Here is an example. It's done in Inline but is just straight vanilla XS. The stack pointer behaves itself over the separate calls.

package Serialize;
use Data::Dump;

sub new { bless {foo=>'bar'}, shift }
sub toString { return Data::Dump::dump($_[0]) } 
my $obj = new Serialize;
my $str = "some string";
print serialize($obj), $/;
print serialize($str), $/;

use Inline Config => FORCE_BUILD => 1, CLEAN_AFTER_BUILD => 0;
use Inline C => <<'END_OF_C_CODE';

void serialize(SV* sv_data) {
    int count;
    dXSARGS;
    sp = mark;
    printf("\nEntry SP is %d\n", sp);
    if (sv_isobject(sv_data)) {
        // this is an object so presume a toString method
        PUSHMARK(sp);
        XPUSHs(sv_data);  // whack it back on the stack
        PUTBACK;
        count = perl_call_pv("toString", G_SCALAR);
        if ( count != 1 ) 
            croak("Expected 1 value got %d\n", count);
        XPUSHs(sv_2mortal(newSVsv(POPs)));
    }
    else {
        // just return SV with something appended for this example
        sv_catpv(sv_data, " - serialized\0");
        XPUSHs(sv_2mortal(newSVsv(sv_data)));  
    }
    PUTBACK;
    printf("Exit  SP is %d\n", sp);
    XSRETURN(1);
}
END_OF_C_CODE

__DATA__
bless({ foo => "bar" }, "Serialize")
some string - serialized

Entry SP is 32203548
Exit  SP is 32203552

Entry SP is 32203548
Exit  SP is 32203552
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