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Inconsistent g_snprintf behaviour (Wireshark 3.2.5)

asked 2020-07-13 11:33:06 +0000

brett323 gravatar image

updated 2020-07-13 11:39:14 +0000

Hello,

I'm continuing working on my dissector and am stumped on this behaviour.

We have a timestamp field in a message that I've written a custom callback function for.

This is the structure of the timestamp, starting at octet 43 within this particular internal message (little endian):

  43  .cal_time                calendar_time_t    STRUCT              8
  43  ..hundredths_of_seconds  bcd_t              byte                 1
  44  ..seconds                bcd_t              byte                 1
  45  ..minutes                bcd_t              byte                 1
  46  ..hours                  bcd_t              byte                 1
  47  ..day                    bcd_t              byte                 1
  48  ..month                  bcd_t              byte                 1
  49  ..year                   long_bcd_t         word                 2

And this is the CF I wrote to output the timestamp correctly:

static void
print_cal_time(gchar *result, guint64 cal_time)
{
    guint16 cal_time_year = (cal_time >> 48);
    gint8 cal_time_month = (cal_time >> 40) & 0xFF;
    gint8 cal_time_day = (cal_time >> 32) & 0xFF;
    gint8 cal_time_hour = (cal_time >> 24) & 0xFF;
    gint8 cal_time_min = (cal_time >> 16) & 0xFF;
    gint8 cal_time_sec = (cal_time >> 8) & 0xFF;
    gint8 cal_time_100ths = cal_time & 0xFF;

    g_snprintf(result, ITEM_LABEL_LENGTH, "%4X-%02X-%02X %02X:%02X:%02X.%02X",
               cal_time_year,
               cal_time_month,
               cal_time_day,
               cal_time_hour,
               cal_time_min,
               cal_time_sec,
               cal_time_100ths);
}

The other relevant bits in my dissector:

proto_tree_add_item(dmx_tree, hf_emb_dx_FCC8_cal_time, payload_tvb, 43, 8, ENC_LITTLE_ENDIAN);

... and ...

{ &hf_emb_dx_FCC8_cal_time,
    {"FCC8 calendar time", "emb.dmx.fcc8.cal_time",
        FT_UINT64, BASE_CUSTOM, CF_FUNC(print_cal_time), 0x0,
        "Calender time of log", HFILL}},

This works most of the time, but sometimes it is giving a weird output for the 1/100s field.

Working okay:

FCC8 calendar time: 2020-06-18 11:47:58.64

0000   64 58 47 11 18 06 20 20

Not working:

FCC8 calendar time: 2020-06-18 11:47:59.FFFFFF96

0000   96 59 47 11 18 06 20 20

The 1/100s field is being output as a dword even though the variable is a single byte and the format string specifies %02X:

gint8 cal_time_100ths = cal_time & 0xFF;

What am I missing? I see the same behaviour in 64-bit 3.2.5 compiled on both Win10 and macOS.

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answered 2020-07-13 12:49:32 +0000

grahamb gravatar image

You're using gint8 as the type for the fields which is signed, so any value greater than 0x7F is treated as negative and sign extended. Try using guint8.

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Comments

Dah! Of course! Much obliged Graham, thanks. I shouldn't be using gint8 at all, so have replaced them all (not just in that function) - much better!

brett323 gravatar imagebrett323 ( 2020-07-13 19:40:21 +0000 )edit

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Asked: 2020-07-13 11:33:06 +0000

Seen: 38 times

Last updated: Jul 13