Snippet

    if (pbuf.len > 1024*5) {
        volatile unsigned char *buf = pbuf.buf;
        Py_ssize_t len = pbuf.len;
Py_ssize_t i = 0;
volatile unsigned char au[100];
volatile unsigned char*x = au;
        fprintf(stderr, "CRC ENTER, buffer=%p\n", buf);
for (i=0; (size_t)i < (size_t)len; ++i) {
    fprintf(stderr, "%ld, buf=%p\n", (signed long)i, buf);
    *x = *buf++;
}

results in

test_big_buffer (test.test_zlib.ChecksumBigBufferTestCase) ... CRC ENTER, buffer=0x1014ab000
0, buf=0x1014ab000