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by brettc (Novice)
on May 23, 2001 at 03:22 UTC ( #82429=perlquestion: print w/replies, xml ) Need Help??
brettc has asked for the wisdom of the Perl Monks concerning the following question:

I am interfacing with a DLL. I am (trying) use Win32::api. One of the calls to the DLL asks for a pointer to address. EG in C:
int test_function( int test_id, size_t& sender_data_size, char*& tagged_result_html, char*& nav_results );
So how do I send the pointer to the address?? How do I populae the call to char*& tagged_results_html? Once I send the pointer to the address correctly the function(dll) will write to that address. How do I access the pointer to the address and get the data out??? Any Ideas or guidance would be cherished. A humble perl guy.

Replies are listed 'Best First'.
Re: Win32::API
by RhetTbull (Curate) on May 23, 2001 at 06:12 UTC
    Reference the Win32::API docs. To use a pointer in a Win32 API call, simply pass in an ordinary perl scalar (not a reference, just a scalar). Note however, that you must make sure the scalar is big enough to hold the results. e.g. if the Win32 API call is expecting a pointer to a string and you know the call will put at most 80 chars in the string, initialize the string (perl scalar) like so:
    $buffer = " " x 80;
    See this writeup on SystemParametersInfo or this one on GetShortPathName for an example of Win32 API calls that take a pointer.
(tye)Re: Win32::API
by tye (Sage) on May 24, 2001 at 02:54 UTC

    Tell Win32::API that your pointers to addresses are of type "P". Then you need to allocate space for the address to be placed: my $addr= '\0' x 4;
    then pass in $addr to Call().

    Then to extract the data, you'll have to use unpack. (APIs that require pointers to addresses often also require you to free buffers that get allocated for you, so don't forget to do that if it is required for your case.)

    If the data that the address points to is of known length, then you use "P$len" as the unpack format. If the data that the address points to is a '\0'-terminated string, then use "p" as the unpack format. If the length of the data is inferred from parts of the data itself, then you'll have to use "P$len" to get enough data to infer the next value to use for $len and repeat until you have all of the data. Using "P$len" with $len too large can cause a "core dump". So, for a worst-case example, if the data is a string terminated by a '$', then you'd have to extract the list via something slow and ugly like:

    my $addr= '\0' x 4; $api->Call( $addr ); my $len= 1; my $string; do { $string= unpack "p$len", $string; } until( '$' eq substr($string,-1) );

            - tye (but my friends call me "Tye")
      Thanks that is the second time you have saved my bacon. Thanks tye Brett

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