Drawing on the code that has so far been presented, it looks to me that you might want: 
SV* test( SV *a, SV *b ) {
    SV * c;
    if(SvREADONLY(a)) {
      c = newSVsv(a);
      sv_catsv(c, b);
      return c;
      }

    sv_catsv(a,b);
    c = newSVsv(a);
    return c;
}

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I don't know how to avoid the "uninitialized value in subroutine entry" warning (apart from the obvious). Predicting whether you're going to get it or not is a little tricky. Of the 2 following subs, only test5() produces the warning for me: 
SV * test4(SV * a, SV * b){return newSVuv(42);}
SV * test5(char * a, char * b){return newSVuv(42);}

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And if I comment out the sv_catsv() call in the code that vkon posted, then the warning disappears. Obviously the exact nature of what the xsub does has a bearing on whether we get that warning.

Cheers,
Rob

In reply to Re^3: XS/Inline::C concat *any* two SVs. by syphilis
in thread XS/Inline::C concat *any* two SVs. by BrowserUk

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