How to force conan to build from source, but only if it is not in the cache?

  c++, conan

I am using conan in an enterprise environment where the operating system is rather old and has an old version of glibc (2.11). As a result, a lot of the pre-built binaries on conan.io do not end up working in my environment. However, conan doesn’t know that and will happily download and install them on my system, resulting in link-time errors.

I have found that if I build from source I can get the libraries to work.

My desired behavior would be as follows:

  • The first time using conan install to install the library (e.g. it is not in my cache) then conan will build from source and place it in my cache, then use it.
  • On subsequent invocations of conan install, conan finds the cached library and uses that without having to rebuild from source.
  • I am invoking conan install as part of an automated build script, so I would like to not have to modify the invocation depending on if this is the first time the library is installed or not (but modifying a configuration file is fine).

I have had troubles obtaining this behavior in practice. Here are the challenges I have run into:

  • If I use conan install --build=thelibrary then conan will rebuild that library from source every time I invoke conan install --build=thelibrary, even if it is already present in my cache.
  • If I use conan install --build=missing, then Ican trick conan into building the library by setting some build options that do not have a pre-built binary associated with them.
    • This is fragile, as it only works for projects with enough build options that it is not tractable to create pre-built options for all combinations.
    • It also doesn’t work if all the build options I need correspond to a pre-built binary.

Here is what I am looking for (and I assume exists but am not able to find):

  • Some setting I can place in my conanfile.txt (or some other configuration file) that tells conan to ignore pre-built binaries for a given library or libraries and instead build from source, but use the cached version if it is available.
    • This ideally should work without me having to tinker with build options.
  • I don’t necessarily want to build all libraries from source, just the ones that won’t run on my ancient OS, but if I have to settle for "all-or-nothing" I will take "all".

Is this possible with conan?

Source: Windows Questions C++

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