mitchm has asked for the wisdom of the Perl Monks concerning the following question:

Hey there,

I'm trying to use perlxs to call a proprietry C library from my perl scripts.
I've gone through the tutorial and such, I've successfully got the extension compiling and I can use it in my perl script and even call a couple of the functions i've already defined in the .xs file.

The problem, however, is I can't figure out how to return a variable from one of the functions. Every way I define it in the .xs file either doesn't compile, or compiles and segfaults when i run the perl script.

The C excerpt I use to call this function is as follows :

    char sError[BUFFLEN];
    QAErrorMessage(error_code, sError, BUFFLEN);

[download]
BUFFLEN being defined as 256 or whatever.
It's supposed to return a nice, helpful error message in sError...

The function doesn't seem to like being called with a char *, and I have no idea how to make xs return the sError to perl as a normal scalar.

Any suggestions would be much appreciated

Replies are listed 'Best First'.
Re: Perlxs help please
by Ovid (Cardinal) on Oct 01, 2003 at 03:57 UTC

    I don't know XS very well, but if you want to create a scalar from a char * and return it to Perl, here's a way to do it in Inline (which is much easier than XS):

    #!/usr/local/bin/perl
use strict;
use warnings;

use Inline C => <<'END_OF_C_CODE';
SV * get_string() {
    char string[] = "test";
    SV *result = newSVpv(string, 0);
    return result;
}
END_OF_C_CODE

my $string = get_string();
print $string;

    [download]

    (It's a bit verbose for clarity)

    Cheers,
    Ovid

    New address of my CGI Course.

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Re: Perlxs help please
by tachyon (Chancellor) on Oct 01, 2003 at 03:21 UTC

    perlman:perlxs OUTLIST RETVAL SV* newSVpv() depending on your context.

    cheers

    tachyon

    s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

[reply]
      This is what I tried, but it's failing with with incompatible types

      in the .h file:
      void QAErrorMessage(int, char *, int);

      [download]

      in the .xs file:
      void
QAErrorMessage(err_code, ret_error, bufflen)
        int     err_code
        SV*     ret_error
        int     bufflen
    OUTPUT:
        ret_error

      [download]

      I'm bviously missing something
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[select]

        How do you want to call the C function out of Perl? I think you are looking for something like: 

        SV *
err_msg( code )
    int code;
  INIT:
    char sError[255];
  CODE:
    QAErrorMessage( code, sError, 255 );
    SV *result = newSVpv(sError, 0);
  OUTPUT:
    result

        [download]

        cheers

        tachyon

        s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

[reply]
[d/l]
        Let me bold some things for you ;) 
        void
QAErrorMessage(err_code, ret_error, bufflen)
        int     err_code
        SV*     ret_error
        int     bufflen
    OUTPUT:
        ret_error
        void and OUTPUT do not mix (void means no output). You should try something like 
        void
QAErrorMessage(error_code)
    CASE:
        int     err_code
    PREINIT:
        int     bufflen = 256;
        char    ret_error[bufflen];
    PPCODE:
    {
         QAErrorMessage(err_code, ret_error, bufflen);
         XPUSHs(sv_2mortal( newSVpvn(ret_error,bufflen) ));
    }

        [download]

        MJD says "you can't just make shit up and expect the computer to know what you mean, retardo!"
        I run a Win32 PPM repository for perl 5.6.x and 5.8.x -- I take requests (README).
        ** The third rule of perl club is a statement of fact: pod is sexy.

[reply]
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