components/guile/guile-snarf.1
author Mike Sullivan <Mike.Sullivan@Oracle.COM>
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'\" t
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.\"
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.\" Copyright (c) 2009, 2011, Oracle and/or its affiliates. All rights reserved.
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.\"
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.\" This man page created by Oracle to provide a reference to the
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.\" Info format documentation for guile provided with the distribution.
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.\"
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.TH guile-snarf 1 "26 May 2008"
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.SH NAME
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guile-snarf \- a tool designed to help guile users to collect subr 
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information from distributed c files
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.SH SYNOPSIS
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/usr/bin/guile-snarf [-o outfile] [cpp-args ...]
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.SH DESCRIPTION
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When writing C code for use with Guile, you typically define a set of 
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C functions, and then make some of them visible to the Scheme world by 
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calling the scm_c_define_gsubr function; a C function published in this
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way is called a subr. If you have many subrs to publish, it can sometimes 
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be annoying to keep the list of calls to scm_c_define_gsubr in sync with
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the list of function definitions. Frequently, a programmer will define
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a new subr in C, recompile the application, and then discover that the
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Scheme interpreter cannot see the subr, because of a missed call to 
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scm_c_define_gsubr.
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.LP
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Guile provides the guile-snarf command to manage this problem. Using this
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tool, you can keep all the information needed to define the subr alongside
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the function definition itself; guile-snarf will extract this information 
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from your source code, and automatically generate a file of calls to 
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scm_c_define_gsubr which you can #include into an initialization function. 
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.LP
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The guile-snarf program will extract initialization actions to outfile or 
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to standard output when no outfile has been specified or when outfile 
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is -. The C preprocessor is called with cpp-args (which usually include 
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an input file) and the output is filtered to extract the initialization
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actions.
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.LP
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If there are errors during processing, outfile is deleted and the program
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exits with non-zero status.
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.LP
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During snarfing, the pre-processor macro SCM_MAGIC_SNARFER is defined. 
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You could use this to avoid including snarfer output files that don't yet 
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exist by writing code like this:
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.LP
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    #ifndef SCM_MAGIC_SNARFER
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    #include "foo.x"
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    #endif
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.LP
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If the environment variable CPP is set, use its value instead of the C 
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pre-processor determined at Guile configure-time. 
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.SH EXAMPLES
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For example, here is how you might define a new subr called clear-image,
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implemented by the C function clear_image:
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.LP
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 #include <libguile.h>
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 SCM_DEFINE (clear_image, "clear-image", 1, 0, 0,
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            (SCM image_smob),
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            "Clear the image.")
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 #define FUNC_NAME s_clear_image
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 {
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   /* C code to clear the image in image_smob... */
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 }
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 #undef FUNC_NAME
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 void
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 init_image_type ()
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 {
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     #include "image-type.x"
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 }
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.LP
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The SCM_DEFINE declaration says that the C function clear_image implements
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a Scheme subr called clear-image, which takes one required argument (of
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type SCM and named image_smob), no optional arguments, and no rest argument.
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See Doc Snarfing, for info on the docstring.
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.LP
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This works in concert with FUNC_NAME to also define a static array of 
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characters named s_clear_image, initialized to the string "clear-image".
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The body of clear_image may use the array in error messages, instead of
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writing out the literal string; this may save string space on some systems.
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.LP
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Assuming the text above lives in a file named image-type.c, you will need 
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to execute the following command to prepare this file for compilation:
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.I guile-snarf -o image-type.x image-type.c
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.LP
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This scans image-type.c for SCM_DEFINE declarations, and writes to
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image-type.x the output:
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.LP
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scm_c_define_gsubr (s_clear_image, 1, 0, 0, (SCM (*)() ) clear_image);
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.LP
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When compiled normally, SCM_DEFINE is a macro which expands to a 
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declaration of the s_clear_image string and the function header for 
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clear_image.
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.LP
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Note that the output file name matches the #include from the input file.
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Also, you still need to provide all the same information you would if you 
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were using scm_c_define_gsubr yourself, but you can place the information 
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near the function definition itself, so it is less likely to become 
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incorrect or out-of-date.
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.LP
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If you have many files that guile-snarf must process, you should consider
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using a fragment like the following in your Makefile:
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 snarfcppopts = $(DEFS) $(INCLUDES) $(CPPFLAGS) $(CFLAGS)
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 .SUFFIXES: .x
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 .c.x:
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        guile-snarf -o $ $< $(snarfcppopts)
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.LP
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This tells make to run guile-snarf to produce each needed .x file from the
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corresponding .c file.
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.LP
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The program guile-snarf passes its command-line arguments directly to the 
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C preprocessor, which it uses to extract the information it needs from 
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the source code. this means you can pass normal compilation flags to 
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guile-snarf to define preprocessor symbols, add header file directories, 
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and so on.