I am trying to avoid copying the buffer redundantly by doing a sv_setpvn every time. Is this the correct way to have a function write directly into the string pointer in the SV? I did look at the source code for sv_setpvn to write this, but I'm not sure if this is the correct way to do it or not, especially with the null character on the end of the buffer beyond the end of the CUR length. Is buffer ST(1) also? My rough testing says this code works, but are there any edge cases where it wouldn't (magic, undef, SVIV instead of SVPV, reference SV, etc)?

I did find out that even though Perl_sv_grow can handle an undef SV/non PV SV and will upgrade it for you, the macro SvGROW (((XPV*) (mySvPtr)->sv_any)->xpv_len < (length) ? Perl_sv_grow(my_perl,mySvPtr,length) : ((mySvPtr)->sv_u.svu_pv)) on Perl 5.10.0 can't handle a SV without a PV part (access violation, null pointer dereference), so I had to add the SvUPGRADE manually, even though its sorta redundant. Hypothetically I thought of replacing the SvGROW with the function call version, or is that forbidden in XS since the functions behind macros aren't frozen in perl? Also I see a performance reduction if the macro is replaced with the function. 

int
ReadStream(handle, buffer, bufferSize, bytesRead)

     unsigned long handle
     char * buffer = NO_INIT
     unsigned long bufferSize
     unsigned long  bytesRead  =  NO_INIT
CODE:
     SvUPGRADE((SV*)ST(1), SVt_PV);
     buffer = SvGROW((SV*)ST(1), bufferSize+1);
     RETVAL = ReadStream(handle, buffer, bufferSize, &bytesRead);
     buffer[bytesRead] = '\0';
     SvCUR_set((SV*)ST(1), bytesRead);
     (void)SvPOK_only_UTF8((SV*)ST(1));
     SvTAINT(ST(1));
     SvSETMAGIC(ST(1));
OUTPUT:   
     RETVAL
     bytesRead

[download]


Also in the source code for sv_setpvn I saw "SV_CHECK_THINKFIRST_COW_DROP(sv);" ? Do I need it? What is it? This is a made up Win32 function, but its irrelavent. I'm more interested in bugs in my code with the SV interaction.

In reply to XS, function writing directly into SV PV pointer, SvGROW doesn't like non PV SVs by patcat88

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.