diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -427,6 +427,7 @@
     libc.src.stdio.fgetc_unlocked
     libc.src.stdio.fgets
     libc.src.stdio.fflush
+    libc.src.stdio.fileno
     libc.src.stdio.fopen
     libc.src.stdio.fputc
     libc.src.stdio.fputs
diff --git a/libc/spec/posix.td b/libc/spec/posix.td
--- a/libc/spec/posix.td
+++ b/libc/spec/posix.td
@@ -1121,6 +1121,11 @@
       [], // Types
       [], // Enumerations
       [
+          FunctionSpec<
+              "fileno",
+              RetValSpec<IntType>,
+              [ArgSpec<FILEPtr>]
+          >,
           FunctionSpec<
               "flockfile",
               RetValSpec<VoidType>,
diff --git a/libc/src/stdio/CMakeLists.txt b/libc/src/stdio/CMakeLists.txt
--- a/libc/src/stdio/CMakeLists.txt
+++ b/libc/src/stdio/CMakeLists.txt
@@ -149,6 +149,19 @@
     libc.src.__support.File.platform_file
 )
 
+add_entrypoint_object(
+  fileno
+  SRCS
+    fileno.cpp
+  HDRS
+    fileno.h
+  DEPENDS
+    libc.src.errno.errno
+    libc.include.stdio
+    libc.src.__support.File.file
+    libc.src.__support.File.platform_file
+)
+
 add_entrypoint_object(
   getc
   SRCS
diff --git a/libc/src/stdio/fileno.h b/libc/src/stdio/fileno.h
new file mode 100644
--- /dev/null
+++ b/libc/src/stdio/fileno.h
@@ -0,0 +1,20 @@
+//===-- Implementation header of fileno -------------------------*- 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_FILENO_H
+#define LLVM_LIBC_SRC_STDIO_FILENO_H
+
+#include <stdio.h>
+
+namespace __llvm_libc {
+
+int fileno(::FILE *stream);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STDIO_FILENO_H
diff --git a/libc/src/stdio/fileno.cpp b/libc/src/stdio/fileno.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/stdio/fileno.cpp
@@ -0,0 +1,28 @@
+//===-- Implementation of fileno ------------------------------------------===//
+//
+// 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/fileno.h"
+#include "src/__support/File/file.h"
+
+#include "src/errno/libc_errno.h"
+#include <stdio.h>
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(int, fileno, (::FILE *__restrict stream)) {
+  auto fd = get_fileno(reinterpret_cast<__llvm_libc::File *>(stream));
+
+  if (fd >= 0) {
+    return fd;
+  } else {
+    libc_errno = EBADF;
+    return -1;
+  }
+}
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/stdio/CMakeLists.txt b/libc/test/src/stdio/CMakeLists.txt
--- a/libc/test/src/stdio/CMakeLists.txt
+++ b/libc/test/src/stdio/CMakeLists.txt
@@ -366,6 +366,18 @@
     libc.src.stdio.fwrite
 )
 
+add_libc_unittest(
+  fileno_test
+  SUITE
+    libc_stdio_unittests
+  SRCS
+    fileno_test.cpp
+  DEPENDS
+    libc.include.stdio
+    libc.src.stdio.fopen
+    libc.src.stdio.fileno
+)
+
 add_libc_unittest(
   ftell_test
   SUITE
diff --git a/libc/test/src/stdio/fileno_test.cpp b/libc/test/src/stdio/fileno_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/stdio/fileno_test.cpp
@@ -0,0 +1,33 @@
+//===-- Unittests for fileno ----------------------------------------------===//
+//
+// 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/__support/File/file.h"
+#include "src/stdio/fileno.h"
+#include "src/stdio/fopen.h"
+#include "test/UnitTest/Test.h"
+#include <stdio.h>
+
+TEST(LlvmLibcFilenoTest, ValidFilenoTest) {
+  ::FILE *file = __llvm_libc::fopen("testdata/test_data.txt", "w");
+  ASSERT_FALSE(file == nullptr);
+
+  // file descriptors associated with the streams stdin, stdout, and stderr are
+  // 0, 1, and 2, respectively. Therefore any newly opened file should have an
+  // integer file descriptor >= 3
+
+  ASSERT_FALSE(__llvm_libc::fileno(file) < 3);
+}
+
+TEST(LlvmLibcFilenoTest, StandardStreamTest) {
+  ASSERT_EQ(__llvm_libc::fileno(reinterpret_cast<FILE *>(__llvm_libc::stdin)),
+            0);
+  ASSERT_EQ(__llvm_libc::fileno(reinterpret_cast<FILE *>(__llvm_libc::stdout)),
+            1);
+  ASSERT_EQ(__llvm_libc::fileno(reinterpret_cast<FILE *>(__llvm_libc::stderr)),
+            2);
+}