Replace

SV* my_c_function(SV* sv) {
   STRLEN len;
   const char* s = SvPVbyte(sv, len);
   ...
   return newSV(...);
}
[download]

with

SV* my_c_function(SV* sv) {
   STRLEN len;
   const char* s = SvPVutf8(sv, len);
   ...
   return newSVpvn_utf8(..., 1);
}
[download]

---

Example:

use strict;
use warnings;
use feature qw( say );

use open ":std", ":encoding(UTF-8)";

use Inline C => <<'__EOS__';


static const char hex_syms[16] = { '0', '1', '2', '3', '4', '5', '6', 
+'7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };

static void my_c_function(char* dst, const char* src, STRLEN n) {
    while (n--) {
        *(dst++) = hex_syms[ ((unsigned char)*src) >> 4 ];
        *(dst++) = hex_syms[ *src & 0xF ];
        *(dst++) = '.';
        ++src;
    }

    dst[-1] = 0;
}

SV* my_xs_function(SV* buf_sv) {
    STRLEN buf_len;
    const char* buf = SvPVutf8(buf_sv, buf_len);

    if (buf_len == 0)
        return newSVpvs("");

    {
        STRLEN hex_len = buf_len * 3 - 1;
        char* hex;
        SV* hex_sv;
        Newx(hex, hex_len, char);
        my_c_function(hex, buf, buf_len);
        hex_sv = newSVpvn_utf8(hex, hex_len, 1);
        Safefree(hex);
        return hex_sv;
    }
}


__EOS__

my $s = "\x{C9}ric";
utf8::downgrade( my $d = $s );  # Let's test with both
utf8::upgrade(   my $u = $s );  #     string storage formats.
say $u eq $d ? "Same" : "Different";

for my $s ($d, $u) {
   say "UCP:   ", sprintf("%vX", $s);   # C9.72.69.63
   say "UTF-8: ", my_xs_function($s);   # C3.89.72.69.63
}
[download]

---

Optimized: (Avoids creating two buffers and copying one into the other. Also protects against memory leaks from long jumps in the C code by mortalizing the allocated memory sooner.)

SV* my_xs_function(SV* buf_sv) {
    STRLEN buf_len;
    const char* buf = SvPVutf8(buf_sv, buf_len);

    if (buf_len == 0)
        return newSVpvs("");

    {
        STRLEN hex_len = buf_len * 3 - 1;
        SV* hex_sv = sv_2mortal(newSV(hex_len));
        SvPOK_on(hex_sv);
        SvCUR_set(hex_sv, hex_len);
        SvUTF8_on(hex_sv);
        my_c_function(SvPVX(hex_sv), buf, buf_len);
        return hex_sv;
    }
}
[download]

---

---

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