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sbcl/.quicklisp/dists/quicklisp/software/trivial-features-20190710-git/SPEC.md

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+TRIVIAL-FEATURES
+================
+
+This is a first *draft* of a description of what symbols should be
+present in `CL:*FEATURES*` for various platforms.  A possible future
+direction of this documentation might be a CDR document, if it turns
+out to be a good idea.  (Making the language of this document much
+more precise will be necessary then.)
+
+We will start by limiting ourselves to OS, CPU and endianness features
+on Windows and POSIX platforms.
+
+There are various possible implementation strategies ranging from null
+implementations (when the host Lisp already pushes the wanted feature)
+to using FFI (e.g. calling uname() to grab system information.
+
+
+Specification
+-------------
+
+### ENDIANNESS
+
+Either `:LITTLE-ENDIAN` or `:BIG-ENDIAN` should present in
+`\*FEATURES\*`.  For the time being, we will not concern ourselves
+with other orderings, switchable endianness, etc.
+
+
+### OPERATING SYSTEM
+
+On Windows, `:WINDOWS` should be present in `*FEATURES*`.
+
+On POSIX systems, the "sysname" information from uname(3) should be
+used to push the appropriate symbol to `*FEATURES*` by upcasing that
+string (or downcasing for the "modern" lisps) and interning it in the
+keyword package.
+
+Examples:
+
+  - `:DARWIN`
+  - `:LINUX`
+  - `:NETBSD`
+  - `:OPENBSD`
+  - `:FREEBSD`
+
+For convenience, `:UNIX` should be pushed when running on
+POSIX/UNIX-like operating system (that doesn't include Windows) and
+`:BSD` should be present when running on BSD-based systems (that
+includes Darwin)
+
+[add `:MACH` too?]
+
+
+### CPU
+
+These features should be mutually exclusive:
+
+  - `:X86`
+  - `:X86-64`
+  - `:PPC`
+  - `:PPC64`
+  - `:MIPS`
+  - `:ALPHA`
+  - `:SPARC`
+  - `:SPARC64`
+  - `:HPPA`
+  - `:HPPA64`
+
+[add more ...]
+
+[note: it's debatable whether `:X86` shouldn't also be exported on
+x86-64, and `:PPC` on ppc64.  SBCL doesn't.  Other ways
+to handle, for example, the x86/x86-64 case would be to export
+something like `:PC386` in both cases or have an additional `:X86-32`. Or
+finally, have just `:X86`, `:PPC`, etc, and add `:32-BIT-CPU` and
+`:64-BIT-CPU` features.]
+
+
+Unreferenced References
+-----------------------
+
+  * [CLHS: Variable \*FEATURES\*][1]
+  * [Maintaining Portable Lisp Programs][2], by Christophe Rhodes
+
+
+[1]: http://www.lispworks.com/documentation/HyperSpec/Body/v_featur.htm
+[2]: http://www-jcsu.jesus.cam.ac.uk/~csr21/papers/features.pdf