Thank you so much for the quick reply. Indeed, your solution is very elegant. I am a newbie in xsubb. I did use the second method. 
 PREINIT:
        unsigned char uuid_arr[16];
    CODE:
        char convert_arr[33];
        int i;
        XGenerateSessionID((uuid_t*) &uuid_arr);
        memset(convert_arr,'\0',33);
        for(i=0; i< 16; i++){
         snprintf(&convert_arr[i*2], sizeof(convert_arr)-(i*2), "%02X"
+, uuid_arr[i]);
        }
        RETVAL = newSVpvn(convert_arr,33);
    OUTPUT:    
         RETVAL

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If I had not converted the unsigned char back to char, the compiler complains of signedness.

In reply to Re^2: Need to pass an unsigned char array from an xsub back to Perl by Anonymous Monk
in thread Need to pass an unsigned char array from an xsub back to Perl by Anonymous Monk

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