# Ccache clang and -fno-pch-timestamp

**URL:** https://discourse.cmake.org/t/ccache-clang-and-fno-pch-timestamp/7253
**Category:** Code
**Created:** [January 16, 2023, 9:24am UTC](https://discourse.cmake.org/t/ccache-clang-and-fno-pch-timestamp/7253 "2023-01-16T09:24:00Z")
**Posts on this page:** 6
**Page:** 1

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### Author: ![silverqx](https://discourse.cmake.org/user_avatar/discourse.cmake.org/silverqx/32/2353_2.png) [@silverqx](https://discourse.cmake.org/u/silverqx)
#### Post date: [January 16, 2023, 9:24am UTC](https://discourse.cmake.org/t/ccache-clang-and-fno-pch-timestamp/7253/1 "2023-01-16T09:24:00Z")

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The latest ccache recommends adding `-fno-pch-timestamp` to the Clang compile command for PCH, but I can’t find anywhere how to do it in CMake.

I’m creating precompiled header like this:

```auto
target_precompile_headers(${TinyOrm_target} PRIVATE $<$<COMPILE_LANGUAGE:CXX>:"pch.h">)

if(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
    target_compile_definitions(${TinyOrm_target} PRIVATE TINYORM_USING_PCH)
endif()

```

My question is how can I add the `-fno-pch-timestamp` compiler option only to this precompiled header compiler command?

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<div class="post-metadata">

### Author: ![craig.scott](https://discourse.cmake.org/user_avatar/discourse.cmake.org/craig.scott/32/20_2.png) [@craig.scott](https://discourse.cmake.org/u/craig.scott)
#### Post date: [January 16, 2023, 8:33pm UTC](https://discourse.cmake.org/t/ccache-clang-and-fno-pch-timestamp/7253/2 "2023-01-16T20:33:06Z")

</div>

The `target_compile_options()` command is probably what you are looking for. The tricky part here though is that you need to precede the option with `-Xclang` because it needs to be passed to the internal compiler, not given to the compiler front end like most options.

```auto
if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
    target_compile_options(${TinyOrm_target} PRIVATE
        "$<$<COMPILE_LANGUAGE:CXX>:SHELL:-Xclang -fno-pch-timestamp>"
    )
endif()

```

We need to use `SHELL:` to keep the two flags together and prevent any de-duplication with other `-Xclang` options that may also appear on the command line.

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<div class="post-metadata">

### Author: ![silverqx](https://discourse.cmake.org/user_avatar/discourse.cmake.org/silverqx/32/2353_2.png) [@silverqx](https://discourse.cmake.org/u/silverqx)
#### Post date: [January 17, 2023, 11:49am UTC](https://discourse.cmake.org/t/ccache-clang-and-fno-pch-timestamp/7253/3 "2023-01-17T11:49:46Z")

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Thx, it compiles. But it also adds it to every compile command, what I understood from this [PR](https://github.com/llvm/llvm-project/commit/533a893fa126d8a9662d836ad28f51afa1d2e74a) then this option should be only added to the compile command for PCH, so compile command that contains `-emit-pch` option. We don’t need to add it to compile commands for \*.cpp files.

I know that CMake uses this variable to set options for the PCH compile command:  
`CMAKE_${lang}_COMPILE_OPTIONS_CREATE_PCH`.

> **You can use this to commands to check if the \`-fno-pch-timestamp\` option is in effect.**
>
> pragma-once2-pch.h:
> 
> ```auto
> #include "pragma-once2.h"
> 
> ```
> 
> pragma-once2.h:
> 
> ```auto
> #pragma once
> 
> inline void f() {}
> 
> ```
> 
> Should fail:
> 
> ```auto
> clang++ -std=c++20 -Winvalid-pch -Xclang -emit-pch -x c++-header -o ./cmake_pch.hxx.pch ./pragma-once2-pch.h
> touch -m -a -t 201008011501 ./pragma-once2.h
> clang++ -std=c++20 -include-pch ./cmake_pch.hxx.pch ./include-timestamp.cpp
> 
> ```
> 
> Should compile ok:
> 
> ```auto
> clang++ -Xclang -fno-pch-timestamp -std=c++20 -Winvalid-pch -Xclang -emit-pch -x c++-header -o ./cmake_pch.hxx.pch ./pragma-once2-pch.h
> touch -m -a -t 201008011501 ./pragma-once2.h
> clang++ -std=c++20 -include-pch ./cmake_pch.hxx.pch ./include-timestamp.cpp
> 
> ```

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<div class="post-metadata">

### Author: ![silverqx](https://discourse.cmake.org/user_avatar/discourse.cmake.org/silverqx/32/2353_2.png) [@silverqx](https://discourse.cmake.org/u/silverqx)
#### Post date: [January 17, 2023, 3:46pm UTC](https://discourse.cmake.org/t/ccache-clang-and-fno-pch-timestamp/7253/4 "2023-01-17T15:46:50Z")

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I have something like this at the end of the day:

```auto
if(NOT MSVC AND CMAKE_CXX_COMPILER_ID MATCHES "Clang")
    list(APPEND CMAKE_CXX_COMPILE_OPTIONS_CREATE_PCH -Xclang -fno-pch-timestamp)
endif()

```

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<div class="post-metadata">

### Author: ![craig.scott](https://discourse.cmake.org/user_avatar/discourse.cmake.org/craig.scott/32/20_2.png) [@craig.scott](https://discourse.cmake.org/u/craig.scott)
#### Post date: [January 17, 2023, 9:44pm UTC](https://discourse.cmake.org/t/ccache-clang-and-fno-pch-timestamp/7253/5 "2023-01-17T21:44:28Z")

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`CMAKE_CXX_COMPILE_OPTIONS_CREATE_PCH` is not a publicly documented part of CMake’s API. It could change at any time without notice, including the way it gets used or formed internally. You would be using that at your own risk.

As far as I can tell, there doesn’t seem to be any harm in having `-fno-pch-timestamp` for all command lines. It doesn’t generate any warnings for non-PCH compiles, at least not that I’ve seen.

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<div class="post-metadata">

### Author: ![silverqx](https://discourse.cmake.org/user_avatar/discourse.cmake.org/silverqx/32/2353_2.png) [@silverqx](https://discourse.cmake.org/u/silverqx)
#### Post date: [January 18, 2023, 7:36am UTC](https://discourse.cmake.org/t/ccache-clang-and-fno-pch-timestamp/7253/6 "2023-01-18T07:36:37Z")

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You are right, everything you wrote is true.