Some functions in .xs file can't be found

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

O Monks, please hear my question.

This concerns the distribution Audio::TagLib and the xs files therein, which interface to the C++ taglib library.

In one of the .xs files, some of the functions that are declared suffer from the (infamous?) error:

Can't locate object method "header" via package "Audio::TagLib::Tag"
[download]

Here's a snip of the .xs file. isEmpty() is recognized, header() and the functions that follow it in the file are not.

bool 
TagLib::ID3v2::Tag::isEmpty()
CODE:
    RETVAL = THIS->isEmpty();
OUTPUT:
    RETVAL

void 
TagLib::ID3v2::Tag::header()
PREINIT:
    TagLib::ID3v2::Header * h;
PPCODE:
    h = THIS->header();
    ST(0) = sv_newmortal();
    sv_setref_pv(ST(0), "Audio::TagLib::ID3v2::Header", (void *)h);
    SvREADONLY_on(SvRV(ST(0)));
    XSRETURN(1);
[download]

Applying nm(1) to the .so file suggests that header() should be located.

t _ZL36XS_Audio__TagLib__ID3v2__Tag_isEmptyP11interpreterP2cv
t _ZL35XS_Audio__TagLib__ID3v2__Tag_headerP11interpreterP2cv
[download]

Everything in this particular .xs that follows isEmpty() suffers the same fate. However, in the .c file compiled from the .xs files (there are quite a few), other functions from other .xs files do not duffer the same fate.

Insight would be welcome.

Comment on Some functions in .xs file can't be found Select or Download Code

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Re: Some functions in .xs file can't be found by syphilis (Archbishop) on Nov 13, 2013 at 02:22 UTC
The XS file from which you quote is xs/id3v2tag.xs (package Audio::TagLib::ID3v2::Tag). There's also a declaration of `TagLib::Tag::isEmpty()` in xs/tag.xs (package Audio::TagLib::Tag) but no such declaration for the "header" function. I suspect this goes at least partway to explaining that isEmpty() can be loaded via package Audio::TagLib::Tag, but header() cannot. Cheers, Rob	[reply] [d/l]
Re: Some functions in .xs file can't be found by BrowserUk (Patriarch) on Nov 13, 2013 at 01:44 UTC
The function is defined as returning void, but then you call `XSRETURN(1)` Change: `void TagLib::ID3v2::Tag::header()` [download] To: `SV* TagLib::ID3v2::Tag::header()` [download] With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l] [select]
Re^2: Some functions in .xs file can't be found by ikegami (Patriarch) on Nov 13, 2013 at 15:05 UTC
No, it should be `void`. The C function doesn't return anything. `#define XSRETURN(off) \ STMT_START { \ const IV tmpXSoff = (off); \ PL_stack_sp = PL_stack_base + ax + (tmpXSoff - 1); \ return; \ } STMT_END` [download] There are two ways of returning to Perl from an XS function: Place it on the Perl stack. `void foo() { ... mXPUSHs(sv); }` [download] Return it from the C function. The typemap will place it on the Perl stack after any conversion and mortalizing it performs. `SV* foo() { ... return sv; }` [download] The OP's code uses the former approach.	[reply] [d/l] [select]
Re^3: Some functions in .xs file can't be found by BrowserUk (Patriarch) on Nov 13, 2013 at 16:59 UTC
Indeed. Thanks for the correction. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply]
Re: Some functions in .xs file can't be found by bulk88 (Priest) on Nov 13, 2013 at 06:37 UTC
xs/tag.xs has in it `#include "tag.h" MODULE = Audio::TagLib PACKAGE = Audio::TagLib::Tag PROTOTYPES: ENABLE ################################################################ # # PUBLIC MEMBER FUNCTIONS # ################################################################ void TagLib::Tag::DESTROY() CODE: /* skip if READONLY flag on */ if(!SvREADONLY(SvRV(ST(0)))) delete THIS;` [download] see "PACKAGE = Audio::TagLib::Tag"? That is where the method calls will wind up in. But since this is C++ XS which I'm not familiar with, not C XS, I think the "TagLib::Tag::" get stripped off. As others said, your header() method in XS exists in a different class than you want it. Check the docs of your C++ library whether the TagLib::Tag class has a header method or not. According to http://taglib.github.io/api/classTagLib_1_1Tag.html it does not. BTW, you should also mention you are the CPAN maintainer of Audio-TagLib ;)	[reply] [d/l]
Re^2: Some functions in .xs file can't be found by geoffleach (Scribe) on Nov 13, 2013 at 23:11 UTC
OK, you got me. I confess I am the maintainer, alas. The good news is that I was shooting myself in the foot by using the wrong abstraction for the file being manipulated. I needed to use Audio::TagLib::MPEG::File rather than Audio::TagLib::FileRef. The latter supports more genereic tag methods, but not the ID3v2 methods, of which header() is one. The bad news is that now I have to write more tests. Sigh. Thanks to all who replied.	[reply]
Re: Some functions in .xs file can't be found by Anonymous Monk on Nov 13, 2013 at 01:40 UTC
What relates TagLib::ID3v2::Tag::header with Audio::TagLib::Tag?	[reply]