[CMake] CCACHE_DIR Environment Variable

[Craig Scott]

Ah, sorry, I misunderstood what CCACHE_DIR was for. Nevertheless, the
method in that linked article could be easily modified to do what you want.
Simply add the setting of the environment variable to the launch scripts.
Since those launch scripts are copied across to the build dir at configure
time and they support variable substitutions, you can even have the value
of CCACHE_DIR be settable from CMake if you want. For example, your
launch-c.in file could look like this:

#!/bin/sh
export CCACHE_CPP2=true
export CCACHE_DIR=${CCACHE_DIR}
exec "${RULE_LAUNCH_COMPILE}" "${CMAKE_C_COMPILER}" "$@"
When you run CMake, it substitutes the value of each of the above variables
as they are set when the configure_file() command is called. Therefore, the
configure step bakes in the value of CCACHE_DIR and it won't need to be set
in your environment when you do the build step. Note that the substitutions
are substituting CMake variables, not environment variables, so if you
wanted to pass through a CCACHE_DIR environment variable instead, you would
do something like this:

#!/bin/sh
export CCACHE_CPP2=true
export CCACHE_DIR=$*ENV*{CCACHE_DIR}
exec "${RULE_LAUNCH_COMPILE}" "${CMAKE_C_COMPILER}" "$@"
Probably better to do that in your CMakeLists.txt instead though and leave
the launch-c.in script as in the first case above. This would mean your
CMakeLists.txt would have a line like this somewhere:

set(CCACHE_DIR $ENV{CCACHE_DIR}).

Note, however, that relying on environment variables for configure or build
steps is not so robust. If, for example, you change a CMakeLists.txt file
or something else in your project requires CMake to be re-run, then CMake
can re-run as part of a build. This then means the environment variables
have to be the same between your CMake runs and your build runs. That's
normally not a problem for most people, but thought I'd mention it just in
case. Personally, I try to avoid relying on environment variables and
instead have such values passed in as CMake cache variables like so:

cmake -G Ninja -DCCACHE_DIR=${CCACHE_DIR} ../src

This saves the value in the cache and then it is preserved regardless of
what environment I have when I do subsequent build steps.




On Fri, Jan 20, 2017 at 7:44 AM, David Lind <davidklind at gmail.com> wrote:

> Scott,
>
> I have find_program implemented to find ccache, as shown below.
>
>     find_program(CCACHE ccache)
>     if(CCACHE)
>         set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE})
>         set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ${CCACHE})
>     endif()
>
> That’s not the issue. The issue is telling ccache where to place it’s
> cache files. If I compiles for toolchain X, Y and X, I need to set
> CCACHE_DIR accordingly. Otherwise the cache will be useless.
>
> —Dave
>
> On Jan 19, 2017, at 12:57 PM, Craig Scott <craig.scott at crascit.com> wrote:
>
> Rather than relying on environment variables, you can use CMake's
> find_program() command to find ccache on your path and then tell CMake to
> use that as a launcher. You can find an article with a detailed explanation
> of how to set this up here:
>
> https://crascit.com/2016/04/09/using-ccache-with-cmake/
>
> An advantage of this approach is that the build will work with or without
> ccache installed. We've been using this in production for some time now and
> it works very smoothly. The technique can probably also be extended to
> support Windows too with clcache <https://github.com/frerich/clcache>,
> but I haven't tried that yet.
>
>
> On Fri, Jan 20, 2017 at 5:00 AM, David Lind <davidklind at gmail.com> wrote:
>
>> I am porting existing makefiles to cmake. One of the key features of
>> these makefiles is setting the CCACHE_DIR environment variable based upon
>> the tool chain selected.
>>
>> I have TC_<toolchain>.cmake files created. Ideally, I would add a line to
>> these files to set the CCACHE_DIR. But, CMake doesn’t have the ability to
>> set environment variables during the build step. So, I’m stuck.
>>
>> Has anyone encountered this kind of situation?
>>
>> —Dave
>> --
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>
>
> --
> Craig Scott
> Melbourne, Australia
> https://crascit.com
>
>
>


-- 
Craig Scott
Melbourne, Australia
https://crascit.com
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