From: Brian Caswell <bmc@...>
Date: 2005-12-28T07:31:32+09:00
Subject: dl/loader ptr issue

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I'm having fits with passing pointers to a function I've imported via 
DL::Importable.  I'm wrapping a simple opaque blob library (part of 
libdnet).  However, the function I'm wrapping doesn't seem to work as I 
expect it to.

     extern "int blob_read(void *blob, void *buf, int size)"

Basically, read the specified amount (size) from the blob, and put it 
in buf.  return status code.  From what I've read, I should be able to 
wrap this with something akin to:

	def read (blob, len)
		ptr = DL.malloc(len)
		ret = Dnet.blob_read(blob, ptr.ref, len)
		return ret.to_s
	end

However, I get a EXC_BAD_ACCESS segv duing the to_s call.  How should I 
be getting access to buf?

My current test is attached.  I have validated that the shared object 
works as expected, when using dlopen/dlsym in C.  (dnet.c works as an 
example usage of the C api)

Brian


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#!ruby
require 'dl/import'

module Dnet
    extend DL::Importable
    dlload "/sw/lib/libdnet.dylib"

    extern "void *blob_new()"
    extern "int blob_read(void *, void *, int)"
    extern "int blob_write(void *, void *, int)"
    extern "int blob_seek(void *, int, int)"

    class Blob
        @blob = nil

        def initialize
            @blob = Dnet.blob_new()
        end

        def write(str)
            ret = Dnet.blob_write(@blob, str.to_ptr, str.length)
        end
      
        def rewind()
            ret = Dnet.blob_seek(@blob, 0, 0)
        end

        def read(len)
            ptr = DL.malloc(len)
            ret = Dnet.blob_read(@blob, ptr.ref, len)
            if ptr.size == 0
                ''
            else
                ptr.to_s
            end
        end
    end
end

if $0 == __FILE__
    blob = Dnet::Blob.new()
    blob.write("hi")
    blob.write("mom")
    blob.rewind()
    a = blob.read(2)
    if a != "hi"
        print "ACK! #{a} should be hi\n"
    end
end

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#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <dlfcn.h>

main()
{
    void *handle;
    void *(*blob_new)(void);
    int (*blob_write)(void *, void *, int);
    int (*blob_read)(void *, void *, int);
    int (*blob_seek)(void *, int, int);
    int (*blob_free)(void *);
    char *c;
    char tmp[32];
    void *blob;


    handle = dlopen("/sw/lib/libdnet.dylib", 1);
    if(dlerror()) { fprintf(stderr, "couldnt open libdnet.dylib\n"); abort(); }

    blob_new = dlsym(handle, "blob_new");
    if(dlerror()) { fprintf(stderr, "couldnt get blob_new symbol\n"); abort(); }

    blob_write = dlsym(handle, "blob_write");
    if(dlerror()) { fprintf(stderr, "couldnt get blob_write symbol\n"); abort(); }

    blob_read = dlsym(handle, "blob_read");
    if(dlerror()) { fprintf(stderr, "couldnt get blob_read symbol\n"); abort(); }

    blob_seek = dlsym(handle, "blob_seek");
    if(dlerror()) { fprintf(stderr, "couldnt get blob_seek symbol\n"); abort(); }

    blob_free = dlsym(handle, "blob_free");
    if(dlerror()) { fprintf(stderr, "couldnt get blob_free symbol\n"); abort(); }

    blob = blob_new();
    blob_write(blob, "foo", 3);
    blob_write(blob, "bar", 3);
    blob_seek(blob, 0, 0);
    blob_read(blob, tmp, 3);
    tmp[4] = 0;
    printf("GOT %s\n", tmp);
    blob_read(blob, tmp, 3);
    printf("GOT %s\n", tmp);
    tmp[4] = 0;
    blob_seek(blob, 0, 0);
    blob_read(blob, tmp, 6);
    tmp[7] = 0;
    printf("GOT %s\n", tmp);
    blob_free(blob);
    dlclose(handle);
}

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