Thanks, I set return type to UV, it works and I can call it back and forth in C:

use strict;
use warnings;
use Devel::Peek;

my $dd = "hello world\n";
Dump $dd;
my $ee = cc($dd);
printf("the dd string address is %x \n", cc($dd));
dd($ee);
use Inline Config => BUILD_NOISY => 1,
                                       CLEAN_AFTER_BUILD => 0;
use Inline 'C' => <<'CODE';

int dd(UV x){

                printf("string is %s \n", (char*)x);
                return 0;
}
UV cc(SV* a){
return SvPVX(a);
}
CODE
###############output###########
andy@andyhome:~/one-off$ perl abe.pl
SV = PV(0x56386170b050) at 0x56386172f008
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x5638617addf0 "hello world\n"\0
  CUR = 12
  LEN = 14
  COW_REFCNT = 1
the dd string address is 5638617addf0
string is hello world
[download]

I pass it back to perl, But I got little confused: in typemap, both unsigned int and UV refer to T_UV and in OUTPUT it does convert to UV:

int             T_IV
unsigned        T_UV
unsigned int    T_UV
UV              T_UV
T_UV
     sv_setuv($arg, (UV)$var);
[download]

Platform:
    osname=linux
    osvers=4.15.0-52-generic
    archname=x86_64-linux
    uname='linux andyhome 4.15.0-52-generic #56-ubuntu smp tue jun 4 2
+2:49:08 utc 2019 x86_64 x86_64 x86_64 gnulinux '
    config_args='-de -Dprefix=/home/andy/perl5/perlbrew/perls/perl-5.3
+0.0 -Aeval:scriptdir=/home/andy/perl5/perlbrew/perls/perl-5.30.0/bin'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=undef
    usemultiplicity=undef
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='cc'
    ccflags ='-fwrapv -fno-strict-aliasing -pipe -fstack-protector-str
+ong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-O2'
    cppflags='-fwrapv -fno-strict-aliasing -pipe -fstack-protector-str
+ong -I/usr/local/include'
    ccversion=''
    gccversion='7.4.0'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
[download]

You are using a typemap to create an SV to return to perl. That SV is being assigned an integer value. The only types of integer values that SVs can have are IVs and UVs. Hence a plain integer gets upgraded to an IV/UV before being assigned to the SV.

Dave.

Hence a plain integer gets upgraded to an IV/UV before being assigned to the SV.

That's what I thought before, but as above, in typemap, unsigned int also use sv_setnv function. Does it mean sv_setnv function wouldn't upgrade an IV to UV silently if it get a unsigned integer?




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