feature-test

feature-test provides foreign feature testing macros and read-syntax. This can be used to alter code at compile-time based on system support for a particular feature.

  1. feature-test
    1. Overview
    2. Determining feature support
      1. feature-test interface
      2. feature-test example
    3. Acting on feature support
    4. Bugs and limitations
    5. About this egg
      1. Source
      2. Author
      3. Version history
      4. License

Overview

Feature testing is a time-honored tradition of C programmers who wish to write portable code.

First, the programmer probes the system with a utility such as configure, generating a header file with #defines such as HAVE_IPV6 or HAVE_ADDRINFO. The programmer may also manually generate a header file by examining the system (e.g. with #ifdef) and then defining appropriate HAVE_... macros.

Second, the HAVE_... macros are used to select the appropriate code path at compile-time. If the feature test is simple or rare, the HAVE_... defines are often omitted.

In Chicken, when writing code that interfaces to C, we can naturally use the same technique inside the C code itself. But what if we want to choose a different Scheme code path based on available system features? For example, say we want to choose between defining the following procedures, depending on whether IPv6-only socket binding is available:

(define (ipv6-v6-only? s) (getsockopt s IPPROTO_IPV6 IPV6_V6ONLY))
(define (ipv6-v6-only? s) (error "IPv6 is not supported"))

In that example, IPPROTO_IPV6 and/or IPV6_V6ONLY may not even be #defined on our system, so we cannot simply create a foreign-variable referring to them, as it could result in a compilation error.

Some possible approaches are:

Feature testing with the feature-test egg proceeds in two phases:

  1. Determine which foreign features are supported by your system by using C header files and the feature-test extension to register features before compile-time.
  2. Alter your generated Scheme code by using cond-expand (effective at compile-time), or by using read syntax from the feature-test-syntax extension (effective at read-time).

Determining feature support

To use feature-test, you

  1. Take any C preprocessor instructions from your main Scheme module and move them into separate include file(s) like myegg.h. Source this file from your module.
  2. Create a Scheme features file such as myegg-features.scm, sourcing the same C header file(s) as your main module (here myegg.h).
  3. Use directives from the feature-test module in myegg-features.scm to select which features you'd like visible to your module.
  4. Compile and execute myegg-features.scm. This will generate a Scheme file on standard output, consisting of one line for each tested feature: either register-feature! if present, or unregister-feature! if absent. Redirect this output to myegg-config.scm.
  5. Compile your main module, adding -X myegg-config.scm to csc. Features will be registered with the compiler before your code is compiled, so they are visible at read-time and compile-time.

feature-test interface

[syntax] (declare-foreign-features FEATURE1 FEATURE2 ...)

For each feature F, tests whether F is #defined in C, and creates a new boolean #define reflecting this. This new #define is prefixed with the declaration-prefix.

For example, using the (default) declaration prefix HAVE_ and the feature AF_UNIX:

(declaration-prefix HAVE_)
(declare-foreign-features AF_UNIX)
/* generates the C code */
#ifdef AF_UNIX
#define HAVE_AF_UNIX 1
#else
#define HAVE_AF_UNIX 0
#endif

The boolean define HAVE_AF_UNIX is now safely visible to a foreign-variable. In contrast, referring to AF_UNIX from Scheme when undefined would result in a compilation error.

[syntax] (register-foreign-features FEATURE1 FEATURE2 ...)

For each feature F, accesses the corresponding boolean #define in C, usually generated by declare-foreign-features. Then, generates code to register or unregister the feature for future compiles.

The boolean define is prefixed with the current declaration-prefix, and the registered feature will be prefixed with the registration-prefix.

For example:

(declaration-prefix "HAVE_")
(registration-prefix "MYEGG_")
(register-foreign-features AF_UNIX)

will expand to code like:

(declare-foreign-variable HAVE_AF_UNIX bool "HAVE_AF_UNIX")
(if HAVE_AF_UNIX (emit-register!   'MYEGG_AF_UNIX)
                 (emit-unregister! 'MYEGG_AF_UNIX))

And when compiled and executed, the following is printed to standard output

(register-feature! 'MYEGG_AF_UNIX)     ;; if AF_UNIX was defined
(unregister-feature! 'MYEGG_AF_UNIX)   ;; if AF_UNIX was not defined
[syntax] (define-foreign-features FEATURE1 FEATURE2 ...)

Equivalent to

(declare-foreign-features FEATURE1 FEATURE2 ...)
(register-foreign-features FEATURE1 FEATURE2 ...)
[syntax] (declaration-prefix X)

Prefix added to the base feature name when declaring a foreign feature. This can be a string or a symbol.

Defaults to HAVE_.

[syntax] (registration-prefix X)

Prefix added to the base feature name when registering a foreign feature. This can be a string or a symbol.

Defaults to the empty string.

feature-test example

In this simple and contrived example, we test for the presence of IPPROTO_IPV6 and IPV6_V6ONLY in C, and register or unregister the corresponding features in future compiles. On Windows 2000 and XP, for example, IPPROTO_IPV6 is defined but IPV6_V6ONLY is not.

Our test module mysock.scm allows you to create an IPv6 socket and test its IPv6 bind-only option status; if the option is unavailable, this is detected at compile time and the test is defined to throw a Scheme error. Our little socket6 procedure makes some assumptions of its own, but it's only for illustration.

A real example can be found in the source code to the socket egg.

/* mysock.h */
#ifdef _WIN32
# include <winsock2.h>
# include <ws2tcpip.h>
#else
# include <netinet/in.h>
# include <sys/socket.h>
#endif
;;; mysock-features.scm
#> #include "mysock.h" <#
(use feature-test)
(declaration-prefix HAVE_)   ;; Exact value not important here.
(registration-prefix "")
(define-foreign-features IPPROTO_IPV6 IPV6_V6ONLY)
;;; mysock.setup
(compile mysock-features.scm)
(run (./mysock-features > mysock-config.scm))
(compile -sJ -X mysock-config.scm mysock.scm)
;; [install-extension is omitted for our test]
;;; mysock.meta
;; Can be left blank for our example 
;; but needs to exist even for chicken-install -n.
;;; mysock-config.scm (generated on linux)
(register-feature! 'IPPROTO_IPV6)
(register-feature! 'IPV6_V6ONLY)
;;; mysock-config.scm (generated on mingw32, pre-Vista)
(register-feature! 'IPPROTO_IPV6)
(unregister-feature! 'IPV6_V6ONLY)
;;; mysock.scm
#> #include "mysock.h" <#
(module mysock (ipv6-v6-only? socket6)
 (import scheme chicken foreign)
 ;; Get integer socket option NAME on fd SOCK at LEVEL
 (define getsockopt
  (foreign-lambda* int ((int sock) (int level) (int name))
		   "int ret; socklen_t sz = sizeof(ret);"
		   "if (getsockopt(sock, level, name, (void *)&ret, &sz) < 0)"
		   "  C_return(-1);"
		   "C_return(ret);"))
 ;; Create an IPv6 TCP socket and return its file descriptor
 ;; for testing purposes.  Assume AF_INET6 and SOCK_STREAM are defined.
 (define socket6 (foreign-lambda* int ()
		   "C_return(socket(AF_INET6,SOCK_STREAM,0));"))

 (cond-expand
  ((and IPPROTO_IPV6 IPV6_V6ONLY)
   (define-foreign-variable _ipproto_ipv6 int "IPPROTO_IPV6")
   (define-foreign-variable _ipv6_v6only int "IPV6_V6ONLY")
   (define (ipv6-v6-only? s)
     (getsockopt s _ipproto_ipv6 _ipv6_v6only)))
  (else
   (define (ipv6-v6-only? s)
     (error "IPv6 only binding is not supported")))))
### Build
$ chicken-install -n mysock.setup

### Test on UNIX
$ csi -R mysock -p "(ipv6-v6-only? (socket6))"
0

### Test on Windows XP
> csi -R mysock -p "(ipv6-v6-only? (socket6))"
Error: IPv6 only binding is not supported

Acting on feature support

cond-expand is the usual way to test and act on feature support. However, cond-expand works at macroexpansion time, as does the default #+ reader macro. Therefore, it generally cannot be used inside macros.

To address this, reader macros that do feature testing at read-time are provided in the feature-test-syntax extension. Use it like:

csc -X feature-test-syntax myegg.scm
[read] #+
#+FEATURE EXPR

Test FEATURE at read-time and, if present, expand to EXPR. FEATURE may be any feature expression permitted in a cond-expand, such as windows or (and windows macosx).

#+ can be used inside macros because it is expanded when the macro form is read, prior to macroexpansion. However, this requires a Chicken version >= 4.6.7, which will omit EXPR if the feature test is false. In earlier versions, the test expands to a (void) form, like the built-in #+. (void) forms are usually illegal inside macro bodies.

Here is an example that assumes Chicken is at least 4.6.7, which will omit EXPR on a false test:

(cond ((eq? x _af_inet) "internet address family")
      #+AF_UNIX
      ((eq? x _af_unix) "unix address family")
      (else "unknown address family")

If AF_UNIX is a registered feature at compile-time, it will be read as:

(cond ((eq? x _af_inet) "internet address family")
      ((eq? x _af_unix) "unix address family")
      (else "unknown address family")

If AF_UNIX is not a registered feature, it will be read as:

(cond ((eq? x _af_inet) "internet address family")
      (else "unknown address family")

However, if AF_UNIX is unregistered and you are using Chicken prior to 4.6.7, it will instead expand into the illegal:

(cond ((eq? x _af_inet) "internet address family")
      (##core#undefined)
      ((eq? x _af_unix) "unix address family")
      (else "unknown address family")

So be careful.

[read] #-

Like #+, but of opposite polarity.

[read] #?
#?(FEATURE CONSEQUENT ALTERNATE)

Perform an if-then test at read-time on FEATURE, expanding to CONSEQUENT if FEATURE is present or ALTERNATE if absent. FEATURE may be any feature expression permitted in a cond-expand, such as windows or (and windows macosx).

#? can be used inside macros because it is expanded when the macro form is read, prior to macroexpansion. It expands correctly irrespective of Chicken version.

#? is similar to the Common Lisp idiom

#+FEATURE CONSEQUENT
#-FEATURE ALTERNATE

and, in Chicken versions >= 4.6.7 it is exactly equivalent, even inside macro bodies. However, in previous versions #+ and #- will not work properly inside macros; see #+ for further explanation.

An example of #? which is essentially equivalent to cond-expand:

(define af/unix #?(AF_UNIX _af_unix #f))
;; is basically the same as
(define af/unix (cond-expand (AF_UNIX _af_unix) (else #f)))

A more powerful example of #?:

(cond ((eq? x _af_inet) "internet address family")
      #?(AF_UNIX
         ((eq? x _af_unix) "unix address family")
         (#f))
      (else "unknown address family")

which, if AF_UNIX is a registered feature, expands into

(cond ((eq? x _af_inet) "internet address family")
      ((eq? x _af_unix) "unix address family")
      (else "unknown address family")

and if not, expands into

(cond ((eq? x _af_inet) "internet address family")
      (#f)
      (else "unknown address family")

In the latter case, the false clause cannot succeed and is hopefully optimized out by the compiler. #+ would be more appropriate, but requires Chicken >= 4.6.7. This technique doesn't work with every macro, but you do what you can.

Bugs and limitations

About this egg

Source

https://github.com/ursetto/feature-test-egg

Author

Jim Ursetto

Version history

0.2.0
Chicken 5 support
0.1.1 Internal release-info fix
0.1
Initial release

License

Copyright (c) 2011-2019 Jim Ursetto.  All rights reserved.

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