Assume in my company, we have same application written in C++, running in machines of RHEL5, 6 and 7.
I want to build from one single build server (which is running RHEL7) to get the executable that runs in old previous versions of RHEL. May I know if it is achievable?
I expect if I am building in RHEL7, with corresponding version of gcc and glibc (and other libs) available in RHEL5, the resulting executable should run in RHEL5. Is my understanding correct? Or are there more things to pay attention to?
I expect if I am building in RHEL7, with corresponding version of gcc and glibc (and other libs) available in RHEL5, the resulting executable should run in RHEL5.
In theory, yes. In practice, installing multiple versions of glibc and other libraries on your RHEL7 system is probably a lost cause - especially the very old ones required by RHEL5, especially glibc, which expects to know quite a lot about the system.
The reverse may be easier - build everything on RHEL5, link statically all you can but glibc (it's essentially impossible to link statically against glibc) and hope that forward binary compatibility holds well enough. This is the route that is usually taken ("build on the oldest Linux distribution you want to support"), but I doubt that forward binary compatibility of glibc will work, as RHEL5 is extremely ancient compared to RHEL7.
Getting back to the original plan, it's probably easier to install RHEL5 and 6 in containers inside the RHEL7 machine and build those versions. After all, it's a bit like installing their gcc and libraries versions on the RHEL7 machine, but in extremely well separated sysroots - but without the overhead of having different build machines (they are all clients of the same kernel).
Finally, the extreme route is to use an alternative libc (that depends only from the kernel, choosing the oldest one you want to support) and compiling everything statically. This can be done e.g. with musl, but you'll have to compile your compiler, your libc and all your dependencies. Still, the nice result is that you'll be able to build completely standalone executables, able to run on virtually any kernel after the one you decided is your minimum requirement.