[libc][hdr-gen] Add a new 'begin' and `end command to control headers

[jhuber6]

Summary

This patch adds a %%begin() and %%end() command to allow the
headergen binary to create custom wrappers and preamples around the
standard header. This does nothing in this patch and is purely NFC. A
future patch will use this to implement GPU wrapper headers as in
https://discourse.llvm.org/t/rfc-implementing-gpu-headers-in-the-llvm-c-library/71523.

[jhuber6]

I'm realizing that a better but more invasive solution would be to have something like the following:

%%begin()
#include <__llvm_libc_common.h>
%%end()

%%public_api()

After trying to base my offloading support off of this I realized that several headers have non-trivial constructions, so what I really need is a hook to wrap around these and insert some GPU headers at the start. So it would look more like

#include <llvm-libc-macros/gpu-macros.h>

#ifdef __OFFLOADING
#include_next <stdheader.h>
#else
#include <__llvm_libc_common.h>
#endif

#ifndef __OFFLOADING
extern SomeType;
#endif

void some_entrypoint.

[jhuber6]

Changing approach

[michaelrj]

I think we should nail down the design before we start making changes. More specifically, what are the begin and end commands going to do?

libc/utils/HdrGen/BeginCommand.cpp

25

nit: should be begin

[jhuber6]

Fixing nit.

I'm going to upload a follow-up patch very soon. I simply wanted to separate all the line noise of updating the headers from that patch.

libc/include/arpa/inet.h.def

 //===-- C standard library header network.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_ARPA_INET_H
 #define LLVM_LIBC_ARPA_INET_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <inttypes.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_ARPA_INET_H

libc/include/assert.h.def

 //===-- C standard library header assert.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
 //
 //===----------------------------------------------------------------------===//
 
+%%begin()
 #include <__llvm-libc-common.h>
+%%end()
 
 // This file may be usefully included multiple times to change assert()'s
 // definition based on NDEBUG.
 
 %%public_api()

libc/include/ctype.h.def

 //===-- C standard library header ctype.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_CTYPE_H
 #define LLVM_LIBC_CTYPE_H
 
+%%begin()
 #include <__llvm-libc-common.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_CTYPE_H

libc/include/dirent.h.def

 //===-- POSIX header dirent.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_DIRENT_H
 #define LLVM_LIBC_DIRENT_H
 
+%%begin()
 #include <__llvm-libc-common.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_DIRENT_H

libc/include/errno.h.def

 //===-- C standard library header errno.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_ERRNO_H
 #define LLVM_LIBC_ERRNO_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #ifdef __linux__
 
 #include <linux/errno.h>
 
 #ifndef ERFKILL
 #define ERFKILL 132

 #else // __linux__
 #include <llvm-libc-macros/generic-error-number-macros.h>
 #endif
 
 #if !defined(__AMDGPU__) && !defined(__NVPTX__)
 extern _Thread_local int __llvmlibc_errno;
 #define errno __llvmlibc_errno
 #endif
+%%end()
 
 #endif // LLVM_LIBC_ERRNO_H

libc/include/fcntl.h.def

 //===-- C standard library header fcntl.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_FCNTL_H
 #define LLVM_LIBC_FCNTL_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/fcntl-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_FCNTL_H 

libc/include/fenv.h.def

 //===-- C standard library header fenv.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_FENV_H
 #define LLVM_LIBC_FENV_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/fenv-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_FENV_H

libc/include/gpu/rpc.h.def

 //===-- GPU header rpc.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_GPU_RPC_H
 #define LLVM_LIBC_GPU_RPC_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-types/rpc_opcodes_t.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_GPU_RPC_H

libc/include/inttypes.h.def

 //===-- C standard library header inttypes.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_INTTYPES_H
 #define LLVM_LIBC_INTTYPES_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <stdint.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_INTTYPES_H

libc/include/math.h.def

 //===-- C standard library header math.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_MATH_H
 #define LLVM_LIBC_MATH_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/math-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_MATH_H

libc/include/pthread.h.def

 //===-- POSIX header pthread.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_PTHREAD_H
 #define LLVM_LIBC_PTHREAD_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #define PTHREAD_STACK_MIN (1 << 14) // 16KB
 
 #define PTHREAD_ONCE_INIT {0}
 
 enum {
   PTHREAD_CREATE_JOINABLE = 0x0,
   PTHREAD_CREATE_DETACHED = 0x1,
 
   PTHREAD_MUTEX_NORMAL = 0x0,
   PTHREAD_MUTEX_ERRORCHECK = 0x1,
   PTHREAD_MUTEX_RECURSIVE = 0x2,
   PTHREAD_MUTEX_DEFAULT = PTHREAD_MUTEX_NORMAL,
 
   PTHREAD_PROCESS_PRIVATE = 0x0,
   PTHREAD_PROCESS_SHARED = 0x1,
 
   PTHREAD_MUTEX_STALLED = 0x0,
   PTHREAD_MUTEX_ROBUST = 0x1,
 };
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_PTHREAD_H

libc/include/sched.h.def

 //===-- C standard library header sched.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_SCHED_H
 #define LLVM_LIBC_SCHED_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/sched-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SCHED_H 

libc/include/setjmp.h.def

 //===-- C standard library header setjmp.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_SETJMP_H
 #define LLVM_LIBC_SETJMP_H
 
+%%begin()
 #include <__llvm-libc-common.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SETJMP_H

libc/include/signal.h.def

 //===-- C standard library header signal.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_SIGNAL_H
 #define LLVM_LIBC_SIGNAL_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #define __need_size_t
 #include <stddef.h>
 
 #include <llvm-libc-macros/signal-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SIGNAL_H

libc/include/spawn.h.def

 //===-- POSIX header spawn.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_SPAWN_H
 #define LLVM_LIBC_SPAWN_H
 
+%%begin()
 #include <__llvm-libc-common.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SPAWN_H 

libc/include/stdio.h.def

 //===-- 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
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/file-seek-macros.h>
 #include <llvm-libc-macros/stdio-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_STDIO_H

libc/include/stdlib.h.def

 //===-- C standard library header stdlib.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_STDLIB_H
 #define LLVM_LIBC_STDLIB_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/stdlib-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_STDLIB_H

libc/include/string.h.def

 //===-- C standard library header string.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_STRING_H
 #define LLVM_LIBC_STRING_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-macros/null-macro.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_STRING_H

libc/include/strings.h.def

 //===-- C standard library header strings.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_STRINGS_H
 #define LLVM_LIBC_STRINGS_H
 
+%%begin()
 #include <__llvm-libc-common.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_STRINGS_H

libc/include/sys/auxv.h.def

 //===-- GNU header auxv.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_SYS_AUXV_H
 #define LLVM_LIBC_SYS_AUXV_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-macros/sys-auxv-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_AUXV_H

libc/include/sys/ioctl.h.def

 //===-- POSIX header ioctl.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_SYS_IOCTL_H
 #define LLVM_LIBC_SYS_IOCTL_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-macros/sys-ioctl-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_IOCTL_H

libc/include/sys/mman.h.def

 //===-- POSIX header mman.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_SYS_MMAN_H
 #define LLVM_LIBC_SYS_MMAN_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-macros/sys-mman-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_MMAN_H

libc/include/sys/prctl.h.def

 //===-- Linux header prctl.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_SYS_PRCTL_H
 #define LLVM_LIBC_SYS_PRCTL_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 // Process control is highly platform specific, so the platform usually defines
 // the macros itself.
 #include <linux/prctl.h>
 
 // TODO: Define the prctl macros.
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_PRCTL_H

libc/include/sys/random.h.def

 //===-- Linux sys/random.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_SYS_RANDOM_H
 #define LLVM_LIBC_SYS_RANDOM_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-macros/sys-random-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_RANDOM_H

libc/include/sys/resource.h.def

 //===-- POSIX header resource.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_SYS_RESOURCE_H
 #define LLVM_LIBC_SYS_RESOURCE_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-macros/sys-resource-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_RESOURCE_H

libc/include/sys/select.h.def

 //===-- Linux sys/select.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_SYS_SELECT_H
 #define LLVM_LIBC_SYS_SELECT_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-macros/sys-select-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_SELECT_H

libc/include/sys/sendfile.h.def

 //===-- Linux sys/sendfile.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_SYS_SENDFILE_H
 #define LLVM_LIBC_SYS_SENDFILE_H
 
+%%begin()
 #include <__llvm-libc-common.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_SENDFILE_H

libc/include/sys/socket.h.def

 //===-- POSIX header sys/socket.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_SYS_SOCKET_H
 #define LLVM_LIBC_SYS_SOCKET_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-macros/sys-socket-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_SOCKET_H

libc/include/sys/stat.h.def

 //===-- POSIX header stat.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_SYS_STAT_H
 #define LLVM_LIBC_SYS_STAT_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-macros/sys-stat-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_STAT_H

libc/include/sys/time.h.def

 //===-- Linux header time.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_SYS_TIME_H
 #define LLVM_LIBC_SYS_TIME_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-types/struct_timeval.h>
 
 #include <llvm-libc-macros/sys-time-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_TIME_H

libc/include/sys/types.h.def

 //===-- Header for sys/types.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_SYS_TYPES_H
 #define LLVM_LIBC_SYS_TYPES_H
 
+%%begin()
 #include <__llvm-libc-common.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_TYPES_H

libc/include/sys/utsname.h.def

 //===-- Linux sys/utsname.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_SYS_UTSNAME_H
 #define LLVM_LIBC_SYS_UTSNAME_H
 
+%%begin()
 #include <__llvm-libc-common.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_UTSNAME_H

libc/include/sys/wait.h.def

 //===-- POSIX header wait.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_SYS_WAIT_H
 #define LLVM_LIBC_SYS_WAIT_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 
 #include <llvm-libc-macros/sys-wait-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_SYS_WAIT_H

libc/include/termios.h.def

 //===-- C standard library header termios.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_TERMIOS_H
 #define LLVM_LIBC_TERMIOS_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/termios-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_TERMIOS_H

libc/include/threads.h.def

 //===-- C standard library header threads.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_THREADS_H
 #define LLVM_LIBC_THREADS_H
 
+%%begin()
 #include <__llvm-libc-common.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_THREADS_H

libc/include/time.h.def

 //===-- C standard library header time.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_TIME_H
 #define LLVM_LIBC_TIME_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/time-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_TIME_H

libc/include/unistd.h.def

 //===-- C standard library header unistd.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_UNISTD_H
 #define LLVM_LIBC_UNISTD_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/file-seek-macros.h>
 #include <llvm-libc-macros/unistd-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_UNISTD_H

libc/include/wchar.h.def

 //===-- C standard library header wchar.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_WCHAR_H
 #define LLVM_LIBC_WCHAR_H
 
+%%begin()
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/wchar-macros.h>
+%%end()
 
 %%public_api()
 
 #endif // LLVM_LIBC_WCHAR_H

libc/utils/HdrGen/BeginCommand.h

+//===-- Class which implements the %%begin command --------------*- 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_UTILS_HDRGEN_BEGIN_COMMAND_H
+#define LLVM_LIBC_UTILS_HDRGEN_BEGIN_COMMAND_H
+
+#include "Command.h"
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+
+#include <string>
+
+namespace llvm_libc {
+
+class BeginCommand : public Command {
+public:
+  static const char Name[];
+
+  void run(llvm::raw_ostream &OS, const ArgVector &Args,
+           llvm::StringRef StdHeader, llvm::RecordKeeper &Records,
+           const Command::ErrorReporter &Reporter) const override;
+};
+
+} // namespace llvm_libc
+
+#endif // LLVM_LIBC_UTILS_HDRGEN_BEGIN_COMMAND_H

libc/utils/HdrGen/BeginCommand.cpp

+//===-- Implementation of BeginCommand ------------------------------------===//
+//
+// 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 "BeginCommand.h"
+
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/SourceMgr.h"
+
+#include <cstdlib>
+
+namespace llvm_libc {
+
+const char BeginCommand::Name[] = "begin";
+
+void BeginCommand::run(llvm::raw_ostream &OS, const ArgVector &Args,
+                       llvm::StringRef StdHeader, llvm::RecordKeeper &Records,
+                       const Command::ErrorReporter &Reporter) const {
+  if (Args.size() != 0)
+    Reporter.printFatalError("%%begin command does not take any arguments.");
+}
+
+} // namespace llvm_libc

libc/utils/HdrGen/CMakeLists.txt

 include(TableGen)
 
 set(LLVM_LINK_COMPONENTS Support)
 
 add_tablegen(libc-hdrgen LIBC
   Command.h
   Command.cpp
   Generator.cpp
   Generator.h
+  BeginCommand.cpp
+  BeginCommand.h
+  EndCommand.cpp
+  EndCommand.h
   IncludeFileCommand.cpp
   IncludeFileCommand.h
   Main.cpp
   PublicAPICommand.cpp
   PublicAPICommand.h
 )
 
 target_include_directories(libc-hdrgen PRIVATE ${LIBC_SOURCE_DIR})
 target_link_libraries(libc-hdrgen PRIVATE LibcTableGenUtil)
 
 add_subdirectory(PrototypeTestGen)

libc/utils/HdrGen/EndCommand.h

+//===-- Class which implements the %%end command ----------------*- 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_UTILS_HDRGEN_END_COMMAND_H
+#define LLVM_LIBC_UTILS_HDRGEN_END_COMMAND_H
+
+#include "Command.h"
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+
+#include <string>
+
+namespace llvm_libc {
+
+class EndCommand : public Command {
+public:
+  static const char Name[];
+
+  void run(llvm::raw_ostream &OS, const ArgVector &Args,
+           llvm::StringRef StdHeader, llvm::RecordKeeper &Records,
+           const Command::ErrorReporter &Reporter) const override;
+};
+
+} // namespace llvm_libc
+
+#endif // LLVM_LIBC_UTILS_HDRGEN_END_COMMAND_H

libc/utils/HdrGen/EndCommand.cpp

+//===-- Implementation of EndCommand --------------------------------------===//
+//
+// 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 "EndCommand.h"
+
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/SourceMgr.h"
+
+#include <cstdlib>
+
+namespace llvm_libc {
+
+const char EndCommand::Name[] = "end";
+
+void EndCommand::run(llvm::raw_ostream &OS, const ArgVector &Args,
+                     llvm::StringRef StdHeader, llvm::RecordKeeper &Records,
+                     const Command::ErrorReporter &Reporter) const {
+  if (Args.size() != 0)
+    Reporter.printFatalError("%%end command does not take any arguments.");
+}
+
+} // namespace llvm_libc

libc/utils/HdrGen/Generator.h

 class Command;
 
 class Generator {
   llvm::StringRef HeaderDefFile;
   const std::vector<std::string> &EntrypointNameList;
   llvm::StringRef StdHeader;
   std::unordered_map<std::string, std::string> &ArgMap;
 
+  std::unique_ptr<Command> BeginCmd;
+  std::unique_ptr<Command> EndCmd;
   std::unique_ptr<Command> IncludeFileCmd;
   std::unique_ptr<Command> PublicAPICmd;
 
   Command *getCommandHandler(llvm::StringRef CommandName);
 
   void parseCommandArgs(llvm::StringRef ArgStr, ArgVector &Args);
 
   void printError(llvm::StringRef Msg);

libc/utils/HdrGen/Generator.cpp

 //===-- Implementation of the main header generation class ----------------===//
 //
 // 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 "Generator.h"
 
+#include "BeginCommand.h"
+#include "EndCommand.h"
 #include "IncludeFileCommand.h"
 #include "PublicAPICommand.h"
 
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/raw_ostream.h"
 
static const size_t ParamNamePrefixSize =
     llvm::StringRef(ParamNamePrefix).size();
 static const char ParamNameSuffix[] = "}";
 static const size_t ParamNameSuffixSize =
     llvm::StringRef(ParamNameSuffix).size();
 
 namespace llvm_libc {
 
 Command *Generator::getCommandHandler(llvm::StringRef CommandName) {
-  if (CommandName == IncludeFileCommand::Name) {
+  if (CommandName == BeginCommand::Name) {
+    if (!BeginCmd)
+      BeginCmd = std::make_unique<BeginCommand>();
+    return BeginCmd.get();
+  } else if (CommandName == EndCommand::Name) {
+    if (!EndCmd)
+      EndCmd = std::make_unique<EndCommand>();
+    return EndCmd.get();
+  } else if (CommandName == IncludeFileCommand::Name) {
     if (!IncludeFileCmd)
       IncludeFileCmd = std::make_unique<IncludeFileCommand>();
     return IncludeFileCmd.get();
   } else if (CommandName == PublicAPICommand::Name) {
     if (!PublicAPICmd)
       PublicAPICmd = std::make_unique<PublicAPICommand>(EntrypointNameList);
     return PublicAPICmd.get();
   } else {