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 {
+    errno = EBADF;
+    return -1;
+  }
+}
+
+} // namespace __llvm_libc