[libc] Implement fileno (#85628)

fixes: #85150

libc/config/linux/x86_64/entrypoints.txt

@@ -208,6 +208,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdio.sscanf
     libc.src.stdio.scanf
     libc.src.stdio.fscanf
+    libc.src.stdio.fileno
 
     # sys/epoll.h entrypoints
     libc.src.sys.epoll.epoll_wait

libc/src/stdio/CMakeLists.txt

@@ -236,6 +236,16 @@ add_entrypoint_object(
     libc.src.stdio.printf_core.vfprintf_internal
 )
 
+add_stdio_entrypoint_object(
+  fileno
+  SRCS
+    fileno.cpp
+  HDRS
+    fileno.h
+  DEPENDS
+    libc.src.stdio.fileno
+)
+
 add_subdirectory(printf_core)
 add_subdirectory(scanf_core)
 

libc/src/stdio/fileno.h

@@ -0,0 +1,21 @@
+//===-- 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 "include/llvm-libc-types/FILE.h"
+
+namespace LIBC_NAMESPACE {
+
+int fileno(::FILE *f);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STDIO_FILENO_H

libc/src/stdio/generic/CMakeLists.txt

@@ -70,6 +70,18 @@ add_entrypoint_object(
     libc.src.__support.File.platform_file
 )
 
+add_entrypoint_object(
+  fileno
+  SRCS
+    fileno.cpp
+  HDRS
+    ../fileno.h
+  DEPENDS
+    libc.include.stdio
+    libc.src.__support.File.file
+    libc.src.__support.File.platform_file
+)
+
 add_entrypoint_object(
   fflush
   SRCS

libc/src/stdio/generic/fileno.cpp

@@ -0,0 +1,21 @@
+//===-- 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 "include/llvm-libc-types/FILE.h"
+#include "src/__support/File/file.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(int, fileno, (::FILE * stream)) {
+  return get_fileno(reinterpret_cast<LIBC_NAMESPACE::File *>(stream));
+}
+
+} // namespace LIBC_NAMESPACE

libc/test/src/stdio/fileop_test.cpp

@@ -30,6 +30,7 @@ TEST(LlvmLibcFILETest, SimpleFileOperations) {
   constexpr char FILENAME[] = "testdata/simple_operations.test";
   ::FILE *file = LIBC_NAMESPACE::fopen(FILENAME, "w");
   ASSERT_FALSE(file == nullptr);
+  ASSERT_EQ(LIBC_NAMESPACE::fileno(file), 3);
   constexpr char CONTENT[] = "1234567890987654321";
   ASSERT_EQ(sizeof(CONTENT) - 1,
             LIBC_NAMESPACE::fwrite(CONTENT, 1, sizeof(CONTENT) - 1, file));