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Is there any way to make CMake "forget" about a file in the dependency tree? My original problem (to avoid the XY situation) is the following: I want to timestamp the build of a set of tools which have complicated dependencies among them and to other tools. Right now, I want to use a pure timestamp, but later I might want add some info from the repository (SVN). Whatever system I end up implementing needs to have the following characteristics (my "X"):

  • No unnecessary rebuilding: the executables should not be rebuilt on every make if the only change would be the timestamp.
  • Update on any change: if any tool is going to be rebuilt or relinked, either by changes to its code or to one of its dependencies, the timestamp needs to be updated.

My current solution goes along the lines of creating a custom command+target that invokes CMake at make time (so the command calls CMake itself with -P script.cmake) to generate a timestamp.h file. The main files of my tools would include that file, and the projects would depend on the target so that it gets rebuilt first.

However, this has its drawbacks: if I do update the timestamp file on every call to make, then CMake's dependency scanner would know about that file even if I do not list it as an explicit dependency of my tools. Thus, every make would trigger at least a recompilation of the respective "main" files and the corresponding relink. With tens of tools, this means slowing down the build when I may be working on just two or three of them at once.

So, I was thinking that my solution would be to somehow make CMake forget about that file when building its dependency tree for the "main" file of each tool. I would keep the dependency on the custom target that does depend on the file, so that it would be regenerated first on each call to make. However, the build tool would not consider that file as relevant to determine whether it is necessary to actually rebuild each individual tool. Thus, tools only with other changes would be rebuilt (satisfying my first criterion), and any change that causes a rebuild of a tool would obviously use the version just generated (fulfilling the second criterion).

To my chagrin, I have not found a way to make the dependency scanner forget about this file, so my solution cannot be put to use. How would I go about doing such a thing? Is it even possible, or is it completely the wrong way to go about this? I am using CMake 3.4, and my code is currently C++, but I would like a solution that did not rely on C/C++ specifics, since I have a different project (written in Fortran) in which I would also like to have build timestamping.

Javier Martín
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  • Such complicated description... Do you want *some program* to output timestamp of its last rebuilding/relinking? For that purpose you want to use *header file* with timestamp, but by generating this file each time, you trigger program's rebuilding whenever its sources are changed or not. **Is my understanding correct?** If so, there are several (at least) SO questions on the same topic. Like [this one](http://stackoverflow.com/questions/13419255/how-to-get-cmake-to-create-timestamp-file-after-an-actual-build-link-do-nothin). – Tsyvarev May 16 '17 at 17:51
  • @Tsyvarev Yes, that is what I want, and your understanding is correct. However, the answer that you linked talks about generating a _separate_ timestamp file external to the executable... I suppose a solution for me could be to modify my programs to read from those "timestamp files", but I find that solution less than ideal. Other SO questions I read deal with the _mechanics_ of creating this timestamp header (custom command, custom target, making the executable targets depend on them, etc.), not with the particular problem of how to avoid rebuilding unnecessarily when using timestamp headers. – Javier Martín May 16 '17 at 18:38

1 Answers1

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I've had almost the same problem than you are. Simply solved by pushing the timestamp header file into standalone target containing only this header generator command. After that you have several choices:

1.. Exclude that project from the build by the IDE you are using. For example, for the Visual Studio you can do it by several ways:

1.1. Project->Project Dependencies...->uncheck project with that header (not always works: Error while removing project dependency in VS2010)

1.2. Build->Configuration Manager...->uncheck project with that header

2.. Create an environment variable and use the condition with that variable around the add_dependencies command in the CMakeLists.txt file.

3.. Generate 2 standalone solutions through the cmake generator with included and with excluded add_dependencies in the CMakeLists.txt file.

I've used particulary [1.2]. When i need build and debug, then i uncheck the dependecy. By default, dependecy always checked, so there is no problem to miss timestamp build for a build server.

Note:

The timestamp header will be included in all projects you want to include that header (for example, through the add_library and add_executable) and you still can observe it in the IDE under a project item menu even if a project depends on the timestamp project indirectly. This is useful if you don't want to search for the timestamp project with the header to open it from there and want to open it from any project which has included that header.

So, in case of removing the timestamp header from the add_library or add_executable you won't have that opportunity.

Andry
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