Hi,

Looks sane to me, but there's a portability issue in that your code requires a C99 compliant compiler.

It therefore won't build for me with any of my Microsoft compilers - though I don't have a recent MS compiler to test with.
(The most recent MS compiler I have is  Microsoft (R) C/C++ Optimizing Compiler Version 14.00.40310.41 for AMD64.)

Even if I pre-declare everything and rewrite your XSub as:

int testing(int channel, SV* byte_ref, int len){
    SV ** elem;
    AV *  bytes = (AV*)SvRV(byte_ref);
    int num_bytes = av_len(bytes) + 1;
    unsigned char buf[num_bytes];
    int i;
    int x;

    if (channel != 0 && channel != 1){
        croak("channel param must be 0 or 1\n");
    }

    if (! SvROK(byte_ref) || SvTYPE(SvRV(byte_ref)) != SVt_PVAV){
        croak("not an aref\n");
    }

    num_bytes = av_len(bytes) + 1;

    if (len != num_bytes){
        croak("$len param != elem count\n");
    }



    for (i=0; i<len; i++){
        elem = av_fetch(bytes, i, 0);
        buf[i] = (int)SvNV(*elem);
    }

    /*
     * here, I'll be passing the char buffer and len
     * to an external C function. For display, I'll
     * just print the elements
     *
     *  if ((spiDataRW(channel, buf, len) < 0){
     *      croak("failed to write to the SPI bus\n");
     *  }
     */

    for (x=0; x<len; x++){
        printf("%d\n", buf[x]);
    }
}
[download]

it still errors out with:

try_pl_1bcb6.xs(10) : error C2057: expected constant expression
try_pl_1bcb6.xs(10) : error C2466: cannot allocate an array of constan
+t size 0
try_pl_1bcb6.xs(10) : error C2133: 'buf' : unknown size
NMAKE : fatal error U1077: 'cl' : return code '0x2'
Stop.
[download]

I would work around that by dynamically allocating buf (Newxz/Safefree) but perhaps there's another way.

Anyway - it's only an issue when (if) your code meets a compiler that's not C99 compliant.

Cheers,
Rob

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