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I've tried searching around but have come up with nothing: How can I determine the bits (32-bit or 64-bit) of my operating system with elisp?

On my 64-bit Linux machine, system-configuration evaluates to "x86_64-unknown-linux-gnu". I don't have a Windows 32-bit machine available, but could I potentially do

(defconst 32-bit (not (null (string-match "^i386-.*" system-configuration))))
(defconst 64-bit (not (null (string-match "^x86_64-.*" system-configuration))))

to detect the bits?

Edit: "Why would you want to know that?" I wanted to write an Elisp script that would download and run the appropriate version of an executable (32 or 64-bit) for the user's OS.

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    Wouldn't that only give you what architecture emacs was built on? You could be running a 32 bit emacs binary on a 64 bit system. I would assume the best way to do this would be (case system-type ...) with calls to a os specific tool that can determine architecture. – Jordon Biondo Apr 30 '15 at 18:44
  • As was mentioned in other comments, you may need this for different purposes: to identify the size of the integer you can have in Emacs, to identify what other programs can receive as an input. If Emacs is 32-bit, but runs on a 64-bit machine, then knowing the system parameters won't help you to identify the size of an integer you can use. Also, you could count on integer overflow, say, (ash 1 31) to find out whether integers will overflow or not. – wvxvw Apr 30 '15 at 23:17
  • Re' your last edit: I don't think there is a cross-platform way to identify that. You would probably need to write OS-specific code for each case you care about and assume the system is the same as Emacs in other cases. – wvxvw May 1 '15 at 8:38
  • There's always /proc/cpuinfo. – Qudit May 2 '15 at 17:08
  • @Qudit No /proc on Windows. – Jackson May 3 '15 at 6:47
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This is quite hacky, and I'm not sure how reliable it is, but the value gc-cons-threshold depends on the underlying word size. It's 800000 on 64-bit systems, and 400000 on 32-bit systems.

(defconst 32-bit (= gc-cons-threshold 400000))
(defconst 64-bit (= gc-cons-threshold 800000))
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The issue of 64/32 bit is just a part of the problem. Look at the larger problem: the user may be running Debian on a machine with an ARM processor, for example. Or she may be running a FreeBSD system. Also, under Windows, if the downloaded program will want to interact with Emacs, you'll probably want a different version depending on whether your Emacs is a Cygwin or a "native" build.

So, system-configuration is probably a good starting point, but you probably want to keep more of it than just the 64/32 bit info. You may actually want to keep the whole of system-configuration.

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Given the use case (taken from a comment I wanted to write an Elisp script that would download and run the appropriate version of an executable (32 or 64-bit) for the user's OS.), the nicest way might be just to ask the user which version to download.

I'm not sure how you'd do that in elisp though

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    Using completing-read would be one way. – Qudit May 3 '15 at 17:15
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Why would you want to now that?

You should be looking at the specific Emacs property/capability and not the hardware attributes.

E.g., most-positive-fixnum will tell you the size of the fixnum (incidentally, it will also tell you the word size: if it is bugger than 2^32, then you have a 64-bit CPU).

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    Calling out to external programs is probably a good enough reason – Squidly Apr 30 '15 at 16:29
  • what external program depends on that but cannot determine it itself? – sds Apr 30 '15 at 16:30
  • Choosing which program to call perhaps? Not everyone writes nice software - some people distributes different binaries under different names depending on 32/64 bit (see e.g. sqldeveloper) – Squidly Apr 30 '15 at 17:05
  • "Why would you want to know that?" I wanted to write an Elisp script that would download and run the appropriate version of an executable (32 or 64-bit) for the user's OS. – Jackson May 1 '15 at 6:39

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