add_subdirectory(signal)
+add_subdirectory(stdio)
 add_subdirectory(stdlib)
 add_subdirectory(string)
 # TODO: Add this target conditional to the target OS.
Index: libc/src/stdio/CMakeLists.txt
===================================================================
--- /dev/null
+++ libc/src/stdio/CMakeLists.txt
@@ -0,0 +1,11 @@
+
+add_entrypoint_object(
+  fwrite
+  SRCS
+    fwrite.cpp
+  HDRS
+    fwrite.h
+  DEPENDS
+    mtx_lock
+    mtx_unlock
+)
Index: libc/src/stdio/FILE.h
===================================================================
--- /dev/null
+++ libc/src/stdio/FILE.h
@@ -0,0 +1,29 @@
+//===-- Internal definition of FILE -----------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STDIO_FILE_H
+#define LLVM_LIBC_SRC_STDIO_FILE_H
+
+#include "include/threads.h"
+#include <stddef.h>
+
+namespace __llvm_libc {
+
+// TOOD: This should return ssize_t but it's defined in sys/types.h
+using cookie_write_function_t = size_t(void *, const char *, size_t);
+
+struct FILE {
+  mtx_t lock;
+
+  void *cookie;
+  cookie_write_function_t *write;
+};
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STDIO_FILE_H
Index: libc/src/stdio/fwrite.h
===================================================================
--- /dev/null
+++ libc/src/stdio/fwrite.h
@@ -0,0 +1,24 @@
+//===-- Implementation header of fwrite and fwrite_unlocked -----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STDIO_FWRITE_H
+#define LLVM_LIBC_SRC_STDIO_FWRITE_H
+
+#include "src/stdio/FILE.h"
+#include <stddef.h>
+
+namespace __llvm_libc {
+
+size_t fwrite_unlocked(const void *__restrict ptr, size_t size, size_t nmeb,
+                       FILE *__restrict stream);
+size_t fwrite(const void *__restrict ptr, size_t size, size_t nmeb,
+              FILE *__restrict stream);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STDIO_FWRITE_H
Index: libc/src/stdio/fwrite.cpp
===================================================================
--- /dev/null
+++ libc/src/stdio/fwrite.cpp
@@ -0,0 +1,30 @@
+//===-- Implementation of fwrite and fwrite_unlocked ------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/stdio/fwrite.h"
+#include "src/stdio/FILE.h"
+#include "src/threads/mtx_lock.h"
+#include "src/threads/mtx_unlock.h"
+
+namespace __llvm_libc {
+
+size_t fwrite_unlocked(const void *__restrict ptr, size_t size, size_t nmeb,
+                       FILE *__restrict stream) {
+  return stream->write(stream->cookie, reinterpret_cast<const char *>(ptr),
+                       size * nmeb);
+}
+
+size_t fwrite(const void *__restrict ptr, size_t size, size_t nmeb,
+              FILE *__restrict stream) {
+  __llvm_libc::mtx_lock(&stream->lock);
+  size_t written = fwrite_unlocked(ptr, size, nmeb, stream);
+  __llvm_libc::mtx_unlock(&stream->lock);
+  return written;
+}
+
+} // namespace __llvm_libc
Index: libc/test/config/linux/x86_64/syscall_test.cpp
===================================================================
--- libc/test/config/linux/x86_64/syscall_test.cpp
+++ libc/test/config/linux/x86_64/syscall_test.cpp
@@ -9,36 +9,45 @@
 #include "config/linux/syscall.h"
 #include "utils/UnitTest/Test.h"
 
-#include <functional>
+namespace __llvm_libc {
+
+template <typename Func> struct function;
+
+template <typename Ret, typename... Params> struct function<Ret(Params...)> {
+  Ret (*func)(Params...);
+
+  template <typename Func> function(Func &&f) : func(f) {}
+};
+
+} // namespace __llvm_libc
 
 TEST(X86_64_SyscallTest, APITest) {
   // We only do a signature test here. Actual functionality tests are
   // done by the unit tests of the syscall wrappers like mmap.
 
-  std::function<long(long)> f([](long n) { return __llvm_libc::syscall(n); });
-  std::function<long(long, long)> f1(
+  __llvm_libc::function<long(long)> f(
+      [](long n) { return __llvm_libc::syscall(n); });
+  __llvm_libc::function<long(long, long)> f1(
       [](long n, long a1) { return __llvm_libc::syscall(n, a1); });
-  std::function<long(long, long, long)> f2(
+  __llvm_libc::function<long(long, long, long)> f2(
       [](long n, long a1, long a2) { return __llvm_libc::syscall(n, a1, a2); });
-  std::function<long(long, long, long, long)> f3(
+  __llvm_libc::function<long(long, long, long, long)> f3(
       [](long n, long a1, long a2, long a3) {
         return __llvm_libc::syscall(n, a1, a2, a3);
       });
-  std::function<long(long, long, long, long, long)> f4(
+  __llvm_libc::function<long(long, long, long, long, long)> f4(
       [](long n, long a1, long a2, long a3, long a4) {
         return __llvm_libc::syscall(n, a1, a2, a3, a4);
       });
-  std::function<long(long, long, long, long, long, long)> f5(
+  __llvm_libc::function<long(long, long, long, long, long, long)> f5(
       [](long n, long a1, long a2, long a3, long a4, long a5) {
         return __llvm_libc::syscall(n, a1, a2, a3, a4, a5);
       });
-  std::function<long(long, long, long, long, long, long, long)> f6(
+  __llvm_libc::function<long(long, long, long, long, long, long, long)> f6(
       [](long n, long a1, long a2, long a3, long a4, long a5, long a6) {
         return __llvm_libc::syscall(n, a1, a2, a3, a4, a5, a6);
       });
 
-  std::function<long(long, void *)> notLongType(
-      [](long n, void *a1) {
-        return __llvm_libc::syscall(n, a1);
-      });
+  __llvm_libc::function<long(long, void *)> notLongType(
+      [](long n, void *a1) { return __llvm_libc::syscall(n, a1); });
 }
Index: libc/test/src/CMakeLists.txt
===================================================================
--- libc/test/src/CMakeLists.txt
+++ libc/test/src/CMakeLists.txt
@@ -1,6 +1,7 @@
 add_subdirectory(assert)
 add_subdirectory(errno)
 add_subdirectory(signal)
+add_subdirectory(stdio)
 add_subdirectory(stdlib)
 add_subdirectory(string)
 add_subdirectory(sys)
Index: libc/test/src/stdio/CMakeLists.txt
===================================================================
--- /dev/null
+++ libc/test/src/stdio/CMakeLists.txt
@@ -0,0 +1,14 @@
+add_libc_testsuite(libc_stdio_unittests)
+
+add_libc_unittest(
+  fwrite_test
+  SUITE
+    libc_stdio_unittests
+  SRCS
+    fwrite_test.cpp
+  DEPENDS
+    fwrite
+    # TODO(sivachandra): remove private dependencies of fwrite
+    mtx_lock
+    mtx_unlock
+)
Index: libc/test/src/stdio/fwrite_test.cpp
===================================================================
--- /dev/null
+++ libc/test/src/stdio/fwrite_test.cpp
@@ -0,0 +1,26 @@
+//===-- Unittests for fwrite ----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/stdio/FILE.h"
+#include "src/stdio/fwrite.h"
+#include "utils/CPP/Array.h"
+#include "utils/UnitTest/Test.h"
+
+TEST(Stdio, FWriteBasic) {
+  __llvm_libc::FILE f;
+  char array[6];
+  f.cookie = array;
+  f.write = +[](void *cookie, const char *ptr, size_t size) {
+    char *buf = reinterpret_cast<char *>(cookie);
+    for (size_t i = 0; i < size; ++i)
+      buf[i] = ptr[i];
+    return size;
+  };
+  EXPECT_EQ(fwrite("hello", 1, 6, &f), 6UL);
+  EXPECT_STREQ(array, "hello");
+}