Been thinking about changing over to 64 bit, but I was just curious about whether or not I'll have to worry about incompatibility with a few 32bit-only applications I use, once I've set up multilib. I'll test it out in vbox when I get home but I wanted to check here to see if I could maybe get a solid yes/no/maybe answer.
thanks

Yes. That's what multilib is for.

thanks. I thought I remembered reading that there were certain cases that it had to actually be a 32 bit OS for certain programs to be able to run.

Last weekend I was upgrading Firefox and didn't initially realize I had downloaded the 32-bit version on my 64-bit machine. I did an upgradepkg, replacing my older 64-bit copy with the latest 32-bit version. I've got multilib setup, and 32-bit Firefox ran without any noticeable difference. I happened to take one more look at the package name and realized it wasn't x86_64, and replaced it just to keep my system clean.

quick question before I do this. are there any reasons a person should go with x86 slackware rather than multilib enabled slackware64?

and will the 32bit-only Adobe Flash Player plugin work in 64bit Firefox?

If you happen to have a 32-bit system, x86_64 will not work. So, first you'll need to have a system that's capable of running 64-bit.
Another reason people have posted about is that they don't want a multi-lib system. Frankly, I don't know why one would not want that, but that's just me I guess. There are 32-bit applications you cannot recompile yourself, and those are not going to stop me from running 64bit in general.

And to confirm: yes: multilib is just about that: running 32bit applications in a 64bit environment. Too bad: but the reverse won't work as easily. You'll need a layer in between (qemu for example) to emulate a 64bit processor...

so if a program is a precompiled .bin application built for 32bit, it won't work even on a 64bit multilib?

It WILL work on a multilib system. What Ramurd is saying is just that you need a multilib system if you are running 64bit and you still want to be able to run 32bit applications.

No, wrong interpretation!

In short: a program will run if it's libraries / dependencies are installed correctly. So if the base 32bit libraries are installed (which is what multilib does) other 32bit applications can be compiled and thus also run.

Read more carefully and look for the context: I replied on the question if there is any reason to keep running x86 versions rather than 64bit multi-libbed. So I quote myself here: That's about the reason to keep runnin x86: for keeping the system "clean" without the 32bit libraries as dependencies installed. I'll stretch that line a bit with a few extra words: There are 32-bit applications you cannot recompile yourself, which I still can run when you run multilib, so those are not stopping me from running 64bit in general.

Is it clear this way?

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Yes if you use nspluginwrapper.

Why would this be a concern though, when there's a 64-bit Flash player plugin that works perfectly well? Just install the "square preview" release, either from Adobe's site, or via Alien Bob's package.

Please remember that is a 32bit "program" has dependencies, those must be 32bit as well

example: "program A", which is only available in 32bit, depends on "program B" + "library 1" + " library 2"
On your brand new slackware64, you only have "library 2" installed, but it's the 64bit version...
... so, for "program A" to work, you have to compile the 32bit version of "program B" + "library 1" + " library 2"

You don't actually have to install all 32bit libraries. You only need enough to satisfy the dependencies of what 32bit programs you want to run (which might be a lot for some things like Firefox). Statically linked 32bit programs shouldn't have any issue.

One interesting question is: what do you get if you compile a C program on a multilib system?

A statically linked program should not have any issues, true. Since most programs are dynamically linked, I did not mention statically linked programs for brevity and keeping things a bit less complex, as there is theory and practice ;-)

What you get if you compile a program on a multilib system depends on how you setup the environment; By default you'd get 64bit programs; For example in Alien Bob's multilib packages (which I assume are the way most people setup their multilib system in Slackware at least) there's a script that sets your environment to point to the 32bit libraries, so that you'd get a 32 bit application.

I think there's more to it than that. But maybe gcc will just generate either 32-bit or 64-bit code based on the environment? What about the other tools? I can believe the linker could figure it out dynamically (and complain if there is an incompatible mix). Don't know about binutils.

But at this point, I'm doing duallib instead of multilib, so I have cleaner semantics.

don't have Alien Bob's multilib packages in front of me, but weren't the binutils there as well; Anyway: if you tell gcc to link against 32bit libraries and build a 32bit binary, the result will be a 32bit binary. If you make either of the two 64bit, you'll get errors;

And what would be the difference between duallib and multilib in your view?