scpi: Rephrase length logic in data block reception, comment/group code

[libsigrok.git] / src / scpi / scpi.c

diff --git a/src/scpi/scpi.c b/src/scpi/scpi.c
index efa839de748972052898156ecd1d4f087be3d3b6..f743f64d6b75d5f072cc2e8af8e8195e5b881113 100644 (file)
--- a/src/scpi/scpi.c
+++ b/src/scpi/scpi.c
@@ -743,55 +743,74 @@ SR_PRIV int sr_scpi_get_block(struct sr_scpi_dev_inst *scpi,
 {
        int ret;
        GString* response;
-
-       char buf[10] = { 0 };
+       char buf[10];
        long llen;
        long datalen;
 
+       /*
+        * Assume an initial maximum length, optionally gets adjusted below.
+        * Prepare a NULL return value for when error paths will be taken.
+        */
        response = g_string_sized_new(1024);
        *scpi_response = NULL;
 
+       /* Get (the first chunk of) the response. */
        ret = sr_scpi_get_data(scpi, command, &response);
        if (ret != SR_OK) {
                g_string_free(response, TRUE);
                return ret;
        }
 
+       /*
+        * SCPI protocol data blocks are preceeded with a length spec.
+        * The length spec consists of a '#' marker, one digit which
+        * specifies the character count of the length spec, and the
+        * respective number of characters which specify the data block's
+        * length. Raw data bytes follow (thus one must no longer assume
+        * that the received input stream would be an ASCIIZ string).
+        *
+        * Get the data block length, and strip off the length spec from
+        * the input buffer, leaving just the data bytes.
+        */
        if (response->str[0] != '#') {
                g_string_free(response, TRUE);
                return SR_ERR_DATA;
        }
-
        buf[0] = response->str[1];
+       buf[1] = '\0';
        ret = sr_atol(buf, &llen);
        if ((ret != SR_OK) || (llen == 0)) {
                g_string_free(response, TRUE);
                return ret;
        }
-
        memcpy(buf, &response->str[2], llen);
+       buf[llen] = '\0';
        ret = sr_atol(buf, &datalen);
        if ((ret != SR_OK) || (datalen == 0)) {
                g_string_free(response, TRUE);
                return ret;
        }
-
-       // strip header
        g_string_erase(response, 0, 2 + llen);
 
+       /*
+        * If the initially assumed length does not cover the data block
+        * length, then re-allocate the buffer size to the now known
+        * length, and keep reading more chunks of response data.
+        */
        if (response->len < (unsigned long)(datalen)) {
                int oldlen = response->len;
                g_string_set_size(response, datalen);
                g_string_set_size(response, oldlen);
        }
-
        while (response->len < (unsigned long)(datalen)) {
-               if (sr_scpi_get_data(scpi, NULL, &response) != SR_OK) {
+               ret = sr_scpi_get_data(scpi, NULL, &response);
+               if (ret != SR_OK) {
                        g_string_free(response, TRUE);
                        return ret;
                }
        }
 
+       /* Convert received data to byte array. */
        *scpi_response = g_byte_array_new_take(
                (guint8*)g_string_free(response, FALSE), datalen);